NEC 240.4(D) Small Conductors — the overcurrent-protection caps that override Table 310.16, why your 12 AWG wire runs a 20 A breaker, and the (E)/(G) carve-outs that exempt taps and motors

The full 240.4(D) lead-in and all seven caps — (D)(1) 18 AWG Cu 7 A, (D)(2) 16 AWG Cu 10 A, (D)(3) 14 AWG Cu 15 A, (D)(4) 12 AWG Al 15 A, (D)(5) 12 AWG Cu 20 A, (D)(6) 10 AWG Al 25 A, (D)(7) 10 AWG Cu 30 A — verbatim, plus the (E) tap and (G) specific-application exceptions, and seven worked examples where every number is computed by the shipped calculator core. Last updated 2026-08-31 (written for PanelWright v1.16).

Runs in your browser. No account, no install, data never leaves your machine. The panel card sizes every branch circuit against its load, the Conductor derating card applies the 310.15 corrections and the 240.4(D) small-conductor caps as the governing ampacity, and the voltage-drop card checks the 3%/5% notes — the exact pipeline this article explains.

The rule in one sentence

For the small conductors the code calls out, the overcurrent-protection device shall not exceed a fixed cap — 14 AWG copper → 15 A, 12 AWG copper → 20 A, 10 AWG copper → 30 A, 12 AWG aluminum → 15 A, 10 AWG aluminum → 25 A (and, under specific conditions, 18 AWG copper → 7 A and 16 AWG copper → 10 A) — no matter what the ampacity column of Table 310.16 says, and that limit applies only "unless specifically permitted in 240.4(E) or (G)." That is why a 12 AWG THHN conductor, whose 90 °C ampacity is 30 A, still lives on a 20 A breaker: the wire can carry 30 A in that column, but the breaker on it is capped at 20 A by 240.4(D). The cap is on the device, not the wire.

The code text (verbatim, with edition notes)

240.4 — Protection of Conductors (lead-in)

The "240.4(A) through (G)" is the map: (A) power-loss hazard, (B) next-standard-size for devices ≤ 800 A, (C) devices over 800 A, (D) small-conductor caps, (E) tap conductors, (F) transformer secondaries, (G) specific applications. (D) is the one that surprises most people, because it overrides the ampacity you just looked up.

240.4(D) — Small Conductors (the seven caps, verbatim)

Two things to notice. First, the caps are on the overcurrent protection, not the ampacity — (D)(3) "14 AWG Copper. 15 amperes" is a breaker/fuse maximum, while Table 310.16 gives 14 AWG copper 20 A of ampacity in the 75 °C column. Second, (D)(1) and (D)(2) — 18 AWG and 16 AWG copper — carry conditions (continuous-load limits and specific device listing requirements) that the five common caps do not. Those two sizes are the appliance-circuit, thermostat, and low-current wire; the five without conditions are the ones you will hit on every residential and light-commercial circuit.

The shipped core encodes these five caps in smallConductorCap(size, mat) — the same function the Conductor derating card uses to set the governing ampacity. EX1 below records every cap against the core. Note the asymmetry that trips people up: aluminum caps sit one ampacity-size lower (12 AWG Al → 15 A vs 12 AWG Cu → 20 A), which is why aluminum branch conductors usually run one wire size larger than copper for the same circuit rating.

240.4(E) — Tap conductors (carve-out #1)

240.4(G) — Overcurrent protection for specific conductor applications (carve-out #2)

These two subsections are the only exceptions the (D) lead-in allows. The practical consequence, worked in EX7: a 14 AWG tap on a 20 A branch circuit is protected at the circuit's 20 A (240.4(E) → 210.19(A)(4)), not capped at 15 A; a motor on 10 AWG is protected per Article 430, not the 30 A (D) cap. For ordinary branch-circuit wiring — receptacles, lighting, small-appliance and laundry circuits — neither carve-out applies, and the (D) caps are absolute.

Worked examples (all computed by the shipped core under node)

Every number below is produced by the shipped cores in app.js (smallConductorCap(), pickConductor31016(), derate31015(), nextStdBreaker()) running under node this session — see income-lab/compute_art17.js → calc_2404d_cited.json — and is asserted in the public test suite (test/run_tests.js in the public repo). Unless a row says otherwise, conductor picks are in the 75 °C column (110.14(C) terminations) at 30 °C ambient with ≤3 current-carrying conductors.

EX1 — the cap table, cross-checked against the core

The core models the five common caps exactly as the code states them, and correctly returns null for sizes outside (D) (8 AWG and larger) and for 16/18 AWG (which the calculator's Table 310.16 does not carry, since those sizes are not used for the branch-circuit and service sizing this tool does). That honest boundary is part of the design: the tool will not silently invent a cap it does not model.

EX2 — the 14 AWG trap: a required 20 A circuit

This is the single most common branch-circuit mistake, and it exists because the conductor test and the breaker test look at different numbers. 14 AWG copper has 20 A ampacity in the 75 °C column — the ampacity test (210.19(A)(1)(a)) passes. But its overcurrent device is capped at 15 A by 240.4(D)(3), and a 15 A device is below the 20 A the circuit requires, so the circuit as built is non-compliant on the OCPD side. One size up — 12 AWG Cu (25 A ampacity, 20 A cap) — the cap covers the requirement. The core emits the warning automatically when it picks a 14 AWG conductor: "240.4(D): overcurrent device for this size is capped at 15 A."

EX3 — the "12 AWG THHN = 30 A" myth (90 °C ampacity vs the 20 A cap)

A 12 AWG THHN conductor has 30 A of ampacity in the 90 °C column — the number that tempts a 30 A breaker. Two independent rules stop it: (1) 110.14(C) termination ratings mean the ampacity you may use for sizing is the 75 °C value (25 A) on most equipment, not the 90 °C base (the 90 °C column exists for derating headroom, not for bigger breakers); and (2) 240.4(D)(5) caps the overcurrent device on 12 AWG copper at 20 A regardless of column. So 12 AWG copper always rides a 20 A breaker. The higher table columns give you derating headroom (310.15), not a bigger breaker — the cap is the ceiling the breaker cannot cross.

EX4 — aluminum / copper-clad: the caps sit one size lower

Aluminum branch conductors carry a lower OCPD cap than the same-size copper: 12 AWG aluminum (20 A ampacity) is capped at 15 A, and 10 AWG aluminum (30 A ampacity) at 25 A. The effect is that an aluminum circuit needing 20 A of OCPD must go to 10 AWG aluminum (25 A cap) — one size larger than the 12 AWG copper that does the same job. Combined with the 110.14(C) termination-rating differences, this is why aluminum branch wiring is sized up relative to copper.

EX5 — where the CAP governs: 14 AWG Cu, normal conditions

In normal conditions (30 °C ambient, ≤3 current-carrying conductors) the 310.15 factors are both 1.00, so the derated ampacity equals the base (20 A). But 240.4(D) caps it at 15 A — so the effective ampacity of a 14 AWG copper branch conductor is 15 A, not 20 A. The core's derate31015() returns exactly this: baseAmp 20, deratedA 20, capA 15, effectiveA 15, with the note "240.4(D) cap (15 A) governs — the derated ampacity (20 A) is NOT usable for a copper 14 AWG." This is the number your load must not exceed.

EX6 — where DERATING governs: 10 AWG Cu in a crowded raceway

The mirror image of EX5: here the derated ampacity (35 × 0.88 × 0.70 = 21.56 A) falls below the 30 A cap, so the derated ampacity is the governing number, not the cap. The cap is a ceiling, not a floor — when derating already brings the usable ampacity under the cap, the cap stops being the constraint. The core's derate31015() returns baseAmp 35, deratedA 21.56, capA 30, effectiveA 21.56, with the note "derated ampacity 21.56 A governs here." The same 10 AWG copper that carries 30 A in a clean, cool run carries only 21.56 A in this raceway.

EX7 — the (E)/(G) carve-outs: where the cap does NOT apply

These are the two exceptions the (D) lead-in allows ("Unless specifically permitted in 240.4(E) or (G)"). A 14 AWG tap that feeds a household range or cooking appliance is protected at the branch-circuit rating — 20 A, not 15 A — because it is a tap conductor under (E). A motor circuit's overcurrent protection is governed by Article 430 (short-circuit/ground-fault per 430.52, overload per 430.32), so the (D) cap is not the sizing rule. For the ordinary branch circuits this article is about — receptacles, lighting, small-appliance, laundry — neither carve-out applies, and the (D) caps are the ceiling. The 210.19(A) continuous-load article works the tap-vs-branch distinction in the branch-circuit context.

Editions: 2014 / 2017 / 2020 / 2023

• Title & structure (all four editions): the section title "Protection of Conductors" (240.4) and the internal (A)–(G) structure are identical in the 2014, 2017, 2020, and 2023 NECs. Verified this session: the up.codes section index names "240.4 Wiring and Protection, Protection of Conductors" identically across all four editions (fetched 2026-08-31), and the official 2017 NFPA text on disk shows the (A)–(G) layout quoted above.
• 240.4(D) itemization: the (D)(1)–(D)(7) structure — including (D)(1) 18 AWG Cu 7 A and (D)(2) 16 AWG Cu 10 A (conditional) — is present in the 2017 and 2020 editions (2017 official text on disk; the 2020-based SunCam PDH on disk cites "240.4(D)(3)" as 14 AWG Cu 15 A). The 2023 PDH change analysis records no 2023 change to the 240.4(D) caps, so the itemization and values carry into 2023.
• The cap values are edition-stable: 14 Cu → 15 A, 12 Cu → 20 A, 10 Cu → 30 A, 12 Al → 15 A, 10 Al → 25 A (and 18 Cu → 7 A, 16 Cu → 10 A) are the same numbers across 2014–2023. What changed in 2023 in Article 240 was the 240.4(B) adjustable-trip sentence (see the conductor-sizing article) — not 240.4(D). Cite by value, not by sub-letter, and you are safe across all four editions.
• One source nuance worth knowing: the 2023-NEC-based study print on disk (codeelec_2023) quotes the Table 310.16 asterisk footnote for the small-conductor caps — the condensed five-value form ("15 amperes for #14 AWG, 20 amperes for #12 AWG, and 30 amperes for #10 AWG copper; or 15 amperes for #12 AWG and 25 amperes for #10 AWG aluminum …"). That footnote is a summary of the five common caps, not the full 240.4(D) section; the full section is the itemized (D)(1)–(D)(7) quoted above (official 2017 NFPA text on disk), which additionally carries the conditional 18 AWG (7 A) and 16 AWG (10 A) entries. The values agree; the footnote just omits the two conditional sizes.

Free panel schedule & service-load tools

The panel card sizes every branch circuit against its load and flags continuous loads; the Conductor derating card applies the 310.15(B)(1) + 310.15(C)(1) corrections and the 240.4(D) small-conductor caps as the governing ampacity (the exact EX5/EX6 pipeline); the 220.82 card sizes the optional dwelling service and its conductors (Table 310.16, 110.14(C) columns); and the voltage-drop card checks any run against the 3%/5% informational notes using Ch. 9 Table 8. Breaker picks use the NEC 240.6(A) standard-size list. All results print in the rollup CSV export and the branded PDF project report.

Sources & verification