Conduit Fill Calculator

NEC 40% fill (3+ wires). Find conduit size that holds N wires of given gauge.

Result

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How to use this calculator

Enter wire count + gauge.
Read smallest conduit that meets 40% fill.

About this calculator

NEC chapter 9 table 1: when 3+ wires share conduit, total wire area cannot exceed 40% of conduit cross-section. Why: heat dissipation, pulling without damaging insulation. THHN wire areas (cross section): 14 AWG 0.0097 in², 12 AWG 0.0133 in², 10 AWG 0.0211 in². EMT conduit ½" allows 0.122 in² fill; ¾" allows 0.213; 1" allows 0.346.

Frequently asked

Why 40% fill?+

NEC 9-1: for 3+ wires. 1 wire allows 53% fill, 2 wires allow 31%. Heat + pulling friction set the limit.

THHN vs THWN?+

Almost identical insulation areas. Use THHN-specific table (most common). Slight differences for XHHW.

Bonding/grounding wire counts?+

Small equipment grounding conductors don't count toward fill. Larger ones do — check NEC 250.

EMT vs PVC vs flex?+

Slightly different inner diameters. Most calculators (this one too) approximate from EMT. Verify with NEC for exact.

Bend penalty?+

Each 90° bend reduces effective fill capacity. Many electricians limit to 360° total in a single run.

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