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Conduit Fill Calculator

This conduit fill calculator totals THHN/THWN-2 wire area against EMT, PVC, or rigid conduit. It shows fill percent, the 53%, 31%, or 40% limit, pass or fail, and the max count of one wire size. It is a planning estimate.

Conductor group 1

Include equipment grounding conductors. They count toward fill. Each group is a wire size and how many of that size.

How many of this one size fit in the conduit at the percent limit.

Examples

Conduit fill

Pass

1/2 in EMT. 9 conductors.

Total wire area
0.1197 in²
Conduit internal area
0.304 in²
Fill
39.38%
Limit
40% (3 or more conductors)
Allowable area
0.122 in²
Max 12 AWG
9 at the 40% limit

0.1197 in² is within the 0.122 in² allowed at 40%.

Planning estimate from NEC Chapter 9 Table 4 internal areas and Table 5 THHN/THWN-2 areas. Confirm the raceway, the conductors, and the fill with the code edition in force and the local inspector.

How to use

  1. Choose EMT, PVC Schedule 40, PVC Schedule 80, or rigid (RMC), then a trade size from 1/2 inch to 4 inches.
  2. Add each conductor group: a THHN/THWN-2 size and how many, including equipment grounding conductors.
  3. Read total wire area, fill percent, the 53%, 31%, or 40% limit, and pass or fail.
  4. Read the max count of the selected wire size that stays within that percent limit.

Formula

Total wire area is the sum of count × the Chapter 9 Table 5 THHN/THWN-2 area. Fill percent is total wire area ÷ the Table 4 internal area × 100. One conductor may use 53% of that area, two conductors 31%, and three or more 40%. The result passes when total wire area is less than or equal to the printed allowable area for that count. Max count is the largest whole number of one wire size that still passes.

Example. Nine 12 AWG THHN/THWN-2 conductors in 1/2 inch EMT total 0.1197 in². Internal area is 0.304 in², the 40% limit allows 0.122 in², and fill is 39.38%. That passes, and 9 is the maximum at the limit.

Example. Three 4/0 THHN/THWN-2 conductors plus one 4 AWG equipment ground in 2 inch EMT total 1.0535 in². The 40% limit allows 1.342 in², so fill is 31.39% and the run passes.

Planning estimate

The areas are the NEC Chapter 9 Table 4 internal areas and Table 5 THHN/THWN-2 areas for the sizes on this page. Pass/fail uses those percent columns. Informative Annex C can list one more identical conductor when Chapter 9 Note 7 rounds a remainder of 0.8 or more up. Nipples 24 inches or shorter are often allowed up to 60% fill, which this page does not use. Ampacity and derating are separate. Confirm the conduit, the conductors, and the fill with the code edition in force and the local inspector.

Frequently asked questions

What is the conduit fill limit?

For more than two conductors the usual limit is 40% of the conduit's internal area. One conductor is 53% and two conductors is 31%.

How many 12 AWG THHN wires fit in 1/2 inch EMT?

Enter the wire and conduit here to get the count for your exact combination.

Does the ground wire count toward fill?

Yes. Equipment grounding conductors count toward conduit fill.

Are short conduit nipples different?

Yes. Nipples 24 inches or shorter are usually allowed up to 60% fill.

Is this a substitute for code review?

No. It is a planning estimate. Your inspector and the code edition in force have the final say.

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