CalcPanel

Conduit Fill Calculator

Check mixed THHN conductors against NEC Chapter 9 fill limits and the smallest trade size that passes. Use it after cable size, breaker size, and voltage drop.

Fill inputs

About this calculator

Fill percent is total THHN area divided by the raceway internal area. Limits are 53%, 31%, and 40%. Continue on the protection calculators hub.

Result

Planning estimate. Confirm the raceway area and insulation before you order conduit.

Assumptions: THHN conductor areas and standard EMT, PVC Schedule 40, or RMC internal areas. Jam ratio and the 24-inch nipple exception are not checked. This sizing screen does not use power factor.

Continue Your Calculation

If fill fails, go back to cable size and reduce the conductor count or upsize the raceway.

Resize cable →

Chapter 9 fill limits

ConductorsMaximum fill
153%
231%
3 or more40%

Formula

Fill % = (sum of conductor areas ÷ internal raceway area) × 100. Example: three 10 AWG THHN in 3/4 EMT is 3 × 0.0211 / 0.533 × 100 ≈ 11.9%, under the 40% limit.

Worked example

Three 10 AWG THHN in 3/4 in EMT: about 11.9% fill, Pass, and 3/4 in is a passing trade size.

FAQ

What NEC fill limits does this use?

53% for one conductor, 31% for two, and 40% for three or more.

Can I mix wire sizes?

Yes. Use the three groups. The fill uses the sum of the areas.

Does this check jam ratio?

No. Jam ratio and the 24-inch nipple 60% exception stay Not checked.