NEC 2023 · checked by a second reviewer · November 7, 2026
NEC Article 240 Practice Questions (10 Free, With Answers)
Overcurrent Protection: 10 original questions on sizing and locating fuses and breakers, small-conductor rules and next-size-up. Click an answer to see the explanation and the NEC section. No sign-up.
NEC Article 240 · 10 questions0 / 10 answered
Which of these is NOT a standard ampere rating for fuses and inverse time circuit breakers?
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Correct answer: C. 45, 110 and 175 A are all on the NEC list of standard ratings; 75 A is not (the list goes 70 A then 80 A). The odd-looking 45 and 175 A are the tempting wrong picks because they seem less common, but both are standard.Reference: NEC 240.6(A)
Feeder conductors have an ampacity of 130 A after all corrections and adjustments. The calculated load is 120 A, and the feeder supplies a panelboard. Using the next-size-up rule, what is the largest standard overcurrent device permitted?
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Correct answer: B. When the conductor ampacity does not match a standard size and the device is 800 A or less, the next higher standard rating may be used, as long as the conductors still carry the load. 130 A is not a standard size, so the next standard size is 150 A. 125 A would also be permitted, but it is not the largest.Reference: NEC 240.4(B); NEC 240.6(A)
A feeder is protected by a 1,000-A circuit breaker. The parallel conductors have a total ampacity of 960 A, and the calculated load is 900 A. Is this permitted?
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Correct answer: A. The next-size-up allowance stops at 800 A. For overcurrent devices rated over 800 A, the conductors must have an ampacity equal to or greater than the device rating, so 960-A conductors cannot be protected at 1,000 A. Rounding up is the tempting choice, but it only works up to 800 A.Reference: NEC 240.4(C)
Tap conductors 20 ft long are taken from a feeder protected at 300 A. They end in a single circuit breaker that limits the load to the tap ampacity. Under the 25-ft tap rule, what is the minimum ampacity of the tap conductors?
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Correct answer: D. Under the 25-ft tap rule, the tap conductors must have an ampacity of at least one-third of the feeder overcurrent device rating: 300 / 3 = 100 A. The 30-A answer uses the one-tenth figure that belongs to the 10-ft tap rule when the tap leaves the enclosure.Reference: NEC 240.21(B)(2)
A circuit breaker serving as an overcurrent device must be installed so the center of the grip of its operating handle, in its highest position, is not more than how high above the floor or working platform?
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Correct answer: D. Overcurrent devices must be readily accessible, and the center of the handle grip at its highest position may not be more than 6 ft 7 in. above the floor or platform. 6 1/2 ft is tempting because it is the working-space headroom, a different rule.Reference: NEC 240.24(A); NEC 404.8(A)
In a warehouse, circuit breakers will be used as the regular on/off switches for 277-volt fluorescent lighting. What must these breakers be?
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Correct answer: B. Breakers used as switches for 120-V and 277-V fluorescent lighting must be listed and marked SWD or HID, which shows they can handle frequent switching. Sizing at 125% relates to continuous loads, which is a separate issue from switching duty.Reference: NEC 240.83(D)
When may Edison-base plug fuses be installed?
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Correct answer: A. Edison-base plug fuses are allowed only as replacements in existing installations that show no sign of overfusing or tampering. New work uses Type S fuses with size-limiting adapters, which is why the 'new circuits 30 A or less' answer is wrong even though plug fuses are limited to 30 A.Reference: NEC 240.51(B)
A circuit breaker is marked with a slash rating of 480Y/277 V. On which system may it be applied?
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Correct answer: C. A slash-rated breaker may be used only on a solidly grounded system where no conductor is more than the lower value (277 V) to ground and no two conductors exceed the higher value (480 V). On corner-grounded or ungrounded delta systems, a conductor can be at 480 V to ground, so only a straight 480-V rated breaker is suitable there.Reference: NEC 240.85
Arc energy reduction (for example, zone-selective interlocking or an energy-reducing maintenance switch) is required for a circuit breaker when its highest continuous trip setting is, or can be adjusted to, at least:
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Correct answer: A. The arc energy reduction requirement applies when a breaker is rated or adjustable to 1,200 A or higher. 800 A is tempting because it is the cutoff for the next-size-up rule, and 1,000 A is the service ground-fault protection threshold; neither is the arc energy reduction trigger.Reference: NEC 240.87
Unless a specific exception applies (such as motor circuits), what is the maximum overcurrent protection for 10 AWG aluminum or copper-clad aluminum conductors?
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Correct answer: D. The small-conductor rule limits 10 AWG aluminum and copper-clad aluminum to 25 A. 30 A is the limit for 10 AWG copper, which is why it is the tempting answer.Reference: NEC 240.4(D)
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