Formula
Allowable area is the conduit’s internal area times the Table 1 percent. One conductor is 53%, two are 31%, three or more are 40%. A nipple of 24 inches or less is 60%.
percent fill = conductor area ÷ conduit area × 100
same-size count = allowable ÷ one conductor, then Note 7
Note 7 rounds a same-size result up when the decimal is 0.8 or more. Mixed conductors are a straight area comparison. Bare conductors use Table 8 circular mils converted by π/4 ÷ 1,000,000. Conduit areas are the 100% column of Table 4.
Worked examples
1/2 in EMT, nine 12 AWG THHN. Area 0.304 in², 40% is 0.1216 in², each wire 0.0133 in². The raw count is 9.14, Note 7 leaves it at 9. Fill is 39.4%.
The same nine wires in a nipple. The fraction becomes 60%, so the allowable area grows and a smaller discussion of “will it fit” gets easier. The nipple has to actually be 24 inches or shorter.
One 12 AWG THHN in 1/2 in EMT. Table 1 allows 53%, not 40%. A single conductor is a different row.
Reference
EMT internal area, 100% column, and how many 12 AWG THHN the 40% column accepts after Note 7.
| Trade | Area in² | 40% | 12 THHN |
|---|---|---|---|
| 1/2 in | 0.304 | 0.122 | 12 |
| 3/4 in | 0.533 | 0.213 | 21 |
| 1 in | 0.864 | 0.346 | 34 |
| 1-1/4 in | 1.496 | 0.598 | 59 |
| 1-1/2 in | 2.036 | 0.814 | 81 |
| 2 in | 3.356 | 1.342 | 133 |
| 2-1/2 in | 5.858 | 2.343 | 233 |
| 3 in | 8.846 | 3.538 | 352 |
| 3-1/2 in | 11.545 | 4.618 | 460 |
| 4 in | 14.753 | 5.901 | 588 |
Questions
How many 12 AWG THHN conductors fit in 1/2 inch EMT?
Nine. Chapter 9 Table 4 gives 1/2 inch EMT an internal area of 0.304 in². Three or more conductors may use 40% of that, 0.122 in². Table 5 gives 12 AWG THHN 0.0133 in². 0.122 ÷ 0.0133 is 9.17, and Note 7 does not round up until the fraction is 0.8, so the count is 9. That is Annex C Table C.1.
When is the fill 53%, 31%, 40%, or 60%?
Chapter 9 Table 1: one conductor 53%, two conductors 31%, more than two 40%. Note 4 allows 60% for a nipple no longer than 24 inches. This page does not apply adjustment factors from 310.15(C)(1), and it does not do box fill.
What is Note 7?
When every conductor is the same size, a decimal of 0.8 or larger rounds up to the next whole conductor. Mixed sizes are compared by area only: the sum of the Table 5 areas has to fit in the allowable area.
Sources
- NFPA 70, Chapter 9, Table 1 (fill), Note 4 (nipples), Note 7 (rounding), Table 4 (conduit area), Table 5 (insulated conductors), Table 8 (bare cmil). The 1/2 in EMT × 12 AWG THHN case matches Annex C Table C.1.
- XHHW-2 and THW areas are the Table 5 columns as commonly reprinted. They were not re-read from the NFPA PDF for this page. THHN 12 AWG is 0.0133 in², which the hand check locks.