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Conduit Fill Calculator

A conduit fill calculator tells you whether your wires fit a conduit within the NEC limit, and how many more you could add. Pick the EMT size, the THHN wire gauge and how many conductors, and it returns the fill percentage against the allowed limit.

Your measurements

Result

Overfilled conduit is hard to pull and traps heat, so the code caps fill at 40% for three or more wires. This checks it instantly and shows the maximum that fits.

How it is calculated

Fill % = (wire area × number of conductors) ÷ conduit internal area. NEC limits are 53% (1 wire), 31% (2) and 40% (3+). Areas are from NEC Chapter 9 tables for EMT and THHN; rigid/PVC conduit and other insulations have different areas.

Steps

Choose the EMT conduit trade size.
Choose the THHN wire gauge.
Enter the number of conductors going in.
Read the fill percentage versus the NEC limit.
See the maximum number of that wire the conduit can take.

Worked example

Three 12 AWG THHN conductors in 1/2 in EMT fill it about 13% — well within the 40% limit (it would take up to 9).

Common questions

What is the conduit fill limit?

40% for three or more conductors, 31% for two, and 53% for a single conductor, per the NEC.

Why limit conduit fill?

So wires can be pulled without damage and so heat can dissipate. Overfilling risks both.

Does wire insulation type matter?

Yes — THHN is compact; THWN-2, XHHW and others have different areas. This uses THHN; check the table for other types.

What about the ground wire?

Count every conductor including the equipment ground when figuring fill.