Overcurrent protection
Overcurrent protection is the breaker or fuse that opens a circuit before an overload, short circuit or ground fault can overheat the conductors. Under NEC Article 240, conductors are generally protected at their ampacity, with a next-size-up allowance for devices of 800 A or less and fixed limits for small conductors (15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper). For continuous loads, the device must be rated at least 125% of the continuous load plus 100% of the noncontinuous load.
All rules below follow the 2023 NEC.
What overcurrent protection does
An overcurrent device protects the conductors and equipment, not the load. Article 100 defines overcurrent as current above the rating of equipment or the ampacity of a conductor, and it can come from three causes:
- Overload: equipment running above its rated current long enough to cause damage or dangerous overheating.
- Short circuit: a low-impedance connection between conductors at different potentials.
- Ground fault: an unintended connection between an ungrounded conductor and metal enclosures, raceways or equipment.
An overload builds heat slowly, while short circuits and ground faults produce very high current almost instantly. A device clears a ground fault only if a low-impedance return path exists, which is why Article 240 depends on grounding and bonding.
The key rules
- Protect conductors at their ampacity (240.4). Use the conductor's ampacity after correction and adjustment factors. See conductor ampacity for how that number is found.
- Next size up (240.4(B)). If the conductor's ampacity does not match a standard device rating, you may use the next higher standard rating, but only if the conductors are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads, and the next higher rating does not exceed 800 A.
- Over 800 A (240.4(C)). Where the device is rated over 800 A, the conductor ampacity must be equal to or greater than the device rating. No rounding up.
- Small conductor rule (240.4(D)). Unless a specific rule permits otherwise, 14 AWG copper is limited to 15 A, 12 AWG copper to 20 A and 10 AWG copper to 30 A. For aluminum and copper-clad aluminum, 12 AWG is limited to 15 A and 10 AWG to 25 A.
- Specific applications (240.4(G)). Motor and air-conditioning circuits follow their own articles (430 and 440) instead of the small conductor rule. See motor circuits.
- Standard ratings (240.6(A)). Fuses and inverse-time breakers come in a fixed set of standard ampere ratings. For example, 110 A and 125 A are standard, but 75 A and 115 A are not.
- Adjustable-trip breakers (240.6(B) and (C)). The rating is the maximum possible setting unless access to the adjustment is restricted.
Continuous loads
A continuous load is one where the maximum current is expected to last 3 hours or more (Article 100). For branch circuits (210.20(A)) and feeders (215.3), the overcurrent device must be rated at least 125% of the continuous load plus 100% of the noncontinuous load. If the result is not a standard rating, go up to the next standard size.
Minimum device = (continuous load × 1.25) + noncontinuous loadThe exception is an assembly, including the device, listed for operation at 100% of its rating. Standard breakers are not 100% rated, which is why a 20 A breaker should carry no more than 16 A of continuous load.
Breaker vs fuse basics
Both breakers and fuses are overcurrent devices, and both must have an interrupting rating at least equal to the fault current available at their line terminals (110.9).
| Feature | Circuit breaker | Fuse |
|---|---|---|
| After it opens | Reset and reuse | Replace |
| How it trips | Inverse time: trips faster as current rises | Element melts; time-delay types ride through inrush |
| Key Article 240 rules | Trip-free (240.80), ON is up on vertical handles (240.81) | Voltage rating at least the circuit voltage; current-limiting fuseholders reject other fuses (240.60) |
| Common exam point | Interrupting rating must be marked if other than 5,000 A (240.83(C)) | Edison-base plug fuses only as replacements (240.51(B)) |
Trip-free means the breaker opens on an overcurrent even if someone holds the handle ON. Dual-element time-delay fuses suit motor circuits because they carry the brief starting inrush without blowing.
Worked examples
Example 1: continuous and noncontinuous load. A feeder supplies 46 A of continuous load and 18 A of noncontinuous load.
46 × 1.25 = 57.5 A57.5 + 18 = 75.5 A
75.5 A is not a standard rating, so the minimum device is the next standard size, 80 A.
Example 2: next size up. An individual branch circuit with no receptacles supplies a 58 A noncontinuous load using 6 AWG copper with 75°C terminations. Table 310.16 gives 65 A for 6 AWG copper at 75°C. 65 A is not a standard rating, so 240.4(B) permits the next standard size, 70 A.
Example 3: over 800 A. A feeder uses three parallel sets of 500 kcmil copper. Table 310.16 gives 380 A for 500 kcmil copper at 75°C.
3 × 380 = 1,140 AA 1,200 A device would need rounding up, which 240.4(C) does not allow above 800 A. The largest standard device whose rating does not exceed 1,140 A is 1,000 A.
Example 4: small conductor rule. A general-purpose circuit uses 12 AWG THHN copper. Table 310.16 gives 30 A at 90°C. With 4 to 6 current-carrying conductors in the raceway, the 80% adjustment factor gives 30 × 0.80 = 24 A. The adjusted ampacity is higher than 20 A, but 240.4(D) still caps the device at 20 A.
Common exam mistakes
- Rounding up above 800 A. The next-size-up rule stops at 800 A.
- Using the 90°C ampacity to justify a 25 A or 30 A breaker on 12 AWG copper.
- Applying the copper limits to aluminum. 10 AWG aluminum is 25 A, not 30 A.
- Forgetting the 125% factor applies only to the continuous portion of the load.
- Treating 75 A, 115 A or 135 A as standard ratings. Check 240.6(A).
- Assuming the small conductor rule applies to motor and air-conditioning circuits.
More continuous-load examples are in the exam calculations guide, and the free practice test covers every topic.
Practice questions
Frequently asked questions
What is the next size up rule in the NEC?
Section 240.4(B) lets you use the next higher standard overcurrent device when a conductor's ampacity does not match a standard rating. It applies only to devices rated 800 A or less, and never to multioutlet branch circuits supplying receptacles for cord-and-plug-connected portable loads.
Can 12 AWG wire be on a 25 A or 30 A breaker?
Not on a general-purpose circuit. Section 240.4(D) limits 12 AWG copper to 20 A protection, even though its table ampacity is higher. Specific loads covered by 240.4(G), such as motors and air-conditioning equipment, can follow different rules from their own articles.
Why is a continuous load sized at 125%?
Standard breakers in enclosures are not designed to carry their full rating for 3 hours or more. Sizing the device at 125% of the continuous load keeps it at or below 80% of its rating during continuous operation. An assembly listed for 100% operation may be sized at 100%.
Is 75 A a standard breaker size?
No. 75 A is not one of the standard ratings in 240.6(A). If a calculation calls for 75 A, the next standard rating is 80 A.