NEC 230.90 — Where Required: Overload Protection for Service Conductors

Every ungrounded service conductor shall have overload protection. Where does it go — and can the service breaker be bigger than the wire?

The question this article answers

The load calc is done, the 230.79 service-disconnect article says what rating the disconnect must be, and the 230.42 article says how big the service-entrance conductors must be. But those are two different requirements — so who polices the match between the breaker and the wires? That is 230.90. It says: every ungrounded service conductor shall have overload protection, by an overcurrent device in series with it whose rating or setting is not higher than the conductor's ampacity — with five exceptions that loosen the rule where the code deliberately allows it (motor starts, the next-standard-size allowance, multi-breaker services, fire pumps, and the 83% dwelling-service rule). And 230.90(B) draws the one hard line nobody crosses: no overcurrent device in the service neutral.

230.90 — Where Required (2017)

The 2020 wording — the same rule, one renumber

The requirements, at a glance

The two most-missed pieces: Exception No. 3's load test — the combined rating may exceed the ampacity, but the calculated load may not, so the conductors still protect themselves against real demand — and (B), which is why a service disconnect is a 2-/3-/4-pole device and the neutral never rides on its own pole. Note what 230.90(A) does not do: it does not size the conductors (that is 230.42 →) and does not set the disconnect rating (that is 230.79 →). It polices the match between the two — and the 83% rule in Exception 5 is a conductor-sizing allowance (the conductors may be smaller than the breaker), not a permission to overload the conductors.

Worked examples (every number from the shipped core)

Inputs are entered into the shipped PanelWright core (serviceLoad22082, serviceLineConductor22082, pickConductor31016, nextStdBreaker) under node via compute_art30.js → calc_23090_cited.json; the tables below are that output. No hand math. Conductor picks use the 75 °C column (110.14(C)(1)(b) — terminations over 100 A; see the 110.14(C) article →); the 83% factor is the code's 310.12 / 310.15(B)(7) value applied to the core's Table 310.16 ampacities (0/168 cells verified against the 2023 print, Session 12).

EX1 — the basic (A) rule: the 100 A one-family service

The ordinary case: the disconnect is 100 A, the conductors carry 100 A, and the (A) rule holds exactly at the boundary (100 ≤ 100). Nothing exotic — this is the default state of every conforming service, and it is the state the whole 230.70–230.80 service story in the 230.79 article → lands on.

EX2 — Exception No. 5 (HEADLINE): the 83% rule, three service sizes

The rule in numbers: for a single-phase 120/240-V, 3-wire dwelling service rated 100 through 400 A, req = 0.83 × rating, then the Table 310.16 75 °C pick (core: pickConductor31016). At 200 A the ungrounded conductors drop from 3/0 Cu to 2/0 Cu (175 A ≥ 166 A); at 400 A from 600 kcmil Cu to 400 kcmil Cu (335 A ≥ 332 A — the 3 A of headroom is the whole margin). The breaker stays at the full service rating; only the wires get the reduction. Scope limits (from 310.15(B)(7)(1) on disk): the service must supply the entire load of the one-family dwelling (or an individual dwelling unit), and the rule covers the ungrounded conductors — the neutral is separately permitted to be sized smaller where 220.61 and 230.42 are met (310.15(B)(7)(4) / 310.12(D)), covered in the 220.61 neutral-load article →.

EX3 — Exception No. 3: two breakers, combined rating exceeds the ampacity

The exception the (A) sentence is built around: two-to-six breakers (or fuse sets) may carry the service, and their combined rating may exceed the conductor ampacity — provided the calculated load does not. The conductors remain the real limit on actual demand; the breakers only need to be able to open it. (The 2–6 count itself is the 230.71(B) territory the lead-in cross-references — see the 230.70–230.80 article →.)

EX4 — 230.90(B): the neutral is never on its own breaker

The same rule at other system geometries: 3-phase 4-wire → 4-pole (all three ungrounded + the neutral share the simultaneous-open device — the neutral is still never on its own pole); 3-phase 3-wire (no neutral) → 3-pole. The neutral's size is governed by 220.61 (maximum unbalanced load) and the 310.12(D) smaller-neutral provision, not by any overcurrent device — see the 220.61 article →.

EX5 — Exception No. 1: motor-starting currents (430.52 / 430.62 / 430.63)

Where the service conductors feed a large motor (an equipment feed where the motor dominates), the starter's OCPD is set by the Article 430 tables, which routinely sit above the conductor ampacity for inrush — Exception No. 1 is what keeps that legal at the service level. Same architecture as 210.23(B)(1)/(D) and 430.52 on the branch/feeder side; the conductor-sizing article walks the feeder-side analog.

EX6 — Exception No. 2: the next-standard-size allowance (240.4(B)/(C) + 240.6)

The non-standard-ampacity case: 2 AWG Cu at 75 °C carries 115 A, which no stock breaker matches. 240.4(B)/(C) (the next-standard-size rule, covered end-to-end in the conductor-sizing article →) lets you go up to the next 240.6(A) rating — 125 A — and 230.90(A) Exception No. 2 incorporates that same allowance at the service. (Cord-and-plug-connected portable loads are excluded from 240.4(B) — the allowance is for loads the conductors serve continuously.)

Where 230.90 fits

• The rating the OCPD must clear: the 230.70–230.80 service-disconnecting-means article → (230.79's rating floors — 15 / 30 / 100 one-family / 60 A — set the disconnect; 230.90(A) then ties that device to the conductor ampacity).
• The conductor ampacity the OCPD must not exceed: the 230.42 service-conductor-sizing article → (230.42 sizes the wires; 230.90(A) is the OCPD-side check on those same wires).
• The 83% rule's home: the 110.14(C) / 310.14 article → (the 2020 ampacity reorganization: 310.15 → 310.14, Table 310.15(B)(16) → Table 310.16, and 310.15(B)(7) → 310.12 — the renumber Exception 5 carries) and the 220.61 neutral-load article → (what the neutral actually carries; the 310.12(D) smaller-neutral provision).
• The ampacity table the picks come from: the Table 310.16 ampacity article → — the 0.83 × rating math lands on its 75 °C column.
• The load that sets everything upstream: the 220.82 optional dwelling service load article → and the standard-method cards; the calculated load is what Exception No. 3's test compares against the ampacity.
• The next-standard-size allowance (Exception No. 2): the conductor-sizing article → (240.4 overcurrent end-to-end, including 240.4(B)/(C) and the 240.6(A) standard sizes).
• 2026 NEC renumber flagged: the 2026 NEC reorganizes service and load-calculation material (Article 220 → the Article 120 area); verify 230.90's cross-references (310.12, 230.71(B), 240.4, 240.6, 430.52) against the adopted edition. This page cites 2017–2023 numbering.

Edition history — what actually changed

Verification basis: verbatim 2017 NFPA text (nec2017_full.txt, lines 15328–15378) diffed word-for-word against the verbatim 2020 full-code scan (slideshare_nec2020.txt, chars 516334–517923) — the two on-disk sources (verify_art30_diff.py; the only residual token differences are the disclosed OCR artifacts). Section title "Where Required" confirmed identical in both on-disk editions; the 2014 edition is not on disk and the up.codes chapter pages render client-side (no static section list this session), so no 2014 title claim is made. The 2023 SunCam change analysis (nec2023_pdh.txt, Article 230) covers the article via 230.30 Part III only — 0 occurrences of "230.90" in the document, so no 2023 change is claimed. The 83% rule's content under its 2020+ home, 310.12 "Single-Phase Dwelling Services and Feeders" (Table 310.12(A)/(B), same 83%-of-rating allowance), was confirmed via the ELR section 1908 page fetched this session; the 2017 310.15(B)(7) body is on disk (nec2017_full.txt lines 25965–26015) and reads: service rated 100 through 400 A supplying the entire one-family (or individual-unit) load — "shall be permitted to have an ampacity not less than 83 percent of the service rating."

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Method, sources & honesty notes