NEC conduit fill rules
NEC Chapter 9, Table 1 limits how much of a conduit's internal area conductors can occupy, so wires can be pulled without damage and heat can escape:
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Fill % = (number of wires × wire area) ÷ conduit internal area × 100
Conduit areas come from Chapter 9, Table 4, and THHN/THWN-2 conductor areas from Table 5. For example, nine 12 AWG THHN wires (0.0133 in² each) in 1/2" EMT (0.304 in²) fill 39.4%, just under the 40% limit. That matches Annex C, which allows nine.
Internal area (in²)
| Trade size | EMT | PVC Sch 40 |
|---|---|---|
| 1/2" | 0.304 | 0.285 |
| 3/4" | 0.533 | 0.508 |
| 1" | 0.864 | 0.832 |
| 1-1/4" | 1.496 | 1.453 |
| 1-1/2" | 2.036 | 1.986 |
| 2" | 3.356 | 3.291 |
NEC Chapter 9, Note 7 lets you round up when the calculated number of same-size conductors has a decimal of 0.8 or more, which is why the maximum can sit slightly above 40%.
Conduit fill chart
The full chart of maximum wires per conduit size, for EMT and PVC, is on the conduit fill chart page, with a table of the smallest conduit for 2 to 12 wires.
How many 12 gauge wires fit in 3/4 inch conduit?
Sixteen 12 AWG THHN conductors in 3/4-inch EMT, or fifteen in 3/4-inch Schedule 40 PVC. More than three current-carrying conductors in one raceway also triggers ampacity derating, so check the wire size after you fill the conduit.
Frequently asked questions
How many 12 AWG wires fit in 1/2 inch EMT?
Nine THHN conductors, which brings the fill to 39.4% of the 40% limit.
Do grounding wires count toward fill?
Yes. Equipment grounding conductors count toward fill, though they do not count as current-carrying conductors for derating.
Does conduit fill apply to short nipples?
A nipple 24 inches or shorter may be filled to 60% under Chapter 9, Note 4, and derating does not apply to it.
Is PVC fill different from EMT?
Yes. Schedule 40 PVC has a slightly smaller internal area than EMT of the same trade size, so it can hold the same number of wires or fewer.
Sources
Reference charts
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