Most-missed NEC mock exam question #2 of 5, from JCR's free 25-question NEC mock exam. 71% of students got this wrong. Try it with your 2020 NEC open, then check your answer below.
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The answer: C, 100 percent
Normally, a branch-circuit overcurrent device must be rated at least the noncontinuous load plus 125% of the continuous load. The exception to 210.20(A) drops the 125% when the assembly, including the overcurrent devices, is listed for operation at 100% of its rating.
This question says the complete assembly is listed for 100%. So the exception applies.
The trap: 80 percent
80 percent was the top wrong answer, picked by 41% of students. It comes from the normal rule: a breaker sized at 125% of a continuous load carries that load at 1 ÷ 1.25 = 80% of its rating.
That math is right for a standard assembly. It doesn't fit here, because the question gives you an assembly listed for 100%.
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What 210.20(A) says
- Continuous load: a load where the maximum current is expected to continue for 3 hours or more (Article 100).
- Normal rule: overcurrent device rating not less than the noncontinuous load plus 125% of the continuous load.
- Exception: where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100% of its rating, the rating may be not less than the continuous load plus the noncontinuous load. A 100%-rated breaker alone isn't enough. The listing has to cover the assembly.
- Conductors: still protected in accordance with 240.4 (210.20(B)).
Worked example
A branch circuit supplies 32 A of continuous load and 8 A of noncontinuous load.
- Standard assembly: 8 + (32 × 125%) = 8 + 40 = 48 A minimum. The next standard size in 240.6(A) is 50 A.
- Assembly listed for 100%: 32 + 8 = 40 A minimum.
Common questions
Do I need a 100% rated breaker for continuous loads?
No. A standard breaker works if you size it at the noncontinuous load plus 125% of the continuous load. The 100% option only lets you skip the extra 25%, and only when the whole assembly is listed for it.
What is a continuous load?
Article 100 defines it as a load where the maximum current is expected to continue for 3 hours or more. Lighting in stores and offices that stays on all day is a common example.
Is a 100% rated breaker enough by itself?
No. The exception in 210.20(A) requires the assembly, including its overcurrent devices, to be listed for 100% operation. Check the equipment's listing and marking before you drop the 125%.
All references are to the 2020 NEC. Order the code book here, or read it online through a paid NFPA LiNK subscription. Our state requirements page lists the edition each state tests.
JCR checked this against the 2020 NEC in NFPA LiNK on October 21, 2026.
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The 5 most-missed NEC mock exam questions
- Pull box size for an angle pull (314.28)
- Continuous loads on a 100%-rated assembly (210.20(A)) (this post)
- Breakers used as switches: SWD and HID (240.83(D))
- Motor overload sizing and service factor (430.32)
- Closet light clearance (410.16(C))
See all 5 most-missed questions on one page.
Free video: the 125% rule
The conductor side of this rule works the same way. Watch JCR's lesson on the 125% rule for conductors (210.19), or see all JCR lessons on YouTube.
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