Conduit Fill Calculator
Enter the number and size of THHN/THWN-2 conductors (mixed sizes allowed) to see the fill percentage in EMT, PVC Schedule 40 or PVC Schedule 80, whether it passes the NEC 53/31/40% rule, and the smallest conduit size that works.
Conduit fill calculator
Conductors (THHN / THWN-2, including grounds):
| Size | Quantity |
|---|---|
Conduit sizes
NEC conduit fill rules
NEC Chapter 9, Table 1 limits how much of a conduit's cross-section the conductors may fill: 53% for one conductor, 31% for two, and 40% for three or more. Add up each conductor's area from Chapter 9, Table 5, and compare it with the conduit's allowed area from Table 4. Equipment grounding conductors count.
Example: six 12 AWG THHN (0.0133 in² each) and one 10 AWG ground (0.0211) total 0.1009 in². ¾" EMT allows 40% × 0.533 = 0.213 in², so it fits at 18.9% fill.
- Nipples of 24 inches or less may be filled to 60%.
- Fill isn't the only limit: more than three current-carrying conductors require ampacity adjustment (NEC 310.15(C)(1)), and pulling long runs or many bends may call for a larger conduit.
- Areas here are from the 2023 NEC Chapter 9 Tables 4 and 5 for EMT, PVC Schedule 40 and 80, and THHN/THWN-2 conductors. Confirm with your code book and your inspector (AHJ).
Common questions
What is the 40% conduit fill rule?
With three or more conductors, their total cross-sectional area can't exceed 40% of the conduit's internal area (NEC Chapter 9, Table 1).
How many 12 AWG wires fit in 1/2-inch EMT?
9 THHN conductors: 0.122 in² allowed ÷ 0.0133 in² each = 9.2.
How many 12 AWG wires fit in 3/4-inch EMT?
16 THHN: 0.213 ÷ 0.0133 = 16.0.
Do ground wires count toward conduit fill?
Yes, equipment grounding and bonding conductors count toward fill.
Why is the limit 31% for two wires?
With two conductors there's a higher risk of jamming during the pull, so the NEC allows less fill than for one or three or more.
Sources
- NFPA 70, National Electrical Code (2023), Chapter 9 Tables 1, 4 and 5
- Steel Tube Institute: NEC conduit and EMT fill tables
Related
General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.