4.3 Branch Circuit Ratings & Overcurrent Protection (0303)
Key Takeaways
- 240.4(D) small-conductor limits (14 AWG Cu = 15 A, 12 AWG Cu = 20 A, 10 AWG Cu = 30 A) override the higher ampacity values in Table 310.16 — except where 240.4(G) sends a specific application, such as a motor circuit, to its own article.
- 240.4(B) permits the next standard device above the conductor ampacity only where the conductor is not a small conductor under 240.4(D), the device does not exceed 800 A, and the circuit does not supply receptacles for cord-and-plug-connected portable loads.
- Feeder taps live in 240.21(B): the 10 ft tap needs ampacity of at least 1/10 of the feeder device rating, and the 25 ft tap needs at least 1/3.
- 110.9 requires each device's interrupting rating to equal or exceed the available fault current at its line terminals; 240.86(C) bars series-rated combinations where connected motor full-load currents exceed 1% of the lower-rated breaker's interrupting rating.
NEC Article 240 — Overcurrent Protection
Article 240 governs overcurrent protective devices (OCPDs) — fuses and circuit breakers — that protect conductors and equipment from overcurrent (overload, short circuit, and ground fault). The inspector verifies OCPD rating, type, location, and interrupting capacity.
Standard OCPD Ratings (240.6)
240.6(A) lists standard ampere ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A. When a calculation yields a nonstandard value, the next standard size is selected.
Conductor Protection (240.4)
240.4 requires conductors to be protected in accordance with their ampacity. Three key subsections:
- 240.4(B) Devices rated 800 A or less (the "next size up" rule): where the conductor ampacity does not correspond to a standard device rating, the next higher standard rating may be used, provided all three conditions are met: the conductors are not part of a multi-outlet branch circuit supplying receptacles for cord-and-plug-connected portable loads; the ampacity does not correspond to a standard rating in 240.6; and the next higher standard rating does not exceed 800 A. Above 800 A, 240.4(C) requires the conductor ampacity to be equal to or greater than the device rating — no rounding up.
- 240.4(D) Small conductors: overcurrent protection must not exceed 15 A for 14 AWG copper, 20 A for 12 AWG copper, and 30 A for 10 AWG copper (with corresponding aluminum limits), regardless of the higher ampacity shown in Table 310.16.
- 240.4(G) Specific conductor applications: this is the escape hatch that makes the whole article make sense. Motor-circuit conductors, air-conditioning conductors, fire-pump conductors, control-circuit conductors, transformer secondary conductors, and several others are protected in accordance with their own articles (430.52, 440.22, 695.6, and so on) rather than 240.4(D). That is why a 12 AWG motor branch-circuit conductor can legitimately sit behind a 35 A breaker.
Standard Sizes & Small-Conductor Limits
| Conductor (Cu) | Max OCPD per 240.4(D) | 75°C Ampacity (Table 310.16) |
|---|---|---|
| 14 AWG | 15 A | 20 A |
| 12 AWG | 20 A | 25 A |
| 10 AWG | 30 A | 35 A |
| 8 AWG | not restricted by 240.4(D) | 50 A |
Note that the 75°C ampacity is higher than the 240.4(D) cap for 14, 12, and 10 AWG, which is exactly why the small-conductor rule overrides the ampacity table for these sizes.
OCPD Location (240.24)
240.24 restricts where overcurrent devices may be installed. Learn the letters, because the exam quotes them:
| Subsection | Rule |
|---|---|
| 240.24(A) | Accessibility — devices must be readily accessible, with the center of the grip of the operating handle in its highest position not more than 6 ft 7 in. (2.0 m) above the floor or working platform (several exceptions apply, including busway devices and supplementary overcurrent devices) |
| 240.24(B) | Occupancy — each occupant must have ready access to all overcurrent devices protecting the conductors supplying that occupancy, with exceptions for buildings under continuous management and for service/feeder devices |
| 240.24(C) | Not exposed to physical damage |
| 240.24(D) | Not in the vicinity of easily ignitible material — for example, in clothes closets |
| 240.24(E) | Not located in bathrooms — of dwelling units, dormitory units, or guest rooms/guest suites |
| 240.24(F) | Not located over steps of a stairway |
Related rules: 240.32 (enclosures in damp or wet locations must be listed for the condition) and 240.33 (enclosures mounted in a vertical position where practicable).
Feeder Tap Rules (240.21)
Normally, the overcurrent device must be at the point where a conductor receives its supply (240.21). Taps are the limited exceptions, and they all live under 240.21(B) Feeder Taps — note that 240.21(A) is branch-circuit conductors, not a tap rule:
| Rule | Length | Minimum tap ampacity | Other key conditions |
|---|---|---|---|
| 240.21(B)(1) | Not over 10 ft | Not less than the combined calculated load, and not less than the rating of the device supplied — and not less than 1/10 of the rating of the device protecting the feeder where the tap leaves the enclosure | Tap conductors enclosed in a raceway; no reduction in size beyond the tap |
| 240.21(B)(2) | Not over 25 ft | Not less than 1/3 of the rating of the device protecting the feeder | Protected from physical damage; terminates in a single device rated no more than the tap ampacity |
| 240.21(B)(3) | Taps supplying a transformer (primary plus secondary not over 25 ft) | Primary ≥ 1/3 of the feeder device; secondary ≥ 1/3 of the feeder device × (primary-to-secondary voltage ratio) | Terminates in a single device |
| 240.21(B)(4) | Over 25 ft (industrial installations only, qualified maintenance) | Not less than 1/3 of the feeder device | Up to 100 ft, engineering supervision |
| 240.21(B)(5) | Outside taps of unlimited length | Sized to the calculated load | Conductors outside the building except at the point of termination; device at the building disconnect |
240.21(C) applies the same idea to transformer secondary conductors, where the primary device does not protect the secondary conductors and the 10 ft, 25 ft, and outside-tap analogues appear again.
Interrupting Rating & Series Ratings (240.83, 110.9, 240.86)
110.9 requires every overcurrent device intended to interrupt fault current to have an interrupting rating sufficient for the current available at its line terminals; 110.10 separately requires the circuit components to withstand that current until the device clears. 110.24 requires service equipment (other than at dwelling units) to be field marked with the maximum available fault current and the date of the calculation. 240.83(C) requires the interrupting rating to be marked on circuit breakers other than 5,000 A.
Where the available fault current exceeds a downstream device's own interrupting rating, a series-rated combination (240.86) — a tested and marked pairing of an upstream and a downstream device — may be used. 240.86(C) bars it where motors are connected on the load side of the higher-rated device and on the line side of the lower-rated device and the sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker, because motor contribution adds fault current the series test did not account for.
Selective Coordination
Selective coordination means only the OCPD nearest the fault opens, leaving the rest of the system energized. It is mandatory for specific applications, including elevators (620.62), the critical branch of healthcare facilities (Article 517), and emergency and legally required standby systems (Articles 700 and 701). A coordinated system avoids nuisance tripping of upstream devices during a downstream fault.
Branch Circuit OCPD (210.20(A))
210.20(A) requires a branch-circuit OCPD rated not less than 125% of the continuous load plus 100% of the noncontinuous load — mirroring 215.3 for feeders.
Per 240.4(D), what is the maximum OCPD rating permitted for a 12 AWG copper conductor?
A feeder tap not over 25 ft long must have an ampacity of at least what fraction of the feeder OCPD rating per 240.21(B)?
What does NEC 110.9 require of every overcurrent protective device?