A conductor takes up area. The conduit has a fixed inside area. Add up the conductor areas and compare the total to a percentage of the conduit area. Over two conductors, the limit is 40%. Table 4 gives the pipe, Table 5 gives the wire, and Annex C reads the count straight off — when every conductor is the same size. Worked example: nine 12 AWG THHN in ½″ EMT.
| Conductors in the raceway | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| Over 2 | 40% |
The fill limit, by conductor count: 1 conductor = 53%, 2 conductors = 31%, over 2 = 40%. Most runs use three or more, so 40% is the number you use most days. Two conductors get the lowest percentage to prevent jamming on a bend.
The conduit inside area, by raceway type and trade size. ½″ EMT is 0.304 in². Table 4 already prints the 40% column — read it, do not multiply. EMT, IMC, RMC, PVC Sch 40 and Sch 80 all differ at the same trade size, so read the correct block.
The conductor area, including insulation. THHN/THWN-2: 14 AWG = 0.0097 in², 12 AWG = 0.0133 in², 10 AWG = 0.0211 in², 8 AWG = 0.0366 in². Insulation type changes the area — read the row that matches the wire in the pipe.
The shortcut. One table per raceway type (C1 is EMT). Read the maximum count directly. Annex C works only when every conductor is the same size and insulation. Mixed sizes require the full Table 4 and Table 5 calculation.
The first reason for 40% — heat. Conductors make heat under load, a packed raceway traps it, and the heat damages insulation. More than three current-carrying conductors in a raceway triggers an ampacity adjustment.
The second reason — the pull. The raceway must permit installation and removal of conductors without damaging the insulation. This is a rule, not a guideline: damage on the pull is a violation even when the percentage passes.
| Step | Source | Value |
|---|---|---|
| ½″ EMT inside area | Table 4 | 0.304 in² |
| Fill limit, over 2 conductors | Table 1 | 40% |
| Budget (0.304 × 0.40) | — | 0.122 in² |
| 12 AWG THHN area | Table 5 | 0.0133 in² |
| Count (0.122 ÷ 0.0133) | — | 9.14 → 9 |
Cross-check: Annex C, Table C1 — EMT, THHN, 12 AWG, ½″ = 9 conductors. The long method and the shortcut agree.
Nipples get 60%. A raceway 24 in. or shorter between boxes may fill to 60% — Chapter 9, Note 4. The 310.15(C)(1) adjustment does not apply either.
Grounds count for fill. An equipment grounding conductor takes up area, so it counts. It is exempt from the adjustment count, not from fill.
Round up at 0.8. When the calculation gives a decimal of 0.8 or larger, round up; otherwise round down — Chapter 9, Note 7.
Multiconductor cable in conduit: use the actual outside diameter of the cable, not the sum of the conductors — Chapter 9, Note 9.
NEC Chapter 9 tables and Annex C (the whole job is in the book). Fish tape and listed pulling lubricant for the pull. Any hand tool = Klein Tools.