10 NEC Mistakes That Fail Electrician Exams

1. Mixing FLC with FLA on motor questions

Branch-circuit conductors use FLC from Tables 430.247-250. Overload protection uses nameplate FLA. Different inputs, same motor. Most candidates use whichever number is given first without checking which sizing decision the question is asking about.

2. Applying 40% conduit fill to a 1-conductor problem

Chapter 9 Table 1: 53% for one conductor, 31% for two, 40% for more than two. Candidates remember 40% (most common) and apply it everywhere. Read the conductor count before reaching for the percentage.

3. Counting pigtails in box fill

Per 314.16(B), conductors that originate AND terminate inside the box don’t count. Pigtails are a zero. The conductors entering from outside count, the splices that stay inside don’t.

4. Using 250.66 for an EGC

Table 250.66 sizes the grounding electrode conductor (system to earth). Table 250.122 sizes the equipment grounding conductor (the safety wire inside circuits). They’re different problems with different inputs. The exam will offer Table 250.66 numbers as wrong answers when the question is really about EGC.

5. Forgetting to upsize the EGC when phase conductors are upsized

Per 250.122(B), if you upsize the phase conductors for voltage drop, you must upsize the EGC proportionally. The exam loves this trap because it requires two table lookups in sequence.

6. Confusing branch circuits and feeders

A branch circuit ends at the load. A feeder ends at another OCPD. The exam will describe a panel-to-panel run and ask for "branch circuit conductor sizing," and the trap is the word "branch." That’s a feeder. Article 215 applies, not Article 210.

7. Studying old NEC editions

GFCI and AFCI requirements expanded across the last three NEC cycles. If you studied from a 2017 prep book, several rules changed. Texas TDLR is on NEC 2023. California uses 2025 CEC (NEC 2023 baseline + state amendments). Verify your prep material’s code edition.

8. Treating informational notes as enforceable

The 3% voltage drop target is an informational note, not a code rule, for branch circuits. Some prep materials present it as a code rule. The exam typically asks the question two ways: what does the NEC require (nothing on voltage drop directly), and what does the informational note recommend (3%).

9. Missing the 80% continuous load rule

Continuous loads are limited to 80% of the OCPD rating. A 20A breaker handles 16A continuous. A 30A breaker handles 24A. The inverse: conductors and OCPD must be at least 125% of the continuous load. Same rule, two directions.

10. Reaching for the book on every question

Half the time the answer is in the question itself: a clue word, a unit, a specific scenario that maps to one article. Reaching for the book on every question burns time. Make a hypothesis first. Verify only if you’re unsure.