Overcurrent Protection: The Rules the Exam Loves
Article 240 questions reward people who know which rule applies before they reach for a table. These five come up over and over.
1. The small-conductor rule beats the ampacity table
For the small sizes, 240.4(D) caps the overcurrent device regardless of what the ampacity table would allow:
| Copper conductor | Maximum OCPD |
|---|---|
| 14 AWG | 15 A |
| 12 AWG | 20 A |
| 10 AWG | 30 A |
Aluminum runs one step lower — the 15 A figure that looks wrong next to 12 AWG belongs to 12 AWG aluminum, and 25 A belongs to 10 AWG aluminum. Exam distractors are built from exactly those copper/aluminum swaps. (The caps have specific-application exceptions in 240.4(E) and (G) — motor circuits being the big one — and question stems usually tell you to set those aside.)
2. The next-size-up allowance — know its conditions
When conductor ampacity doesn't land on a standard device rating, 240.4(B) lets you round up to the next standard size — but only if the device doesn't exceed 800 A and the circuit isn't supplying receptacles for cord-and-plug portable loads. Classic exam setup: conductors with 115 A of allowable ampacity. 115 isn't a standard rating; the next standard size up is 125 A, and that's the answer — provided the stem satisfies the conditions. Choosing 110 A "to be safe" is the engineered wrong answer.
3. Standard ratings: know the list well enough to spot a fake
240.6(A) lists the standard ampere ratings for fuses and inverse-time breakers. The exam's favorite trick is planting a plausible non-member — 85 A, for instance, is not a standard rating, while 70, 90, and 100 all are. You don't need the whole list cold, but you should know where it lives and how the steps run so a fake jumps out.
4. Continuous loads: 125% on the continuous part only
The OCPD must be sized at no less than the noncontinuous load plus 125% of the continuous load. Worked example from our practice set: 8 A noncontinuous + (20 A continuous × 1.25) = 33 A minimum → next standard size, 35 A. The wrong answers are always the same two moves: forgetting the 1.25 factor (28 A) or applying it to everything. The factor multiplies the continuous portion only.
5. Two quick ones that show up constantly
Handle height: a breaker used as a switch or disconnect may sit no higher than 6 ft 7 in to the center of the grip of the operating handle in its highest position — a number that also appears in the snap-switch rule, so it's worth owning once and recognizing everywhere.
Switching-duty marking: breakers switching 120 V or 277 V fluorescent lighting must be listed and marked SWD or HID. HACR is the heating/air-conditioning marking — a pure terminology-confusion distractor.
Put it to work
Questions 1 and 5 in the free set hit the continuous-load factor and the next-size-up allowance. Work them with the book open and a timer running — the procedure matters more than the answer.