Conductors and overcurrent

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1. 1 · Conductors and overcurrent · NEC Knowledge

   A main-lug panelboard has a 100 A bus. The only overcurrent device ahead of it is a 150 A feeder breaker. The helper says the bus is fine because no single branch breaker is over 30 A. What is unprotected?

   1. A. The panelboard bus. The device protecting that bus cannot exceed the bus rating. A 150 A feeder breaker does not protect a 100 A bus.Keyed answer
   2. B. Nothing. The branch breakers protect the bus.
   3. C. Only the equipment grounding bar has to match the bus rating.
   4. D. The feeder breaker has to be larger than the bus or the panel will nuisance-trip.

   Article 408 requires the panelboard bus to be protected at not more than its rating. Branch breakers protect the branch circuits. They do not make a 150 A feeder breaker acceptable ahead of a 100 A bus. Article 240 is the overcurrent device. Confirm both in NEC 2026.

   Open NEC 2026 408.36

2. 2 · Conductors and overcurrent · NEC Knowledge

   A calculation needs 105 A. The conductor ampacity is 115 A, so a helper uses a 125 A breaker and says the next standard size is always allowed. On another feeder the device would be 900 A, and the helper wants the same habit. Where does the habit stop?

   1. A. The next-higher standard size is a limited permission. It is not available once the overcurrent device is over 800 A, so a 900 A device is past it. A 125 A device still has to meet the other conditions in the section.Keyed answer
   2. B. Next-size-up is allowed at every rating, including 900 A.
   3. C. Next-size-up is forbidden under 800 A and required over 800 A.
   4. D. A 115 A conductor may be protected at 200 A on any feeder.

   Article 240 lets you move to the next standard overcurrent device only inside the conditions it states, and that permission stops above 800 A. It also does not apply to every kind of circuit. Confirm the section in NEC 2026 before you treat next-size-up as automatic.

   Open NEC 2026 240.4

3. 3 · Conductors and overcurrent · NEC Knowledge

   A panelboard is main-lug-only. The breaker that protects the feeder to that panel is in the upstream equipment. In this arrangement, what keeps the panelboard bus within its rating?

   1. A. The grounding electrode at the service
   2. B. The largest branch breaker in the main-lug panel
   3. C. A bonding jumper between the neutral bar and the enclosure
   4. D. The feeder overcurrent device ahead of the panel, sized within the panel ratingKeyed answer

   A main-lug panel still needs overcurrent protection not greater than the panelboard rating. That device can be the feeder breaker upstream. Article 408 covers the panelboard. Article 240 covers the feeder device. Confirm both in NEC 2026 before you leave a main-lug panel unprotected.

   Open NEC 2026 Article 408

4. 4 · Conductors and overcurrent · NEC Knowledge

   Inside the boiler room the feeder conductors pass through air that stays much hotter than the office where the helper stood. He applies the office thermostat as the ambient and says the ampacity table already assumes a warm building. What temperature is the correction actually based on?

   1. A. The office temperature, because that is the room where someone reads the panel directory.
   2. B. No ambient at all, because the ampacity table already includes every boiler room.
   3. C. The ambient around the conductors, which is the boiler room. The office thermostat is a different place and does not set this correction.Keyed answer
   4. D. The outdoor design temperature, because indoor conductors are corrected as if they were on the roof.

   Article 310 corrects ampacity for the ambient where the conductors are installed. A cooler room nearby does not replace the boiler room. Confirm the correction factors in NEC 2026. Do not invent a factor for this answer.

   Open NEC 2026 Article 240

5. 5 · Conductors and overcurrent · NEC Knowledge

   The panelboard bus is rated 100 amperes and the feeder breaker ahead of this main-lug panel is rated 200 amperes. The helper says a main-lug panel needs no protection of its own because the calculated load is under 100 amperes. What did that load figure fail to do?

   1. A. It failed to protect the panelboard. The feeder overcurrent device still cannot exceed the panelboard rating just because the calculated load is smaller.Keyed answer
   2. B. It legalized the 200-ampere feeder breaker, because a main-lug panel is protected by the load calculation alone.
   3. C. It limited only the number of branch breakers, not the feeder device that supplies the bus.
   4. D. It showed that panelboard protection is required only when the panel is used as service equipment.

   Article 408 requires overcurrent protection for the panelboard and does not permit that protection to exceed the panelboard rating. A smaller calculated load does not raise the bus. Confirm the protection rule in NEC 2026.

   Open NEC 2026 Article 240

6. 6 · Conductors and overcurrent · NEC Knowledge

   The helper parallels a 1/0 copper conductor with a larger copper conductor on the same phase because the two ampacities add up to more than the breaker. Length and metal match. He says any sizes may share a lug if the sum is enough. What match is still missing?

   1. A. Nothing. Any sizes may be paralleled when the sum of the ampacities covers the breaker.
   2. B. A requirement that parallels are legal only for aluminum, so this copper set is illegal for being copper.
   3. C. A requirement that one conductor of the pair be smaller so it can act as a spare.
   4. D. The same size. Parallel conductors of one phase have to match in size. Adding two different ampacities is not the parallel rule.Keyed answer

   Article 310 permits paralleled conductors only when the set matches, including size. A larger partner does not legalize a smaller one. Confirm the parallel conditions in NEC 2026.

   Open NEC 2026 Article 240

7. 7 · Conductors and overcurrent · NEC Knowledge

   This multi-receptacle branch circuit supplies several cord-and-plug receptacle outlets, and its conductor ampacity falls between two standard breaker ratings. The helper always takes the next size larger and says that permission applies to every circuit he installs. Where does that habit fail here?

   1. A. It fails only on motor circuits, and every receptacle circuit is required to take the next larger breaker.
   2. B. It fails on this multioutlet branch circuit. Next-size-up is not a universal permission, and this receptacle circuit is one of the cases where it does not apply.Keyed answer
   3. C. It fails because standard ratings may be ignored and any larger frame is close enough.
   4. D. It fails because the conductor ampacity is allowed to sit below the continuous load whenever the breaker is larger.

   Article 240 permits the next higher standard overcurrent device only in the cases that rule lists. A branch circuit supplying more than one cord-and-plug receptacle is outside that permission. Confirm the limits in NEC 2026.

   Open NEC 2026 Article 240

8. 8 · Conductors and overcurrent · NEC Knowledge

   The feeder neutral was cut two sizes smaller than the ungrounded conductors because the helper says neutrals never carry full current. This feeder is almost entirely line-to-neutral load. What is wrong with that habitual reduction?

   1. A. Every feeder neutral is required to be two sizes smaller than the ungrounded conductors.
   2. B. The neutral should be omitted, because the equipment ground carries line-to-neutral return current.
   3. C. The neutral must always be larger than the ungrounded conductors, with no calculation either way.
   4. D. A neutral is reduced only when the neutral load permits it. A fixed two-size cut is not that calculation, and this line-to-neutral feeder does not justify the cut.Keyed answer

   Article 215 sizes the feeder grounded conductor from the neutral load. A feeder that is mostly line-to-neutral does not get an automatic reduction. Confirm the neutral rule in NEC 2026.

   Open NEC 2026 Article 240

9. 9 · Conductors and overcurrent · NEC Knowledge

   A 14 AWG copper conductor feeds one luminaire from a 20-ampere breaker. The lamp draws about one ampere. The helper says the small-conductor limit is aimed at receptacle circuits only, so a lighting load may use the larger breaker. What limit did the lamp current miss?

   1. A. No limit. One ampere of load permits any breaker size on 14 AWG copper.
   2. B. The small-conductor limit on the overcurrent device for this copper size. A small lamp does not raise the breaker that protects 14 AWG copper on a general circuit.Keyed answer
   3. C. A requirement that 14 AWG copper on a luminaire must be protected at 30 amperes.
   4. D. A rule that the lamp nameplate replaces the breaker entirely.

   Article 240 sets the small-conductor overcurrent limit by conductor size on a general circuit. The load being small does not move that limit. Confirm the ampere cap in NEC 2026.

   Open NEC 2026 Article 240

10. 10 · Conductors and overcurrent · NEC Knowledge

    An ordinary inverse-time breaker is called 100 percent rated because the helper says any breaker may be loaded continuously to its handle rating. Nothing on the assembly says it is listed for 100 percent continuous operation. What does the missing listing leave in place?

    1. A. A rule that every breaker is 100 percent rated for continuous loads by definition.
    2. B. A prohibition on continuous loads, so the feeder may not supply one at any size.
    3. C. A requirement to size the continuous load at half the breaker and ignore the listing.
    4. D. The ordinary continuous-load rule. Calling a standard breaker 100 percent rated does not make the assembly listed for that duty.Keyed answer

    Article 215 sizes the feeder, and Article 240 sizes the overcurrent device, for continuous load unless the assembly is actually listed for 100 percent continuous operation. Speech is not that listing. Confirm the rule in NEC 2026.

    Open NEC 2026 Article 240

11. 11 · Conductors and overcurrent · NEC Knowledge

    The circuit directory says lights on every breaker, including the range, the dryer, and spare positions that still contain unused breakers. The helper says a directory is optional if the occupant can flip handles until the right load goes dead. What must the directory actually do?

    1. A. Nothing. Directories are prohibited because labels confuse occupants during an emergency.
    2. B. Use the word lights for every circuit, which is the required text in a dwelling.
    3. C. Identify each circuit so it can be told from the others, and describe spare positions that contain unused overcurrent devices. One repeated word does not do that.Keyed answer
    4. D. Label only the empty screw holes, and leave installed breakers unidentified on purpose.

    Article 408 requires a legible directory that states the specific purpose of each circuit and identifies spare positions that contain unused overcurrent devices. Confirm the directory rule in NEC 2026.

    Open NEC 2026 Article 240

12. 12 · Conductors and overcurrent · NEC Knowledge

    The helper counted the equipment grounding conductor as a current-carrying conductor and applied an adjustment the power conductors alone would not have triggered. That grounding conductor is not a neutral and carries no load current in this circuit. What should have been left out of the count?

    1. A. The equipment grounding conductor. It is not a current-carrying conductor for this adjustment.Keyed answer
    2. B. An ungrounded conductor, because only green wires count toward adjustment.
    3. C. Nothing. The equipment grounding conductor always counts, and power conductors never do.
    4. D. The whole raceway, because adjustment is based on the number of conduits on the job, not on conductors.

    Article 310 counts current-carrying conductors when it adjusts ampacity. An equipment grounding conductor is not added to that count just because it is in the raceway. Confirm the count in NEC 2026.

    Open NEC 2026 Article 240

13. 13 · Conductors and overcurrent · NEC Knowledge

    Free-air ampacities were copied onto conductors pulled through a raceway. The helper says a metal raceway sheds heat better than open air, so the free-air table is conservative and always allowed for conduit. Which table did he apply to the wrong installation?

    1. A. The raceway table, which is what he used, and free air is only a nickname for conduit.
    2. B. Either table, because copper ampacity does not depend on whether the conductors are enclosed.
    3. C. The free-air table. Conductors in a raceway take the raceway ampacity. His cooling theory does not move them to the free-air table.Keyed answer
    4. D. The aluminum free-air column, which is required for every copper conductor in conduit.

    Article 310 assigns different ampacities to conductors in free air and conductors in a raceway. The installation picks the table. Confirm which table applies in NEC 2026.

    Open NEC 2026 Article 240

14. 14 · Conductors and overcurrent · NEC Knowledge

    The dwelling allowance that permits smaller service or feeder conductors for a qualifying dwelling was applied to a feeder that supplies only a garage panel, not the whole dwelling. The helper says the house is a dwelling, so every feeder on the property gets that allowance. What did he misclassify?

    1. A. The garage feeder. That allowance is for the dwelling service or a dwelling feeder the article actually covers, not for every downstream feeder on the lot.Keyed answer
    2. B. The service. The allowance is required for every garage feeder and prohibited on the dwelling service.
    3. C. A branch circuit. The allowance means every dwelling feeder must be smaller than its load.
    4. D. The grounding electrode conductor, which is the only conductor that allowance sizes.

    Article 310 limits where a dwelling service or feeder conductor allowance may be used. A feeder that supplies only a garage panel is not automatically that feeder. Confirm whether this feeder qualifies in NEC 2026.

    Open NEC 2026 Article 240

15. 15 · Conductors and overcurrent · Calculations

    A feeder carries 40 A of noncontinuous load and 20 A of continuous load. For this drill, minimum conductor ampacity is the noncontinuous load plus 125% of the continuous load. What ampacity do you need?

    1. A. 60 A
    2. B. 65 AKeyed answer
    3. C. 70 A
    4. D. 75 A

    40 + (20 × 1.25) = 65 A. Only the continuous part gets the extra 25% in this drill. Confirm the feeder rule in NEC 2026 Article 215.

    Open NEC 2026 Article 215

16. 16 · Conductors and overcurrent · Calculations

    A feeder carries 55 A of noncontinuous load and 24 A of continuous load. For this drill, minimum ampacity is the noncontinuous load plus 125% of the continuous load. What do you need?

    1. A. 79 A
    2. B. 99 A
    3. C. 85 AKeyed answer
    4. D. 70 A

    24 × 1.25 = 30 A. Add the 55 A noncontinuous load to get 85 A. Applying 125% to the whole 79 A load lands near 99 A. Article 215 is the feeder ampacity rule. The split between continuous and noncontinuous is printed in the stem.

    Open NEC 2026 Article 215

17. 17 · Conductors and overcurrent · Calculations

    A feeder conductor's table ampacity is 80 A. Apply ambient 0.91 and a current-carrying adjustment of 0.8. The continuous load is 40 A and needs 125% of that current. How many amperes above the required ampacity is the corrected conductor? Use a negative result only if you must; these rows are above.

    1. A. 8.2 AKeyed answer
    2. B. 58.2 A
    3. C. 50 A
    4. D. 14 A

    80 × 0.91 × 0.8 = 58.2 A corrected. Required is 40 × 1.25 = 50 A. The margin is 8.2 A. Skipping ambient leaves a different margin. Article 310 is the correction. Article 215 is the feeder continuous-load factor. Both are printed in the stem.

    Open NEC 2026 Article 310

18. 18 · Conductors and overcurrent · Calculations

    A heat feeder is 64 A continuous and 15 A noncontinuous. Required ampacity is 125% of the continuous load plus the noncontinuous load. This drill then allows the next standard overcurrent size at or above that ampacity. Ratings: 90, 100, 110, 125. What size do you select?

    1. A. 90 A
    2. B. 100 AKeyed answer
    3. C. 95 A
    4. D. 79 A

    64 × 1.25 + 15 = 95 A. The smallest listed rating at or above that is 100 A. 90 A does not cover the required ampacity. Confirm the feeder calculation in Article 215 and the standard-size rule in NEC 2026 Article 240.

    Open NEC 2026 Article 240

19. 19 · Conductors and overcurrent · Calculations

    A short tap leaves a 450 A feeder. This drill requires the tap conductors to have ampacity at least 0.1 times the feeder overcurrent device, and the tap is 6 ft against a 10 ft length limit. What minimum tap ampacity does the fraction require?

    1. A. 450 A
    2. B. 405 A
    3. C. 45 AKeyed answer
    4. D. 22.5 A

    450 × 0.1 = 45 A. That is the ampacity floor in this drill. The length comparison is a second tap condition, not a substitute for the ampacity. Article 240 is the tap rule. Confirm the real tap conditions in NEC 2026. The fraction is printed in the stem.

    Open NEC 2026 Article 240

20. 20 · Conductors and overcurrent · Calculations

    A feeder's non-range unbalanced current is 64 A minus 36 A. Separately, a household range demand of 4,800 W is on the feeder, and this drill takes the range neutral at 70% of that demand, then divides by 240 V to get amperes. Add that range neutral to the non-range unbalance. What neutral current do you get?

    1. A. 28 A
    2. B. 20 A
    3. C. 100 A
    4. D. 42 AKeyed answer

    Non-range unbalance is 64 − 36 = 28 A. Range neutral is 4,800 × 0.70 / 240 = 14 A. Added, 42 A. Using the full range watts without the 70% overstates the neutral. Article 215 is the feeder neutral. The range neutral factor is an Article 120 step and is printed in the stem.

    Open NEC 2026 Article 215