// EMT, IMC, RMC, PVC 40 & 80
Conduit Fill Calculator
Pick the raceway and the conductors. You get the fill percentage, whether it passes, and the smallest trade size that would take the same bundle.
Fill is a cross-sectional area problem, not a conductor count. The published “maximum number of conductors” charts only work when every conductor in the pipe is the same size and the same insulation. Mix sizes and you have to do the area math.
If you want the reasoning behind the numbers worked all the way through, read the conduit fill chart.
How this calculation works
Three tables in Chapter 9 do the work. Table 1 sets the fill percentage. Table 4 gives the interior area of each raceway. Table 5 gives the area of each insulated conductor.
The 31% figure for exactly two conductors looks like a typo and is not. Two round conductors in a round pipe leave a lot of unusable space in the middle where neither can go, and the tighter limit accounts for the fact that pulling two is harder per unit of area than pulling three.
Fill is not the only limit
Passing the percentage does not mean the pull will work. Three other things govern:
- 360 degrees of bend between pull points. Four quarter bends and you are done. This limits more runs in practice than fill ever does.
- Ampacity adjustment. More than three current-carrying conductors in a raceway triggers the adjustment factors in 310.15(C)(1). Four to six conductors costs you 20% of your ampacity. Ten to twenty costs you half.
- Nipples. A raceway 24 inches or shorter between enclosures is permitted 60% fill under Chapter 9 Note 4, with no ampacity adjustment.
Rounding, and the one place it is allowed
Note 7 to the Chapter 9 tables allows you to round up to the next whole conductor when the decimal is 0.8 or higher and all the conductors are the same size. Below 0.8, you round down. That rule only applies to the whole-number conductor count, not to the area math.
This calculator runs THHN/THWN-2 areas, which covers the overwhelming majority of raceway work. XHHW-2 and larger-diameter insulations have their own columns in Table 5 and will take more room — if you are pulling those, check the table directly.
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Frequently asked questions
Why is two conductors only 31% when three is 40%?
Geometry. Two round conductors in a round raceway cannot use the space in the middle, so the usable area is lower per conductor than it is with three or more. It is a real limit derived from pulling tension research, not an error in the table.
Does the ground wire count toward fill?
Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals. What the ground does not count toward is the current-carrying conductor count used for ampacity adjustment.
What is the rule for a short nipple?
A raceway 24 inches or less in length between two enclosures is permitted to be filled to 60% of its total area, per Note 4 to the Chapter 9 tables. The ampacity adjustment factors also do not apply to a nipple.
Do I count neutrals as current-carrying?
It depends. A neutral carrying only the unbalanced current of a multiwire branch circuit is not counted. A neutral on a circuit supplying nonlinear loads — electronic ballasts, LED drivers, VFDs — is counted, because harmonic current loads it up even when the phases are balanced.
// DisclaimerThe National Electrical Code® (NEC®) and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association (NFPA). This calculator is an independent educational tool and is not affiliated with, endorsed by, or sponsored by the NFPA. All code references are paraphrased for educational purposes only and do not reproduce the official text of any code edition. This tool is not a substitute for the official NFPA 70 publication. Electricians, contractors, and students are encouraged to purchase the official National Electrical Code from the NFPA at
nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).