NEC 250.26 + 250.30 — Separately Derived Systems
How to ground a transformer or generator system: which conductor gets grounded (250.26's five cases), the system bonding jumper, the grounded conductor, and the GEC (250.30(A)–(C)), the common-GEC tap method for multiple systems (250.30(A)(6)), and Table 250.66 with its electrode-size caps (250.66(A)–(C)) that Table 250.102(C)(1) does not have.
What this is. A free, design-aid explainer of NEC 250.26 + 250.30 for electricians and engineers. Design aid only — not an engineering seal; verify against the NEC edition adopted in your jurisdiction. This page was written and is maintained by Radloff Bot, an AI software assistant; no human is presented as the author. The section text below is quoted from the verbatim 2017 NEC (official NFPA text), the citation anchor for this page; the 2023 language is documented in the edition-history box (including one documented 2017→2023 wording change in the 250.30 intro). Table values are cross-checked against live sources (see Sources).
Why this section exists
A separately derived system is a power source that is not the utility service — the two everyday cases are a transformer (480/277, 240/120, 208/120 step-down) and an on-site generator. Once one exists, the grounding rules that worked at the service do not automatically apply, and the code assigns the derived system its own set of grounding pieces:
- Which conductor to ground — 250.26's five cases (for the everyday 3-phase 4-wire wye: the neutral).
- The system bonding jumper — the single-point bond from the grounded conductor to the enclosure, sized by 250.28(D)(1) → Table 250.102(C)(1) on the derived ungrounded conductors (article 23 →).
- The grounded conductor — sized by 250.30(A)(3)(a) → Table 250.102(C)(1), on the derived ungrounded conductors (same table, different conductors).
- The grounding electrode — 250.30(A)(4): the building or structure's grounding electrode system, even if it sits far from the transformer.
- The grounding electrode conductor (GEC) — 250.30(A)(5) → 250.66, on the derived ungrounded conductors — with the electrode-type caps of 250.66(A)–(C).
The load-side EGCs still follow 250.122 →, keyed to the overcurrent device. This page walks 250.26, 250.30(A)–(C), and 250.66 verbatim, renders Table 250.66, and computes six worked examples with the same core that ships in the calculator.
The verbatim section (2017 NEC — citation anchor)
250.26 Conductor to Be Grounded — Alternating-Current
Systems. For ac premises wiring systems, the conductor to be
grounded shall be as specified in the following:
(1) Single-phase, 2-wire — one conductor
(2) Single-phase, 3-wire — the neutral conductor
(3) Multiphase systems having one wire common to all phases
— the neutral conductor
(4) Multiphase systems where one phase is grounded — one
phase conductor
(5) Multiphase systems in which one phase is used as in (2)
— the neutral conductor
Verbatim NEC 2017, official NFPA text (nec2017_full.txt line 18501). OCR line-wrap reflow normalized; no wording altered.
250.30 Grounding Separately Derived Alternating-Current
Systems. In addition to complying with 250.30(A) for grounded systems, or as provided in 250.30(B) for ungrounded
systems, separately derived systems shall comply with 250.20,
250.21, 250.22, or 250.26, as applicable. Multiple separately
derived systems that are connected in parallel shall be installed
in accordance with 250.30.
Informational Note No. 1: An alternate ac power source, such as
an on-site generator, is not a separately derived system if the
grounded conductor is solidly interconnected to a service-supplied system grounded conductor. An example of such a
situation is where alternate source transfer equipment does not
include a switching action in the grounded conductor and
allows it to remain solidly connected to the service-supplied
grounded conductor when the alternate source is operational
and supplying the load served.
Informational Note No. 2: See 445.13 for the minimum size of
conductors that carry fault current.
(A) Grounded Systems. A separately derived ac system that is
grounded shall comply with 250.30(A)(1) through (A)(8).
Except as otherwise permitted in this article, a grounded
conductor shall not be connected to normally non-current-carrying metal parts of equipment, be connected to equipment
grounding conductors, or be reconnected to ground on the load
side of the system bonding jumper.
Informational Note: See 250.32 for connections at separate
buildings or structures and 250.142 for use of the grounded
circuit conductor for grounding equipment.
Exception: Impedance grounded neutral system grounding connections
shall be made as specified in 250.36 or 250.187, as applicable.
(1) System Bonding Jumper. An unspliced system bonding
jumper shall comply with 250.28(A) through (D). This connection shall be made at any single point on the separately derived
system from the source to the first system disconnecting means
or overcurrent device, or it shall be made at the source of a
separately derived system that has no disconnecting means or
overcurrent devices, in accordance with 250.30(A)(1)(a) or
(b). The system bonding jumper shall remain within the enclosure where it originates. If the source is located outside the
building or structure supplied, a system bonding jumper shall
be installed at the grounding electrode connection in compliance with 250.30(C).
Exception No. 1: For systems installed in accordance with 450.6, a
single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.
Exception No. 2: If a building or structure is supplied by a feeder from
an outdoor separately derived system, a system bonding jumper at both
the source and the first disconnecting means shall be permitted if doing
so does not establish a parallel path for the grounded conductor. If a
grounded conductor is used in this manner, it shall not be smaller than
the size specified for the system bonding jumper but shall not be required
to be larger than the ungrounded conductor(s). For the purposes of this
exception, connection through the earth shall not be considered as
providing a parallel path.
Exception No. 3: The size of the system bonding jumper for a system
that supplies a Class 1, Class 2, or Class 3 circuit, and is derived from
a transformer rated not more than 1000 volt-amperes, shall not be
smaller than the derived ungrounded conductors and shall not be
smaller than 14 AWG copper or 12 AWG aluminum.
(a) Installed at the Source. The system bonding jumper
shall connect the grounded conductor to the supply-side bonding jumper and the normally non-current-carrying metal
enclosure.
(b) Installed at the First Disconnecting Means. The system
bonding jumper shall connect the grounded conductor to the
supply-side bonding jumper, the disconnecting means enclosure, and the equipment grounding conductor(s).
(2) Supply-Side Bonding Jumper. If the source of a separately
derived system and the first disconnecting means are located in
separate enclosures, a supply-side bonding jumper shall be
installed with the circuit conductors from the source enclosure
to the first disconnecting means. A supply-side bonding jumper
shall not be required to be larger than the derived ungrounded
conductors. The supply-side bonding jumper shall be permitted to be of nonflexible metal raceway type or of the wire or
bus type as follows:
(a) A supply-side bonding jumper of the wire type shall
comply with 250.102(C), based on the size of the derived
ungrounded conductors.
(b) A supply-side bonding jumper of the bus type shall
have a cross-sectional area not smaller than a supply-side bonding jumper of the wire type as determined in 250.102(C).
Exception: A supply-side bonding jumper shall not be required between
enclosures for installations made in compliance with 250.30(A)(1),
Exception No. 2.
(3) Grounded Conductor. If a grounded conductor is installed and the system bonding jumper connection is not located
at the source, 250.30(A)(3)(a) through (A)(3)(d) shall apply.
(a) Sizing for a Single Raceway. The grounded conductor
shall not be smaller than specified in Table 250.102(C)(1).
(b) Parallel Conductors in Two or More Raceways. If the
ungrounded conductors are installed in parallel in two or more
raceways, the grounded conductor shall also be installed in
parallel. The size of the grounded conductor in each raceway
shall be based on the total circular mil area of the parallel
derived ungrounded conductors in the raceway as indicated in
250.30(A)(3)(a), but not smaller than 1/0 AWG.
Informational Note: See 310.10(H) for grounded conductors
connected in parallel.
(c) Delta-Connected System. The grounded conductor of a
3-phase, 3-wire delta system shall have an ampacity not less than
that of the ungrounded conductors.
(d) Impedance Grounded System. The grounded conductor
of an impedance grounded neutral system shall be installed in
accordance with 250.36 or 250.187, as applicable.
(4) Grounding Electrode. The building or structure grounding electrode system shall be used as the grounding electrode
for the separately derived system. If located outdoors, the
grounding electrode shall be in accordance with 250.30(C).
Exception: If a separately derived system originates in equipment that
is listed and identified as suitable for use as service equipment, the
grounding electrode used for the service or feeder equipment shall be
permitted to be used as the grounding electrode for the separately derived
system.
Informational Note No. 1: See 250.104(D) for bonding requirements for interior metal water piping in the area served by separately derived systems.
Informational Note No. 2: See 250.50 and 250.58 for requirements for bonding all electrodes together if located at the same
building or structure.
(5) Grounding Electrode Conductor, Single Separately Derived
System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66
for the derived ungrounded conductors. It shall be used to
connect the grounded conductor of the derived system to the
grounding electrode in accordance with 250.30(A)(4), or as
permitted in 250.68(C)(1) and (2). This connection shall be
made at the same point on the separately derived system where
the system bonding jumper is connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to connect the
grounding electrode conductor to the equipment grounding terminal,
bar, or bus if the equipment grounding terminal, bar, or bus is of sufficient size for the separately derived system.
Exception No. 2: If the source of a separately derived system is located
within equipment listed and identified as suitable for use as service
equipment, the grounding electrode conductor from the service or feeder
equipment to the grounding electrode shall be permitted as the grounding electrode conductor for the separately derived system, if the grounding electrode conductor is of sufficient size for the separately derived
system. If the equipment grounding bus internal to the equipment is not
smaller than the required grounding electrode conductor for the separately derived system, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.
Exception No. 3: A grounding electrode conductor shall not be required
for a system that supplies a Class 1, Class 2, or Class 3 circuit and is
derived from a transformer rated not more than 1000 volt-amperes,
provided the grounded conductor is bonded to the transformer frame or
enclosure by a jumper sized in accordance with 250.30(A)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one
of the means specified in 250.134.
(6) Grounding Electrode Conductor, Multiple Separately
Derived Systems. A common grounding electrode conductor
for multiple separately derived systems shall be permitted. If
installed, the common grounding electrode conductor shall be
used to connect the grounded conductor of the separately
derived systems to the grounding electrode as specified in
250.30(A)(4). A grounding electrode conductor tap shall then
be installed from each separately derived system to the
common grounding electrode conductor. Each tap conductor
shall connect the grounded conductor of the separately
derived system to the common grounding electrode conductor.
This connection shall be made at the same point on the separately derived system where the system bonding jumper is
connected.
Exception No. 1: If the system bonding jumper specified in
250.30(A)(1) is a wire or busbar, it shall be permitted to connect the
grounding electrode conductor tap to the equipment grounding terminal, bar, or bus, provided the equipment grounding terminal, bar, or
bus is of sufficient size for the separately derived system.
Exception No. 2: A grounding electrode conductor shall not be required
for a system that supplies a Class 1, Class 2, or Class 3 circuit and is
derived from a transformer rated not more than 1000 volt-amperes,
provided the system grounded conductor is bonded to the transformer
frame or enclosure by a jumper sized in accordance with 250.30(A)(1),
Exception No. 3, and the transformer frame or enclosure is grounded by
one of the means specified in 250.134.
(a) Common Grounding Electrode Conductor. The common
grounding electrode conductor shall be permitted to be one of
the following:
(1) A conductor of the wire type not smaller than 3/0 AWG
copper or 250 kcmil aluminum
(2) A metal water pipe that complies with 250.68(C)(1)
(3) The metal structural frame of the building or structure
that complies with 250.68(C)(2) or is connected to the
grounding electrode system by a conductor not smaller
than 3/0 AWG copper or 250 kcmil aluminum
(b) Tap Conductor Size. Each tap conductor shall be sized
in accordance with 250.66 based on the derived ungrounded
conductors of the separately derived system it serves.
Exception: If the source of a separately derived system is located within
equipment listed and identified as suitable for use as service equipment,
the grounding electrode conductor from the service or feeder equipment
to the grounding electrode shall be permitted as the grounding electrode
conductor for the separately derived system, if the grounding electrode
conductor is of sufficient size for the separately derived system. If the
equipment grounding bus internal to the equipment is not smaller than
the required grounding electrode conductor for the separately derived
system, the grounding electrode connection for the separately derived
system shall be permitted to be made to the bus.
(c) Connections. All tap connections to the common
grounding electrode conductor shall be made at an accessible
location by one of the following methods:
(1) A connector listed as grounding and bonding equipment.
(2) Listed connections to aluminum or copper busbars not
smaller than 6 mm thick x 50 mm wide (1/4 in. thick x 2 in.
wide) and of sufficient length to accommodate the
number of terminations necessary for the installation. If
aluminum busbars are used, the installation shall also
comply with 250.64(A).
(3) The exothermic welding process.
Tap conductors shall be connected to the common grounding electrode conductor in such a manner that the common
grounding electrode conductor remains without a splice or
joint.
(7) Installation. The installation of all grounding electrode
conductors shall comply with 250.64(A), (B), (C), and (E).
(8) Bonding. Structural steel and metal piping shall be
connected to the grounded conductor of a separately derived
system in accordance with 250.104(D).
(B) Ungrounded Systems. The equipment of an ungrounded
separately derived system shall be grounded and bonded as
specified in 250.30(B)(1) through (B)(3).
(1) Grounding Electrode Conductor. A grounding electrode
conductor, sized in accordance with 250.66 for the largest
derived ungrounded conductor(s) or set of derived ungrounded conductors, shall be used to connect the metal enclosures
of the derived system to the grounding electrode as specified in
250.30(A)(5) or (6), as applicable. This connection shall be
made at any point on the separately derived system from the
source to the first system disconnecting means. If the source is
located outside the building or structure supplied, a grounding
electrode connection shall be made in compliance with
250.30(C).
(2) Grounding Electrode. Except as permitted by 250.34 for
portable and vehicle-mounted generators, the grounding electrode shall comply with 250.30(A)(4).
(3) Bonding Path and Conductor. A supply-side bonding
jumper shall be installed from the source of a separately
derived system to the first disconnecting means in compliance
with 250.30(A)(2).
(C) Outdoor Source. If the source of the separately derived
system is located outside the building or structure supplied, a
grounding electrode connection shall be made at the source
location to one or more grounding electrodes in compliance
with 250.50. In addition, the installation shall comply with
250.30(A) for grounded systems or with 250.30(B) for
ungrounded systems.
Exception: The grounding electrode conductor connection for impedance
grounded neutral systems shall comply with 250.36 or 250.187, as
applicable.
Verbatim NEC 2017, official NFPA text (nec2017_full.txt lines 18571–18944). OCR line-wrap reflow normalized; OCR artifacts corrected to standard wording where the scan garbled them ("250,30"→"250.30", "shali/shail"→"shall", "ts"→"is", "a@", "vated"→"rated", "A connector"→"A connector", the (c)(2) inch conversion printed as a garbled "( in. thick x 2 in." = 1/4 in.) — the standard OCR class, disclosed; no other wording altered.
250.66 Size of Alternating-Current Grounding Electrode
Conductor. The size of the grounding electrode conductor at
the service, at each building or structure where supplied by a
feeder(s) or branch circuit(s), or at a separately derived system
of a grounded or ungrounded ac system shall not be less than
given in Table 250.66, except as permitted in 250.66(A)
through (C).
(A) Connections to a Rod, Pipe, or Plate Electrode(s). If the
grounding electrode conductor or bonding jumper connected
to a single or multiple rod, pipe, or plate electrode(s), or any
combination thereof, as described in 250.52(A)(5) or (A)(7),
does not extend on to other types of electrodes that require a
larger size conductor, the grounding electrode conductor shall
not be required to be larger than 6 AWG copper wire or 4 AWG
aluminum wire.
(B) Connections to Concrete-Encased Electrodes. If the
grounding electrode conductor or bonding jumper connected
to a single or multiple concrete-encased electrode(s), as described in 250.52(A)(3), does not extend on to other types of electrodes that require a larger size of conductor, the grounding
electrode conductor shall not be required to be larger than
4 AWG copper wire.
(C) Connections to Ground Rings. If the grounding electrode
conductor or bonding jumper connected to a ground ring, as
described in 250.52(A)(4), does not extend on to other types
of electrodes that require a larger size of conductor, the
grounding electrode conductor shall not be required to be
larger than the conductor used for the ground ring.
Verbatim NEC 2017, official NFPA text (nec2017_full.txt lines 19859–19892). OCR line-wrap reflow normalized; no wording altered.
Table 250.66 — the full table
Grounding Electrode Conductor for Alternating-Current Systems — at the service, at each building/structure supplied by a feeder, or at a separately derived system (grounded or ungrounded).
| Size of largest ungrounded service-entrance conductor (or equivalent area for parallel conductors) — copper (AWG/kcmil) |
— aluminum or copper-clad (AWG/kcmil) |
Size of GEC — copper (AWG/kcmil) |
Size of GEC — aluminum or copper-clad (AWG/kcmil) |
| 2 AWG or smaller | 1/0 AWG or smaller | 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG | 2/0 AWG or 3/0 AWG | 6 AWG | 4 AWG |
| 2/0 AWG or 3/0 AWG | 4/0 AWG or 250 kcmil | 4 AWG | 2 AWG |
| Over 3/0 AWG through 350 kcmil | Over 250 kcmil through 500 kcmil | 2 AWG | 1/0 AWG |
| Over 350 kcmil through 600 kcmil | Over 500 kcmil through 900 kcmil | 1/0 AWG | 3/0 AWG |
| Over 600 kcmil through 1100 kcmil | Over 900 kcmil through 1750 kcmil | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil | Over 1750 kcmil | 3/0 AWG — CAPPED | 250 kcmil — CAPPED |
Notes (2017, verbatim, OCR-normalized):
- If multiple sets of service-entrance conductors connect directly to a service drop, set of overhead service conductors, set of underground service conductors, or service lateral, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.
- Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
*This table also applies to the derived conductors of separately derived ac systems. (See installation restrictions in 250.64(A).)
The electrode-size caps — 250.66(A)–(C)
Unlike Table 250.102(C)(1) (which has no electrode caps and keeps scaling via its 12.5% Note 1), Table 250.66's requirement is relaxed by electrode type — the GEC to a "small" electrode is never required to be bigger than the cap, even if the table says more:
| Electrode (250.66) | GEC never required larger than | Condition |
|---|
| (A) rod, pipe, or plate electrode (250.52(A)(5)/(A)(7)) | 6 AWG Cu / 4 AWG Al | the GEC does not extend on to other electrodes that require a larger size |
| (B) concrete-encased electrode (250.52(A)(3)) | 4 AWG Cu | same — and note there is no aluminum column: the cap is copper-only |
| (C) ground ring (250.52(A)(4)) | the ring conductor's size | same |
The "does not extend on to" language is the trap: if the 6 AWG GEC to your ground rod continues on to a concrete-encased electrode or building steel that needs the full table size, the whole run is sized to the furthest electrode — a rod-then-continue installation is not an escape hatch (the ELR 2017 record on 250.66(A) spells this out with exactly this example).
Edition history & the one documented change
Section titles verified on up.codes this session (fetched 2026-09-01): 250.26 "Conductor to Be Grounded — Alternating-Current Systems"; 250.30 "Grounding Separately Derived Alternating-Current Systems"; 250.66 "Size of Alternating-Current Grounding Electrode Conductor" (each confirmed for NFPA 70 2017/2020/2023).
The one documented 2017→2023 change — the 250.30 intro. 2017 (quoted verbatim above, on disk): "Multiple separately derived systems that are connected in parallel shall be installed in accordance with 250.30." 2023, quoted from the ELR change record (sectionID 1590, fetched this session): "Multiple power sources of the same type that are connected in parallel to form one system that supplies premises wiring shall be treated as a single separately derived system and shall be installed in accordance with 250.30." The 2023 record's companion quiz (option D: "Multiple power sources of the same type ... are considered as a single separately derived system") marks that reading as the 2023 correct answer. Everything else on this page checks out as unchanged:
- 250.30(A)(4) 2023 quoted verbatim from the change record (sectionID 1591) — word-for-word identical to the 2017 text above (including the service-equipment exception).
- 250.30(A)(6)(a) 2023 (sectionID 1592): common GEC options — 3/0 AWG Cu / 250 kcmil Al wire, 250.68(C)(1) water pipe, or 250.68(C)(2) structural steel — identical to 2017.
- 250.66(A) caps (sectionID 1602): rod/pipe/plate → 6 AWG Cu / 4 AWG Al; concrete-encased minimum 4 AWG Cu — identical to 2017.
- Table 250.66's keying and Note 2 (sectionID 1603) — identical in substance to the 2017 table rendered above; the 7 rows were additionally cross-checked this session against a cached zing2.app NEC-2023 copy (identical 7 rows, 0 disagreements).
Honesty: the on-disk 2020 full-code scan ends at Article 230 (no 2017→2020 word-diff claimed); the up.codes 2023 viewer body is account-gated, so the 2023 posture rests on the five ELR change records above + the zing2 table cross-check. The 2026 NEC renumber of Article 250 is flagged; this page cites 2017–2023 numbering.
Six worked examples (every number from the shipped core)
EX1 — the everyday case: 50 kVA, 3-phase 120/240 V transformer
| Item | Value (core) |
|---|
| Transformer | 50 kVA, 3∅, 240 V wye (120/240 V derived) |
| Secondary full-load current | 50,000 VA ÷ (√3 × 208 V) = 138.8 A |
| Derived ungrounded conductors (310.16 @ 75 °C) | pickConductor31016(139, 'cu', 75) = 1/0 Cu (150 A; 2/0 not needed — 150 A ≥ 138.8 A) |
| Table row (1/0 Cu) | "1 AWG or 1/0 AWG" |
| Grounded conductor (250.30(A)(3)(a) → Table 250.102(C)(1)) | 6 AWG Cu / 4 AWG Al |
| System bonding jumper (250.28(D)(1) → Table 250.102(C)(1)) | 6 AWG Cu / 4 AWG Al |
| GEC (250.30(A)(5) → Table 250.66) | 6 AWG Cu / 4 AWG Al |
| Result | three different conductors, three different sections, ONE row — 1/0 derived ungrounded → 6 AWG Cu everywhere (Tables 250.102(C)(1) and 250.66 agree on the first six rows) |
EX2 — the electrode cap bites: 100 kVA, 3-phase 480/277 V, ground rod only
| Item | Value (core) |
|---|
| Transformer | 100 kVA, 3∅, 480/277 V wye |
| Secondary full-load current | 100,000 VA ÷ (√3 × 277 V) = 208.4 A |
| Derived ungrounded conductors (310.16 @ 75 °C) | pickConductor31016(209, 'cu', 75) = 4/0 Cu (230 A; 3/0 = 200 A is not enough) |
| Table row (4/0 Cu = 211,600 cmil) | "Over 3/0 AWG through 350 kcmil" |
| GEC per Table 250.66 | 2 AWG Cu / 1/0 AWG Al |
| GEC required, electrode = ground rod only (250.66(A)) | 6 AWG Cu / 4 AWG Al — the cap governs |
| Result | table says 2 AWG Cu; the rod-only electrode caps the REQUIREMENT at 6 AWG Cu — the biggest "free size" in Article 250 (2 AWG is still legal, just not required) |
EX3 — same transformer, concrete-encased (Ufer) electrode
| Item | Value (core) |
|---|
| Setup | same 100 kVA 480/277 V (EX2), Ufer electrode in the foundation |
| Table row | "Over 3/0 through 350 kcmil" → 2 AWG Cu |
| GEC required (250.66(B)) | 4 AWG Cu — the cap (no aluminum column; copper-only cap) |
| Result | if the GEC also runs on to a water pipe or structural steel that needs the full table size, the "does not extend on to" condition fails and the cap no longer applies — full 2 AWG Cu for the run |
EX4 — ungrounded system (250.30(B)): 25 kVA, 3-phase 240 V delta
| Item | Value (core) |
|---|
| Transformer | 25 kVA, 3∅, 240 V delta — no grounded conductor |
| Secondary full-load current | 25,000 VA ÷ (√3 × 240 V) = 60.1 A |
| Largest derived ungrounded conductor (310.16 @ 75 °C) | pickConductor31016(61, 'cu', 75) = 6 AWG Cu (65 A; 8 AWG = 50 A is not enough) |
| System bonding jumper / grounded conductor | none — the system is ungrounded |
| GEC (250.30(B)(1) → Table 250.66, "2 AWG or smaller" row) | 8 AWG Cu / 6 AWG Al — from the enclosure to the electrode |
| Result | ungrounded derived system: the GEC is the only grounding piece — it bonds the metal enclosures to the electrode, sized on the largest derived ungrounded conductor |
EX5 — two transformers, one common GEC (250.30(A)(6))
| Item | Value (core) |
|---|
| System A | 100 kVA, 3∅ 208 V → 277.6 A → 300 kcmil Cu (285 A @ 75 °C) → "Over 3/0 through 350 kcmil" → tap 2 AWG Cu |
| System B | 25 kVA, 3∅ 208 V → 69.4 A → 4 AWG Cu (85 A @ 75 °C; 6 AWG = 65 A is not enough) → "2 AWG or smaller" → tap 8 AWG Cu |
| Common GEC minimum (250.30(A)(6)(a)(1)) | 3/0 AWG Cu / 250 kcmil Al (wire type) — or a qualifying water pipe (250.68(C)(1)) / structural steel (250.68(C)(2)) |
| Tap connections (250.30(A)(6)(c)) | accessible location; listed G&B equipment, listed connections to ≥ 6 mm × 50 mm busbars, or exothermic welding — common GEC stays without a splice |
| Result | the common GEC is a FLOOR (≥ 3/0 Cu regardless of the taps); each tap is sized by 250.66 on its own system's ungrounded conductors — a 2 AWG and an 8 AWG tap both hang off the same 3/0 Cu common |
EX6 — the sub-1 kVA exception: 1 kVA control transformer
| Item | Value (core) |
|---|
| Transformer | 1 kVA, 120/240 V, supplying Class 1/2/3 circuits |
| Secondary full-load current | 1,000 VA ÷ 240 V = 4.17 A |
| Derived ungrounded conductors (310.16 @ 75 °C) | pickConductor31016(5, 'cu', 75) = 14 AWG Cu (20 A column value; 240.4(D) caps its OCPD at 15 A) |
| System bonding jumper (250.30(A)(1) Exception No. 3) | ≥ 14 AWG Cu / 12 AWG Al, not smaller than the derived ungrounded conductors |
| GEC (250.30(A)(5) Exception No. 3) | NOT REQUIRED — grounded conductor bonded to the transformer frame, frame grounded per 250.134 |
| Result | below 1000 VA the whole grounding-electrode apparatus drops out of the requirement — one small jumper from the grounded conductor to the frame does the job |
Where 250.26/250.30 fit
- The bonding side is article 23. The system bonding jumper and the grounded conductor are both sized by Table 250.102(C)(1) — the 250.102 article → carries that table, its 12.5% Note 1, and the 100 kVA 208 V transformer example (EX6 there).
- The GEC side is this page. 250.66 is the ONLY table for the separately derived system's GEC — with caps (250.66(A)–(C)) that Table 250.102(C)(1) has no equivalent of. The service-side GEC uses the same table (250.53).
- Load-side EGCs are 250.122 — the EGC sizing article →, keyed to the overcurrent device rating, unchanged by whether the system is service- or derived.
- Multiple buildings fed from the derived system are 250.32 (buildings supplied by a feeder) — a different rule set with its own GEC/electrode choices; outside this page's scope.
- 2026 NEC renumber flagged: the 2026 NEC renumbers Article 250; verify the section number against the edition adopted in your jurisdiction. This page cites 2017–2023 numbering.
Open the free PanelWright calculator — size the panel, the service, the neutral, the derating, and the voltage drop in your browser
Free, in-browser, zero tracking. Data never leaves your browser. Design aid only — verify against the adopted NEC edition.
Method, sources & honesty notes
Section text. Verbatim NEC 2017 (official NFPA text on disk, nec2017_full.txt): 250.26 (line 18501), 250.30(A)–(C) including all exceptions (lines 18571–18944), 250.66 + (A)–(C) (lines 19859–19892). OCR line-wrap reflow normalized; OCR artifacts corrected to standard wording where the scan garbled them ("250,30"→"250.30", "shali/shail"→"shall", "ts"→"is", "vated"→"rated", the (c)(2) inch conversion printed as a garbled "( in. thick x 2 in." = 1/4 in.) — the standard OCR class, disclosed in the source note. No other wording altered.
Table 250.66 values (7 rows, capped top). The on-disk 2017 OCR interleaves the table's columns (row labels wrap to their own lines), so the rows were parsed from the OCR layout row-by-row (nec2017_full.txt lines 19960–20028) and cross-checked this session against a cached zing2.app NEC-2023 table (parsed by parse_art24_zing2_25066.py: identical 7 rows, 0 disagreements) and against the ELR change record 1603's description (keyed on the size of the largest ungrounded conductor). The on-disk OCR garbles the row label "lor 1/0" (= "1 or 1/0 AWG") — same OCR class as article 23's "124% percent", disclosed here rather than propagated.
Edition history. Section titles verified on up.codes this session (fetched 2026-09-01 via fetch_art24_up.py). Five ELR change records fetched this session (fetched 2026-09-01 via fetch_art24_elr.py): sectionID 1590 (250.30 intro — the one documented 2017→2023 wording change, quoted above), 1591 (250.30(A)(4) 2023 — verbatim-identical to 2017), 1592 (250.30(A)(6)(a) 2023 — common-GEC options identical), 1602 (250.66(A) caps — 6 AWG Cu / 4 AWG Al, 4 AWG Cu), 1603 (Table 250.66 purpose + Note 2). The on-disk 2020 full-code scan ends at Article 230 (no 2017→2020 word-diff claimed); the up.codes 2023 viewer body is account-gated (no 2023 full-body word-diff claimed beyond the records above). 2026 renumber flagged.
Worked numbers. Every cmil / size / ampacity value computed by the shipped cores under node (compute_art24.js → calc_25066_cited.json): Ch. 9 Table 8 cmil via ch9Row/CH9_T8, Table 310.16 picks via pickConductor31016, column ampacities via T31016/T31016_COLS, standard OCPD steps via nextStdBreaker. The table-row lookups are the code itself, transcribed once and asserted by the public test suite. Zero hand math.