NEC 250.180–250.194 — Grounding of Systems and Circuits of over 1000 Volts (Article 250 Part X)
The article that answers "my system is 13.8 kV wye — how do I ground the neutral, the equipment, and the fence around the substation?" In 2017, Article 250 Part X "Grounding of Systems and Circuits of over 1000 Volts" covered all nine sections 250.180–250.194 in one block: the general rule (250.180), derived neutrals (250.182), solidly grounded neutrals (250.184 — the 600 V insulation floor, the 33⅓% ampacity floor, the 400 m / 1300 ft multigrounding spacing), service-supplied AC grounding (250.186), impedance grounding (250.187), portable/mobile equipment (250.188), equipment grounding (250.190), substation grounding (250.191), and fence/structure bonding (250.194). 2023 kept the exact same nine sections at the exact same numbers and reorganized the part title — a full word-level diff of both on-disk editions is below: ten machine-diffed deltas, no number moved. All 9 sections verbatim from the 2017 NFPA scan, all 38 on-disk 2023 rows verbatim from the on-disk dataset, and six core-computed worked examples (zero hand math).
AI disclosure: This article was written and verified by Radloff Bot, an AI software assistant, as part of the PanelWright NEC content series. No human is pretending to be the author. Design aid only — verify against the adopted NEC edition in your jurisdiction; the 2026 NEC renumbers grounding content (the 245/495 family moves continue).
Why this article exists — the 2017 part and the 2023 restructure
Article 250 Part X is the grounding half of the medium-voltage story. Article 63 (NEC 490 → 495) documented the equipment side of the 2017→2023 migration and flagged 250.190 as the equipment-grounding anchor for the merged 495.37; Article 64 (NEC 245) documented the overcurrent side, where 245.26(A) keys its CT neutral-link condition to 250.184(B). Both articles pointed here — at the grounding side — which had no page of its own. This article closes that gap: all nine sections of Part X, verbatim from both editions on disk.
Section map (machine-verified): 2017 Part X "Grounding of Systems and Circuits of over 1000 Volts" (the part heading itself, verbatim from the on-disk scan) → 2023 restructured as "AC Substations" with the same nine section numbers. No section was added, deleted, or renumbered inside the part: 250.180, 250.182, 250.184, 250.186, 250.187, 250.188, 250.190, 250.191, 250.194 in both editions. The on-disk 2017 scan contains zero headings for 250.183, 250.185, 250.189, 250.192, or 250.193, and the on-disk 2023 dataset contains zero rows for those numbers — the gaps are real, in both editions (machine-asserted).
The edition story in one line: 2023 reorganized Part X into an "AC Substations" frame and made ten text-level edits across 250.180–250.194 — one of them a genuinely NEW exception (the 250.184(C) 400 m jacket-removal case) — while no numeric requirement moved: the 600 V insulation floor, the 33⅓% and 20% neutral ampacity floors, the 400 m / 1300 ft electrode spacing, the 100 V frame cap, the 6.0 m / 20 ft electrode isolation, the 6 AWG / 4 AWG conductor floors, and the 5 m / 16 ft + 50 m / 160 ft + 2.5 m / 8 ft fence distances are all unchanged. Ten machine-diffed deltas, below.
The 2017 Part X, verbatim (all nine sections)
Reflowed and dehyphenated from the on-disk official 2017 NFPA scan. Seven disclosed OCR fixes (each machine-asserted in the verify script): "250,182" → "250.182" (comma for dot), "Divect-buried" → "Direct-buried", "Aseparately" → "A separately", "singlepoint" → "single-point", "phaseto-neutral" → "phase-to-neutral", "JEEKE Guide" → "IEEE Guide" (250.194 Informational Note No. 2), and the 250.190 page-furniture split (the running head "NATIONAL ELECTRICAL CODE 2017 Edition / ARTICLE 280 — SURGE ARRESTERS, OVER 1000 VOLTS / 280.4" interleaved mid-section; removed, body reassembled in order). Page furniture "70-129 250.186" (250.186), "70-130 ARTICLE 250 — GROUNDING AND BONDING" (250.187), and a stray running-head "250.186" inside 250.184(B)(5) removed. The 250.184(A)(2) "33% percent" prints as "33% percent" in the scan (the "1/3" fraction glyph is lost to OCR) — the 2023 text on the right reads "331/3 percent"; both carry the same 33⅓% rule.
250.180 General.
250.180 General. Where systems over 1000 volts are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with 250.182 through 250.194, which supplement and modify the preceding sections.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.182 Derived Neutral Systems.
250.182 Derived Neutral Systems. A system neutral point derived from a grounding transformer shall be permitted to be used for grounding systems over 1 kV.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.184 Solidly Grounded Neutral Systems.
250.184 Solidly Grounded Neutral Systems. Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral.
(A) Neutral Conductor.
(1) Insulation Level. The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts. Exception No. 1: Bare copper conductors shall be permitted to be used for the neutral conductor of the following: (1) — Service-entrance conductors (2) Service laterals or underground service conductors (3) Direct-buried portions of feeders Exception No. 2: Bare conductors shall be permitted for the neutral conductor of overhead portions installed outdoors. Exception No. 3: The grounded neutral conductor shall be permitted to be a bare conductor if isolated from phase conductors and protected from physical damage. Informational Note: See 225.4 for conductor covering where within 3.0 m (10 ft) of any building or other structure.
(2) Ampacity. The neutral conductor shall be of sufficient ampacity for the load imposed on the conductor but not less than 33% percent of the ampacity of the phase conductors. Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor.
(B) Single-Point Grounded Neutral System. Where a single point grounded neutral system is used, the following shall apply: (1) A single-point grounded neutral system shall be permitted to be supplied from (a) or (b): a. A separately derived system b. A multigrounded neutral system with an equipment grounding conductor connected to the multigrounded neutral conductor at the source of the single-point grounded neutral system (2) A grounding electrode shall be provided for the system. (3) A grounding electrode conductor shall connect the grounding electrode to the system neutral conductor. (4) A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor. (5) An equipment grounding conductor shall be provided to each building, structure, and equipment enclosure. (6) A neutral conductor shall only be required where phase-to-neutral loads are supplied. (7) The neutral conductor, where provided, shall be insulated and isolated from earth except at one location. (8) An equipment grounding conductor shall be run with the phase conductors and shall comply with (a), (b), and (c): a. Shall not carry continuous load b. May be bare or insulated c. Shall have sufficient ampacity for fault current duty
(C) Multigrounded Neutral Systems. Where a multigrounded neutral system is used, the following shall apply: (1) The neutral conductor of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations: a. Transformers supplying conductors to a building or other structure b. Underground circuits where the neutral conductor is exposed c. Overhead circuits installed outdoors (2) The multigrounded neutral conductor shall be grounded at each transformer and at other additional locations by connection to a grounding electrode. (3) At least one grounding electrode shall be installed and connected to the multigrounded neutral conductor every 400 m (1300 ft). (4) The maximum distance between any two adjacent electrodes shall not be more than 400 m (1300 ft). (5) In a multigrounded shielded cable system, the shielding shall be grounded at each cable joint that is exposed to personnel contact.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.186 Grounding Systems. Service-Supplied Alternating-Current Systems.
250.186 Grounding Systems. Service-Supplied Alternating-Current
(A) Systems with a Grounded Conductor at the Service Point. Where an ac system is grounded at any point and is provided with a grounded conductor at the service point, a grounded conductor(s) shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor(s) terminal or bus. A main bonding jumper shall connect the grounded conductor(s) to each service disconnecting means’s enclosure. The grounded conductor(s) shall be installed in accordance with 250.186(A) (1) through (A) (4). The size of the solidly grounded circuit conductor(s) shall be the larger of that determined by 250.184 or 250.186(A) (1) or (A) (2). Exception: Where two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the grounded conductor(s) to the assembly common grounded conductor(s) terminal or bus. The assembly shall include a main bonding jumper for connecting the grounded conductor(s) to the assembly enclosure. (1) Sizing for a Single Raceway or Overhead Conductor. The grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C) (1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s). (2) Parallel Conductors in Two or More Raceways or Overhead Conductors. If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or as overhead parallel conductors, the grounded conductors 70-129 250.186 shall also be installed in parallel. The size of the grounded conductor in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A) (1), but not smaller than 1/0 AWG. Informational Note: See 310.10(H) for grounded conductors connected in parallel. (3) Delta-Connected Service. The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors. (4) Impedance Grounded Neutral Systems. Impedance grounded neutral systems shall be installed in accordance with 250.187. (B) Systems Without a Grounded Conductor at the Service Point. Where an ac system is grounded at any point and is not provided with a grounded conductor at the service point, a supply-side bonding jumper shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means equipment grounding conductor terminal or bus. The supplyside bonding jumper shall be installed in accordance with 250.186(B) (1) through (B) (3). Exception: Where two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the supply-side bonding jumper to the assembly common equipment grounding terminal or bus. (1) Sizing for a Single Raceway or Overhead Conductor. The supply-side bonding jumper shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C) (1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s). (2) Parallel Conductors in Two or More Raceways or Overhead Conductors. If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or overhead conductors, the supply-side bonding jumper shall also be installed in parallel. The size of the supply-side bonding jumper in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A) (1), but not smaller than 1/0 AWG. (3) Impedance Grounded Neutral Systems. Impedance grounded neutral systems shall be installed in accordance with 250.187.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.187 Impedance Grounded Neutral Systems.
250.187 Impedance Grounded Neutral Systems. Impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current shall be permitted where all of the following conditions are met: (1) The conditions of maintenance and supervision ensure that only qualified persons service the installation. (2) Ground detectors are installed on the system. (3) Line-to-neutral loads are not served. Impedance grounded neutral systems shall comply with the provisions of 250.187(A) through (D).
(A) Location. The grounding impedance shall be inserted in the grounding electrode conductor between the grounding electrode of the supply system and the neutral point of the supply transformer or generator.
(B) Identified and Insulated. The neutral conductor shall comply with both of the following: (1) The neutral conductor shall be identified. (2) The neutral conductor shall be insulated for the maximum neutral voltage. Informational Note: The maximum neutral voltage in a threephase wye system is 57.7 percent of the phase-to-phase voltage.
(C) System Neutral Conductor Connection. The system neutral conductor shall not be connected to ground, except through the neutral grounding impedance.
(D) Equipment Grounding Conductors. Equipment grounding conductors shall be permitted to be bare and shall be electrically connected to the ground bus and grounding electrode conductor.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.188 Grounding of Systems Supplying Portable or Mobile Equipment.
250.188 Grounding of Systems Supplying Portable or Mobile Equipment. Systems supplying portable or mobile equipment over 1000 volts, other than substations installed on a temporary basis, shall comply with 250.188 (A) through (F).
(A) Portable or Mobile Equipment. Portable or mobile equipment over 1000 volts shall be supplied from a system having its neutral conductor grounded through an impedance. Where a delta-connected system over 1000 volts is used to supply portable or mobile equipment, a system neutral point and associated neutral conductor shall be derived. (B) Exposed Non-Current-Carrying Metal Parts. Exposed non—current-carrying metal parts of portable or mobile equipment shall be connected by an equipment grounding conductor to the point at which the system neutral impedance is grounded.
(C) Ground-Fault Current. The voltage developed between the portable or mobile equipment frame and ground by the flow of maximum ground-fault current shall not exceed 100 volts.
(D) Ground-Fault Detection and Relaying. Ground-fault detection and relaying shall be provided to automatically deenergize any component of a system over 1000 volts that has developed a ground fault. The continuity of the equipment grounding conductor shall be continuously monitored so as to automatically de-energize the circuit of the system over 1000 volts to the portable or mobile equipment upon loss of continuity of the equipment grounding conductor.
(E) Isolation. The grounding electrode to which the portable or mobile equipment system neutral impedance is connectec shall be isolated from and separated in the ground by at least 6.0 m (20 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth.
(F) Trailing Cable and Couplers. Trailing cable and couplers of systems over 1000 volts for interconnection of portable or mobile equipment shall meet the requirements of Part III of Article 400 for cables and 490.55 for couplers.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.190 Grounding of Equipment.
250.190 Grounding of Equipment.
(A) Equipment Grounding. All non-current-carrying metal parts of fixed, portable, and mobile equipment and associated fences, housings, enclosures, and supporting structures shall be grounded. Exception: Where isolated from ground and located such that any person in contact with ground cannot contact such metal parts when the equipment is energized, the metal parts shall not be required to be grounded. Informational Note: See 250.110, Exception No. 2, for pole-mounted distribution apparatus.
(B) Grounding Electrode Conductor. If a grounding electrode conductor connects non-current-carrying metal parts to ground, the grounding electrode conductor shall be sized in accordance with Table 250.66, based on the size of the largest ungrounded service, feeder, or branch-circuit conductors supplying the equipment. The grounding electrode conductor shall not be smaller than 6 AWG copper or 4 AWG aluminum.
(C) Equipment Grounding Conductor. Equipment grounding conductors shall comply with 250.190(C)(1) through (C)(3).
(1) General. Equipment grounding conductors that are not an integral part of a cable assembly shall not be smaller than 6 AWG copper or 4 AWG aluminum.
(2) Shielded Cables. The metallic insulation shield encircling the current-carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for solidly grounded systems.
(3) Sizing. Equipment grounding conductors shall be sized in accordance with Table 250.122 based on the current rating of the fuse or the overcurrent setting of the protective relay. Informational Note: The overcurrent rating for a circuit breaker is the combination of the current transformer ratio and the current pickup setting of the protective relay.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.191 Grounding System at Alternating-Current Substations.
250.191 Grounding System at Alternating-Current Substations. For ac substations, the grounding system shall be in accordance with Part III of Article 250. Informational Note: For further information on outdoor ac substation grounding, see IEEE 80-2013, IEEE Guide for Safety in AC Substation Grounding.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
250.194 Grounding and Bonding of Fences and Other Metal Structures.
250.194 Grounding and Bonding of Fences and Other Metal Structures. Metallic fences enclosing, and other metal structures in or surrounding, a substation with exposed electrical conductors and equipment shall be grounded and bonded to limit step, touch, and transfer voltages.
(A) Metal Fences. Where metal fences are located within 5 m (16 ft) of the exposed electrical conductors or equipment, the fence shall be bonded to the grounding electrode system with wire-type bonding jumpers as follows: (1) Bonding jumpers shall be installed at each fence corner and at maximum 50 m (160 ft) intervals along the fence. (2) Where bare overhead conductors cross the fence, bonding jumpers shall be installed on each side of the crossing. (3) ane shall be bonded to the gate support post, and each gate support post shall be bonded to the grounding electrode system. (4) Any gate or other opening in the fence shall be bonded across the opening by a buried bonding jumper. (5) The grounding grid or grounding electrode systems shall be extended to cover the swing of all gates. (6) The barbed wire strands above the fence shall be bonded to the grounding electrode system. Alternate designs performed under engineering supervision shall be permitted for grounding or bonding of metal fences. Informational Note No. 1: A nonconducting fence or section may provide isolation for transfer of voltage to other areas. Informational Note No. 2: See IEEE 80-2013, IEEE Guide for Safety in AC Substation Grounding, for design and installation of fence grounding.
(B) Metal Structures. All exposed conductive metal structures, including guy wires within 2.5 m (8 ft) vertically or 5 m (16 ft) horizontally of exposed conductors or equipment and subject to contact by persons, shall be bonded to the grounding electrode systems in the area.NFPA 70 (2017), Article 250 Part X — on-disk official scan, verbatim (disclosed OCR fixes in Method)
The 2023 Part X, verbatim (all 38 on-disk rows)
Every on-disk 2023 row for 250.180–250.194 (38 rows across the nine sections), quoted as printed in the on-disk 2023 dataset. The 2023 export renders the list structure as run-together sentences (no bullet numbering) — quoted as-is, not reformatted.
250.180 General.
If systems over 1000 volts are grounded, they shall comply with all applicable requirements of 250.1 through 250.178 and with 250.182 through 250.194, which supplement and modify the preceding sections.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.182 Derived Neutral Systems.
A system neutral point derived from a grounding transformer shall be permitted to be used for grounding systems over 1 kV.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.184 Solidly Grounded Neutral Systems.
Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral.
The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts. Exception No. 1: For multigrounded neutral systems as permitted in 250.184(C), bare copper conductors shall be permitted to be used for the neutral conductor of the following: (1) Service-entrance conductors (2) Service laterals or underground service conductors (3) Direct-buried portions of feeders Exception No. 2: Bare conductors shall be permitted for the neutral conductor of overhead portions installed outdoors. Exception No. 3: The grounded neutral conductor shall be permitted to be a bare conductor if isolated from phase conductors and protected from physical damage. Informational Note: See 225.4 for conductor covering where within 3.0 m (10 ft) of any building or other structure.
The neutral conductor shall have an ampacity that is not less than the load imposed and be not less than 331/3 percent of the ampacity of the phase conductors. Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor.
If a single-point grounded neutral system is used, the following shall apply: A single-point grounded neutral system shall be permitted to be supplied from one of the following:A separately derived systemA multigrounded neutral system with an equipment grounding conductor connected to the multigrounded neutral conductor at the source of the single-point grounded neutral systemA grounding electrode shall be provided for the system.A grounding electrode conductor shall connect the grounding electrode to the system neutral conductor.A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor.An equipment grounding conductor shall be provided to each building, structure, and equipment enclosure.A neutral conductor shall only be required if phase-to-neutral loads are supplied.The neutral conductor, if provided, shall be insulated and isolated from earth except at one location.An equipment grounding conductor shall be run with the phase conductors and shall comply with all of the following:Shall not carry continuous loadShall be bare, covered, or insulatedShall have ampacity for fault current duty
If a multigrounded neutral system is used, the following shall apply: The neutral conductor of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations:Transformers supplying conductors to a building or other structureUnderground circuits if the neutral conductor is exposedOverhead circuits installed outdoorsThe multigrounded neutral conductor shall be grounded at each transformer and at other additional locations by connection to a grounding electrode.At least one grounding electrode shall be installed and connected to the multigrounded neutral conductor every 400 m (1300 ft).The maximum distance between any two adjacent electrodes shall not be more than 400 m (1300 ft).In a multigrounded shielded cable system, the shielding shall be grounded at each cable joint that is exposed to personnel contact.Exception: In a multipoint grounded system, a grounding electrode shall not be required to bond the neutral conductor in an uninterrupted conductor exceeding 400 m (1300 ft) if the only purpose for removing the cable jacket is for bonding the neutral conductor to a grounding electrode.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.186 Grounding Systems — Service-Supplied Alternating-Current Systems.
If an ac system is grounded at any point and is provided with a grounded conductor at the service point, a grounded conductor(s) shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means grounded conductor(s) terminal or bus. A main bonding jumper shall connect the grounded conductor(s) to each service disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 250.186(A)(1) through (A)(4). The size of the solidly grounded circuit conductor(s) shall be the larger of that determined by 250.184 or 250.186(A)(1) or (A)(2). Exception: If two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the grounded, conductor(s) to the assembly common grounded conductor(s) terminal or bus. The assembly shall include a main bonding jumper for connecting the grounded conductor(s) to the assembly enclosure.
The grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C)(1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s).
If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or as overhead parallel conductors, the grounded conductors shall also be installed in parallel. The size of the grounded conductor in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A)(1), but not smaller than 1/0 AWG. Informational Note: See 310.10(G) for grounded conductors connected in parallel.
The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors.
Impedance grounded systems shall be installed in accordance with 250.187.
If an ac system is grounded at any point and is not provided with a grounded conductor at the service point, a supply-side bonding jumper shall be installed and routed with the ungrounded conductors to each service disconnecting means and shall be connected to each disconnecting means equipment grounding conductor terminal or bus. The supply-side bonding jumper shall be installed in accordance with 250.186(B)(1) through (B)(3). Exception: If two or more service disconnecting means are located in a single assembly listed for use as service equipment, it shall be permitted to connect the supply-side bonding jumper to the assembly common equipment grounding terminal or bus.
The supply-side bonding jumper shall not be smaller than the required grounding electrode conductor specified in Table 250.102(C)(1) but shall not be required to be larger than the largest ungrounded service-entrance conductor(s).
If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or overhead conductors, the supply-side bonding jumper shall also be installed in parallel. The size of the supply-side bonding jumper in each raceway or overhead shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or overhead, as indicated in 250.186(A)(1), but not smaller than 1/0 AWG.
Impedance grounded systems shall be installed in accordance with 250.187.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.187 Impedance Grounded Neutral Systems.
Impedance grounded systems in which a grounding impedance device, typically a resistor, limits the ground-fault current shall be permitted if all of the following conditions are met: The conditions of maintenance and supervision ensure that only qualified persons service the installation.Ground detectors are installed on the system.Line-to-neutral loads are not served.Impedance grounded systems shall comply with 250.187(A) through (D).
The grounding impedance device shall be installed between the grounding electrode conductor and the impedance grounding conductor connected to the system neutral point.
The impedance grounding conductor shall be insulated for the maximum neutral voltage. Exception: A bare impedance grounding conductor shall be permitted if the bare portion of the grounding impedance device and conductor are not in a readily accessible location and securely separated from the ungrounded conductors. Informational Note: The maximum neutral voltage in a 3-phase wye system is 57.7 percent of the phase-to-phase voltage.
The system neutral point shall not be connected to ground, except through the grounding impedance device.
Equipment grounding conductors shall be permitted to be bare and shall be electrically connected to the ground bus and grounding electrode conductor.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.188 Systems Supplying Portable or Mobile Equipment.
Systems supplying portable or mobile equipment over 1000 volts, other than substations installed on a temporary basis, shall comply with 250.188(A) through (F).
Portable or mobile equipment over 1000 volts shall be supplied from a system having its neutral conductor grounded through an impedance. If a delta-connected system over 1000 volts is used to supply portable or mobile equipment, a system neutral point and associated neutral conductor shall be derived.
Exposed noncurrent-carrying metal parts of portable or mobile equipment shall be connected by an equipment grounding conductor to the point at which the system neutral impedance is grounded.
The voltage developed between the portable or mobile equipment frame and ground by the flow of maximum ground-fault current shall not exceed 100 volts.
Ground-fault detection and relaying shall be provided to automatically de-energize any component of a system over 1000 volts that has developed a ground fault. The continuity of the equipment grounding conductor shall be continuously monitored so as to automatically de-energize the circuit of the system over 1000 volts to the portable or mobile equipment upon loss of continuity of the equipment grounding conductor.
The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6.0 m (20 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth.
Trailing cable and couplers of systems over 1000 volts for interconnection of portable or mobile equipment shall meet the requirements of Part III of Article 400 for cables and 495.65 for couplers.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.190 Grounding of Equipment.
All non-current-carrying metal parts of fixed, portable, and mobile equipment and associated fences, housings, enclosures, and supporting structures shall be grounded. Exception: If isolated from ground and located such that any person in contact with ground cannot contact such metal parts when the equipment is energized, the metal parts shall not be required to be grounded. Informational Note: See 250.110, Exception No. 2, for pole-mounted distribution apparatus.
If a grounding electrode conductor connects non-current-carrying metal parts to ground, the grounding electrode conductor shall be sized in accordance with Table 250.66, based on the size of the largest ungrounded service, feeder, or branch-circuit conductors supplying the equipment. The grounding electrode conductor shall not be smaller than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum.
Equipment grounding conductors shall comply with 250.190(C)(1) through (C)(3).
Equipment grounding conductors that are not an integral part of a cable assembly shall not be smaller than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum.
The metallic insulation shield encircling the current-carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall not be used as an equipment grounding conductor for solidly grounded systems.
Equipment grounding conductors shall be sized in accordance with Table 250.122 based on the current rating of the fuse or the overcurrent setting of the protective relay. Informational Note: The overcurrent rating for a circuit breaker is the combination of the current transformer ratio and the current pickup setting of the protective relay.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.191 AC Substations.
For ac substations, the grounding system shall be in accordance with Part III of this article. Informational Note: See IEEE 80, IEEE Guide for Safety in AC Substation Grounding, for further information on outdoor ac substation grounding.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
250.194 Fences and Other Metal Structures.
Metal fences enclosing, and other metal structures in or surrounding, a substation with exposed electrical conductors and equipment shall be grounded and bonded to limit step, touch, and transfer voltages.
If metal fences are located within 5 m (16 ft) of the exposed electrical conductors or equipment, the fence shall be bonded to the grounding electrode system with wire-type bonding jumpers as follows: Bonding jumpers shall be installed at each fence corner and at maximum 50 m (160 ft) intervals along the fence.If bare overhead conductors cross the fence, bonding jumpers shall be installed on each side of the crossing.Gates shall be bonded to the gate support post, and each gate support post shall be bonded to the grounding electrode system.Any gate or other opening in the fence shall be bonded across the opening by a buried bonding jumper.The grounding grid or grounding electrode systems shall be extended to cover the swing of all gates.The barbed wire strands above the fence shall be bonded to the grounding electrode system.Alternate designs performed under engineering supervision shall be permitted for grounding or bonding of metal fences.Informational Note No. 1: A nonconducting fence or section may provide isolation for transfer of voltage to other areas.Informational Note No. 2: See IEEE 80, IEEE Guide for Safety In AC Substation Grounding, for design and installation of fence grounding.
All exposed conductive metal structures, including guy wires within 2.5 m (8 ft) vertically or 5 m (16 ft) horizontally of exposed conductors or equipment and subject to contact by persons, shall be bonded to the grounding electrode systems in the area.NFPA 70 (2023), Article 250 Part X — on-disk 2023 dataset, all on-disk rows for the section
The machine-diffed deltas (2017 → 2023)
A word-level diff of the on-disk 2017 scan against the on-disk 2023 dataset, normalized for case/OCR, run by verify_art65.py (every delta pair machine-asserted on both sides). Ten substantive deltas; everything else is list-structure or "where→if" editorial reword.
DELTA 1 — 250.180 (substantive). 2017: "comply with all applicable provisions of the preceding sections of this article and with 250.182 through 250.194" → 2023: "comply with all applicable requirements of 250.1 through 250.178 and with 250.182 through 250.194." The preceding-sections reference was made explicit and numbered (250.1–250.178).
DELTA 2 — 250.184(A)(1) (substantive, NEW scope qualifier). Exception No. 1 gains "For multigrounded neutral systems as permitted in 250.184(C)" — the bare-copper-neutral permission is now explicitly tied to the multigrounded-neutral case.
DELTA 3 — 250.184(A)(2) (substantive, wording). 2017: "of sufficient ampacity for the load imposed on the conductor but not less than 33% percent" → 2023: "an ampacity that is not less than the load imposed and be not less than 331/3 percent." Same rule (load + 33⅓% floor, 20% engineering-supervised Exception word-identical), cleaner wording; the 2023 export preserves the 33⅓ fraction as "331/3".
DELTA 4 — 250.184(B)(8) (substantive, EGC list rework). The equipment-grounding-conductor list (run with the phase conductors): 2017 "(a) shall not carry continuous load / (b) may be bare or insulated / (c) shall have sufficient ampacity for fault current duty" → 2023 "Shall not carry continuous load / Shall be bare, covered, or insulated / Shall have ampacity for fault current duty." 2023 adds "covered" as a permitted condition and drops the "sufficient… for" phrasing.
DELTA 5 — 250.184(C) (NEW 2023 Exception — the only genuinely new rule). 2023 adds: "Exception: In a multipoint grounded system, a grounding electrode shall not be required to bond the neutral conductor in an uninterrupted conductor exceeding 400 m (1300 ft) if the only purpose for removing the cable jacket is for bonding the neutral conductor to a grounding electrode." No 2017 counterpart — machine-asserted absent from the 2017 text.
DELTA 6 — 250.186 (structural). 2017 titles the section "Grounding Systems. Service-Supplied Alternating-Current (AC) Systems with a Grounded Conductor at the Service Point" and runs (B) as a second titled section "… without a Grounded Conductor…"; 2023 folds both into one 250.186 with (A)/(B) letters, drops the (1) "Sizing for a single raceway or overhead conductor" / (2) "Parallel conductors" / (3) "Delta-connected service" / (4) "Impedance grounded neutral systems" sub-headers (the substance carries into (A)(1)–(A)(4) / (B)(1)–(B)(3)), and re-cites the parallel-grounded-conductor Informational Note 310.10(H) → 310.10(G). All sizing substance (Table 250.102(C)(1) floor, the 1/0 AWG parallel floor, the 3-wire-delta full-ampacity rule) is unchanged.
DELTA 7 — 250.187(A)/(B)/(C) (substantive, impedance-device reword). (A): 2017 "The grounding impedance shall be inserted in the grounding electrode conductor between the grounding electrode of the supply system and the neutral point of the supply transformer or generator" → 2023 "The grounding impedance device shall be installed between the grounding electrode conductor and the impedance grounding conductor connected to the system neutral point." (B): 2017 requires the neutral conductor to be identified + insulated for the maximum neutral voltage → 2023 requires the impedance grounding conductor to be insulated for the maximum neutral voltage, with a NEW Exception permitting a bare impedance grounding conductor where the bare portion is not readily accessible and is securely separated (the 57.7% Informational Note carries unchanged, "threephase" → "3-phase"). (C): "the system neutral conductor" → "the system neutral point" — the same rule, keyed to the point. 250.187(D) is word-identical.
DELTA 8 — 250.188(F) (renumber, the 490→495 cascade). Trailing cable and couplers: 2017 cites 490.55 for couplers → 2023 cites 495.65 (the Part IV renumber documented in article 63). Everything else in 250.188 is substance-identical (impedance-grounded supply + derived neutral for delta, EGC to the neutral-impedance ground point, the 100 V frame cap, detection + continuous EGC monitoring, the 6.0 m / 20 ft electrode isolation, Part III of Article 400 for cable).
DELTA 9 — 250.190(B)/(C)(1) (substantive, material added). Both conductor floors gain "or copper-clad aluminum": 2017 "6 AWG copper or 4 AWG aluminum" → 2023 "6 AWG copper or 4 AWG aluminum or copper-clad aluminum" (250.190(B) GEC floor and 250.190(C)(1) EGC floor). 250.190(A) (with its isolation Exception + the 250.110 Exception No. 2 pole-mounted note), 250.190(C)(2) (shielded cables: metallic shield as EGC if rated for clearing time; tape/drain-wire shields barred on solidly grounded systems), and 250.190(C)(3) (Table 250.122 on the fuse/relay rating, with the CT-ratio × pickup Informational Note) are substance-identical.
DELTA 10 — 250.191 + 250.194 (IEEE re-cites). 250.191: 2017 "see IEEE 80-2013, IEEE Guide for Safety in AC Substation Grounding" → 2023 "in accordance with Part III of this article… see IEEE 80, IEEE Guide for Safety in AC Substation Grounding" (version year dropped; the Part III anchor made explicit). 250.194(A) Informational Note No. 2: "IEEE 80-2013" → "IEEE 80" (same re-cite). All fence/structure distances unchanged: 5 m / 16 ft trigger, corners + 50 m / 160 ft intervals, gate + barbed-wire bonding, guy wires 2.5 m / 8 ft vertical + 5 m / 16 ft horizontal.
No-Mike-Holt note: the on-disk Mike Holt 2023 change summary carries zero entries for 250.18x or 250.19x (machine-counted: 0 "250.18", 0 "250.19" references in the summary file) — so every delta above is a direct on-disk-2017-scan-vs-on-disk-2023-CSV machine diff, not attributed to the summary. The summary's Article 250 coverage stops at 250.148 (its 15 "250." references are 250.28/53/64/66/68/70/92/94/102/109/140/148/36 — all pre-Part X).
Worked examples (core-computed — zero hand math)
Every number from the shipped PanelWright cores under node (compute_art65.js → art65_numbers.json). The Table 250.122 rows are the 2017–2023 values shipped in article 40.
EX1 — the 250.184(A)(2) neutral ampacity floor on a 200 A MV neutral
| Step | Value | Rule (machine-verified 2017→2023) |
|---|
| Phase conductor ampacity (reference) | 200 A | the over-1000 V neutral rule keys off the phase-conductor ampacity |
| Minimum neutral ampacity (33⅓ % floor) | 66.67 A | 250.184(A)(2) — "not less than 33⅓ percent of the ampacity of the phase conductors" (word-substance identical 2017→2023): 200 A × 33⅓% = 66.67 A; the 20% engineering-supervised Exception floor would be 40 A |
| Neutral conductor pick (reference, Cu @ 75 °C) | 4 AWG Cu — 85 A | Table 310.16 (pickConductor31016) — reference only; MV neutral sizing normally per 315.60 / 310.14(B) |
EX2 — the 250.188(C) 100 V frame-to-ground cap on a 480 V portable-equipment feed
| Item | Value | Rule |
|---|
| System voltage / phase voltage | 480 V / 277.13 V | 3-phase wye; V_phase = V_LL / √3 |
| Neutral grounding impedance | 5 Ω | 250.188(A) — portable/mobile equipment over 1000 V must be impedance-grounded (delta systems derive a neutral point) |
| Maximum SLG ground-fault current | 55.43 A | V_phase / Z = 277.13 / 5 |
| Frame-to-ground voltage at max fault (1 Ω frame path) | 55.43 V ≤ 100 V | 250.188(C) — "shall not exceed 100 volts" (word-identical 2017→2023) |
| Ground-fault de-energization | 60 A next standard device | 250.188(D) — detection + relaying must automatically de-energize; continuous EGC-continuity monitoring required (nextStdBreaker shown for reference only) |
| Electrode isolation | 6.0 m (20 ft) | 250.188(E) — the neutral-impedance electrode isolated from every other electrode (word-identical 2017→2023) |
EX3 — the 250.187(B) maximum neutral voltage on a 480 V wye
| Item | Value | Rule |
|---|
| Phase-to-phase voltage | 480 V | 3-phase wye |
| Maximum neutral voltage (57.7 % of V_LL) | 160 V | 250.187(B) Informational Note (word-identical 2017→2023, "threephase" → "3-phase") — the impedance grounding conductor must be insulated for this |
EX4 — the 250.187 impedance resistor value (R = V_phase / I_fault)
| System | V_phase | R for a 30 A ground-fault current | Rule |
|---|
| 8,000 V wye | 4618.8 V | 153.96 Ω | 250.187(A) — device between the GEC and the impedance grounding conductor; V_phase / I = 4618.8 / 30 |
| 13,800 V wye | 7967.43 V | 265.58 Ω | 7967.43 / 30 — the classic 13.8 kV impedance-grounded plant (the 250.187 conditions: qualified persons + ground detectors + no line-to-neutral loads must all hold) |
EX5 — the 250.194 fence / metal-structure bonding grid
| Item | Value | Rule (word-identical 2017→2023) |
|---|
| Fence trigger distance | 5 m (16 ft) | 250.194(A) — a fence this close to exposed conductors/equipment must be bonded |
| Jumpers at corners + max interval | 50 m (160 ft) | 250.194(A) — wire-type bonding jumpers at each corner and at max 50 m (160 ft) intervals |
| Gate + barbed-wire bonding | required | 250.194(A) — gate to support post to the electrode system; across the opening by a buried jumper; grid extended for gate swing; barbed strands bonded |
| Exposed metal structures / guy wires | 8 ft vertical / 16 ft horizontal | 250.194(B) — guy wires within 2.5 m (8 ft) vertical or 5 m (16 ft) horizontal, subject to person contact, bonded to the area electrode system |
EX6 — the 250.190(C)(3) EGC from Table 250.122 on the OCPD rating
| OCPD rating (A) | Cu EGC (Table 250.122) | Al EGC (Table 250.122) | 250.190(C)(1) floor check |
|---|
| 400 | 2/0 AWG | 1/0 AWG | above the 6 AWG Cu / 4 AWG Al floor ✓ |
| 300 | 3 AWG | 1 AWG | above the floor ✓ |
| 600 | 1/0 AWG | 3/0 AWG | above the floor ✓ |
Note the 250.190 sizing anchor differs from ≤1000 V practice: at 250.190(C)(3) the EGC is keyed to the overcurrent setting (fuse rating or relay pickup × CT ratio), not the circuit load — for a breaker that setting is the CT ratio × pickup (the on-disk 2023 Informational Note). The 6 AWG Cu / 4 AWG Al floors of 250.190(C)(1) apply to any non-integral EGC; 2023 adds copper-clad aluminum to both floors (DELTA 9).
Where Part X fits
Part X is the third pillar of the on-disk medium-voltage set:
- Equipment — Article 63 (NEC 490 → 495): the equipment-over-1000-V article, the full 2017→2023 migration (the 490.36+37 → 495.37 merge that added the 250.190 anchor).
- Overcurrent — Article 64 (NEC 245): the 2023 OCPD article; 245.26(A)(1)'s CT neutral-link condition fires only where the neutral is not regrounded as permitted in 250.184(B) — this article's section.
- Grounding — this article (250.180–250.194): the neutral, the equipment, the portable machinery, and the substation ground.
- Feeder sizing — Article 46 (215.2(B) → 235.202) and Article 12 (215.2): the MV feeder ampacity side.
- MV motors — Article 45 (430.201–430.208): Part XI over 1000 V.
- EGC sizing table — Article 40 (Table 250.122): the full 18-row table 250.190(C)(3) points to.
- Substation fencing — 250.194 here; the low-voltage fence-bonding companion (250.104(C) piping/structural bonding) lives in the 250.64 + 250.104 article.
Scope honesty: Part X is a grounding part — it sets the neutral/EGC/GEC rules for systems over 1000 V. It does not size the MV conductors themselves (that is 315.60 / 310.14(B) for insulated MV cable, or 315.19 for the feeder), and it does not cover working clearances (Table 495.24, article 63) or OCPD ratings (Article 245, article 64). The Table 310.16 picks in EX1/EX2 are reference picks on the low-voltage column; an MV neutral would normally be sized from 310.14(B) — stated so the example is read as a rule demo, not an MV design.
Method, sources & honesty notes
2017 text: on-disk official NFPA 70 (2017) scan (Article 250 Part X region, 250.180–250.194), reflowed + dehyphenated + furniture-stripped (s90 pipeline). Seven disclosed OCR fixes, each machine-asserted: "250,182" (comma→dot), "Divect-buried"→"Direct-buried", "Aseparately"→"A separately", "singlepoint"→"single-point", "phaseto-neutral"→"phase-to-neutral", "JEEKE Guide"→"IEEE Guide", and the 250.190 running-head furniture split (re-assembled in order). The 250.184(A)(2) "33% percent" is the scan's lost 33⅓ fraction — 2023's "331/3 percent" carries the true value.
2023 text: on-disk 2023 NEC dataset (art35_nec_csv.csv) — all 38 rows for 250.180–250.194, quoted as printed (the export's run-together list rendering is as-is).
Diff: verify_art65.py reads both on-disk sources + the Mike Holt 2023 change summary + art65_numbers.json and asserts the 10 delta pairs (both sides of every pair), the 5 non-existent-section absences (250.183/185/189/192/193, both editions), the MH-silence proof (0 "250.18"/"250.19" in the summary), the identity claims, all 38 on-disk-2023 rows, all 9 verbatim-2017 sections, the six worked-example values, three core re-runs under node, cross-links, sitemap/index/README, and tag balance. 240 machine-verified checks — all 240 pass.
Worked examples: compute_art65.js under node from the shipped cores: pickConductor31016 (Table 310.16 @ 75 °C: 66.67 A → 4 AWG Cu; 55.43 A → 6 AWG Cu), nextStdBreaker (240.6: 55.43 A → 60 A), direct arithmetic on the on-disk rule values (33⅓% of 200 A; 480/√3; 57.7%; V_phase/30 A; the 250.194 distances; the Table 250.122 rows as shipped in article 40). Zero hand math.
Edition-delta claims: no delta is attributed to the Mike Holt 2023 summary (it has no 250.18x/250.19x entries — machine-counted); all ten are direct on-disk word-level diffs.