Conduit Fill on the Electrician Exam: Chapter 9 Table 1 and Annex C
"How many 12 AWG THHN can I put in 3/4 inch EMT?" is one of the most searched code questions there is, and some version of it is on nearly every journeyman and master exam. The good news: it is a lookup, not a judgment call, once you know which table to open.
Study aid only, not legal or design advice. Confirm every value in the edition your state has adopted and in your own code book.
The rule: Chapter 9, Table 1
| Number of conductors | Max fill of raceway area |
|---|---|
| 1 | 53% |
| 2 | 31% |
| Over 2 | 40% |
Two conductors get the lowest percentage because two round wires jam more easily on a pull. Most exam questions are "over 2," so 40% is the number you will use most.
Fast path: Annex C (all conductors the same size and type)
If every conductor is the same size and insulation, skip the math. Annex C has a table for each raceway type (Table C.1 is EMT, others cover ENT, FMC, IMC, LFMC, RMC, PVC and so on) that already applies the 40% rule. Find the insulation type, the wire size, and read across to the trade size.
- Annex C, EMT, 12 AWG THHN: 1/2 in. = 9, 3/4 in. = 16.
Exam tip: the raceway type matters. The same trade size of PVC Schedule 80 holds fewer wires than EMT because its inside diameter is smaller. Open the table for the raceway in the question.
Mixed sizes: Tables 4 and 5
When the conductors are different sizes or types, Annex C will not work. Do it by area:
- Table 5: look up the area (in.²) of each conductor by insulation type and size. Use Table 8 for bare conductors.
- Add up the total conductor area.
- Table 4: find your raceway type and pick the smallest trade size whose 40% column (or 31% / 53%) is equal to or greater than your total.
Worked example
How many 12 AWG THHN fit in 3/4 in. EMT, by the math?
- Table 5: 12 AWG THHN = 0.0133 in².
- Table 4, EMT, 3/4 in., 40% column = 0.213 in².
- 0.213 ÷ 0.0133 = 16.0.
- Answer: 16, which matches Annex C. Good habit: when the conductors are all the same, check your math against Annex C.
The Chapter 9 notes that show up on exams
- Equipment grounding conductors count toward fill (Note 3). Use their actual size and area.
- Nipples 24 in. or shorter between boxes may be filled to 60% (Note 4), and derating for more than three current-carrying conductors does not apply there.
- Rounding: when all conductors are the same size, a calculated decimal of 0.8 or larger can round up to the next whole number (Note 7).
- Fill is not ampacity. Sixteen conductors may fit, but more than three current-carrying conductors triggers the 310.15(C)(1) adjustment. Exam writers combine these two on purpose.
Exam traps
- Using the EMT table when the question says rigid, IMC or PVC.
- Using Annex C when sizes or insulation types are mixed.
- Forgetting the ground wire.
- Using 40% for a two-wire question (it is 31%).
How to tab it
Tab Chapter 9 Table 1, Table 4, Table 5 and the start of Annex C. Highlight the 40% column in Table 4 for EMT, PVC 40 and RMC. Conduit fill questions become 60-second lookups.
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More guides: How to navigate the NEC fast · 2026 NEC changes for exam takers · Massachusetts exam: 527 CMR and the NEC · How to tab and mark your NEC · 30-day journeyman study plan · Box fill calculations (314.16) · Conductor ampacity and derating · Grounding conductor sizing (250.66 vs 250.122) · Voltage drop calculations · Article 120 load calculations (2026 NEC) · 240.4(D) small-conductor rule · NEC exam questions answered · Conduit fill (Chapter 9, Annex C)