Electrical
Conduit Fill Calculator
Enter your wire mix and get the conduit trade size, the fill percentage, and the NEC Chapter 9 math behind it. EMT, IMC, RMC, ENT, LFNC, LFMC, and PVC.
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Conduit
Fill mode
Standard NEC Chapter 9 Table 1 limits: 53% for one conductor, 31% for two, 40% for three or more. Use this for typical raceway runs.
Wire groups
EGC: 1 x 10 AWG Copper THHN/THWN
Calculations use NEC Chapter 9 Table 4 for conduit dimensions and Table 5 for conductor cross-sectional areas. Fill limits in Auto mode are 53% for one conductor, 31% for two, and 40% for three or more (NEC Chapter 9 Table 1). Nipple = 60% (Note 4). Seal-off = 25% (NEC 501.15(C)(1)).
Conduit fill
27.8
%4 conductors in EMT 1/2"(auto)
NEC compliant
Comfortably within the 40% fill limit. Room for future conductors.
Heads up: with more than 3 current-carrying conductors in a raceway, NEC 310.15(C)(1) ampacity adjustment may apply. Check separately.
About this calculator
NEC-compliant. Free to use. No watermarks.
Conduit fill comes down to one rule: the combined cross-sectional area of everything inside the raceway cannot exceed the NEC's allowable percentage for that conduit. 40% for three or more conductors, 31% for exactly two, 53% for a single conductor.
The math is simple. The lookup is the painful part. You are cross-referencing wire size, insulation type, and conductor count against three separate NEC tables for every job. This calculator does that for you, including the equipment grounding conductor, which is the piece most people forget.
Conduit fill chart: THHN in EMT
The common combinations, at the 40% fill limit for three or more conductors. These match NEC Annex C, including the Note 7 rounding explained below. For anything mixed, use the calculator above.
| Wire size | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|
| #14 THHN | 12 | 22 | 35 | 61 | 84 | 138 |
| #12 THHN | 9 | 16 | 26 | 45 | 61 | 101 |
| #10 THHN | 5 | 10 | 16 | 28 | 38 | 63 |
| #8 THHN | 3 | 6 | 9 | 16 | 22 | 36 |
| #6 THHN | 2 | 4 | 7 | 12 | 16 | 26 |
| #4 THHN | 1 | 2 | 4 | 7 | 10 | 16 |
| #2 THHN | 1 | 2 | 3 | 5 | 7 | 11 |
Copper THHN/THWN, 40% fill. Counts include every conductor in the raceway, so subtract your equipment grounding conductor from the number of current-carrying conductors you can run.
The 0.8 rule that makes charts disagree
If you have ever computed a fill by hand and landed one conductor below the published table, this is why. NEC Chapter 9, Note 7 permits the next higher whole number when the calculated count includes a decimal of 0.8 or larger, for conductors that are all the same size.
A 3/4 inch EMT gives 0.213 square inches at 40% fill. A #14 THHN is 0.0097 square inches, so the arithmetic says 21.98 conductors. Round down and you get 21. Apply Note 7 and you get the 22 that Annex C actually publishes. The chart above applies the rule.
Nipples get 60%
A raceway 24 inches or shorter between two enclosures is a nipple, and NEC Chapter 9, Note 4 permits 60% fill in one. The run is too short for heat to build up and the pull is easy, so the usual derating concerns do not apply. The calculator has a nipple mode for exactly this case.
NEC references
- NEC Chapter 9, Table 1: Percent of Cross Section for Conductor Fill
- NEC Chapter 9, Table 4: Dimensions and Percent Area of Conduit
- NEC Chapter 9, Table 5: Dimensions of Insulated Conductors
- NEC Chapter 9, Note 4: Nipples (24 in. or less) permit 60% fill
- NEC Chapter 9, Note 7: Next higher whole number when the decimal is 0.8 or larger
- NEC Annex C: Conduit and Tubing Fill Tables for same-size conductors
The same calculations, running live as you design.
Solar Design Lab runs this calculator (and a dozen more) live as you build plansets. First planset free, then from $29 per planset.
Conduit fill questions, answered straight.
What is the maximum conduit fill allowed by the NEC?
It depends on how many conductors are in the raceway. NEC Chapter 9, Table 1 allows 40 percent fill for three or more conductors, 31 percent for exactly two, and 53 percent for a single conductor. The limits exist so conductors can be pulled without damage and can shed heat once installed.
How many #12 THHN conductors fit in 3/4 inch EMT?
Sixteen. A 3/4 inch EMT has 0.533 square inches of internal area, 40 percent of that is 0.213 square inches, and a #12 THHN occupies 0.0133 square inches. The quick-reference table on this page covers the other common combinations, and the calculator handles mixed wire sizes.
Do grounding conductors count toward conduit fill?
Yes. Every conductor in the raceway counts, including the equipment grounding conductor, whether it is insulated or bare. This is one of the most common ways a fill calculation comes out wrong: the EGC gets left out and the conduit ends up over-filled on paper and in the field.
What is the 0.8 rounding rule in conduit fill?
NEC Chapter 9, Note 7 permits the next higher whole number of conductors when the calculated result includes a decimal of 0.8 or larger, for conductors all of the same size. It is why Annex C lists 22 #14 THHN in a 3/4 inch EMT even though the strict area math gives 21.98. Tables that ignore the rule report one conductor fewer than the NEC actually allows.
What is a conduit nipple and why does it allow 60 percent fill?
A nipple is a raceway 24 inches or shorter between two enclosures. NEC Chapter 9, Note 4 permits 60 percent fill in a nipple because the run is too short for heat buildup to matter and the pull is trivially easy.
Does this calculator work for solar and PV wire?
Yes. It handles PV wire and USE-2 alongside THHN, XHHW, and the other common insulations, and it counts the equipment grounding conductor for you. Sizing the conduit for a PV circuit is the same fill math as any other circuit; the difference is the insulation types and the ambient conditions those circuits usually run in.
Sizing conduit for a solar circuit? The same fill math runs automatically inside the free single-line editor, alongside the rest of the design checks.