New to this? Plain-English guide

Conduit — the pipe that protects electrical wires (PVC or metal), sized in "trade sizes" like 1/2", 3/4", 1".

Fill Percentage — how much of the pipe's cross-section the wires occupy. Code caps it (40% for 3+ wires) so cables pull through easily and don't overheat.

Wire Gauge (AWG) — the wire's thickness. Smaller AWG numbers are thicker wires (8 AWG is thicker than 14 AWG); thicker wires need more conduit space.

Conduit

Conduit Type

Conduit Size (trade)

Conductors (THHN/THWN-2)

Wire Group 1 Size

Quantity

Wire Group 2 Size

Quantity

Wire Group 3 Size

Quantity

Wire Group 4 Size

Quantity

Method: NEC Chapter 9, Table 1 fill limits — 1 conductor: 53%, 2 conductors: 31%, 3 or more: 40% of internal area. Conductor areas from Table 5 (THHN/THWN-2), conduit areas from Table 4.

Guide

Why Fill Limits Exist

Overfilled conduit traps heat (conductors derate), makes pulls damage insulation, and violates NEC Chapter 9 Table 1. The limits — 53% for one conductor, 31% for two, 40% for three or more — leave room for heat dissipation and safe pulling tension.

Derating Reminder

Fill is only half the story: more than three current-carrying conductors in a raceway requires ampacity derating per NEC 310.15(C)(1) — 4-6 conductors to 80%, 7-9 to 70%, 10-20 to 50%. Neutrals carrying only unbalanced current and grounds do not count.

Practical Tips

The 40% limit is a maximum, not a target — long runs and runs with multiple bends pull far easier at 25-30% fill. If the calculator puts you right at the limit, go up one size. Conduit is cheap; a failed pull is not.

Results

Total Conductors

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Total Conductor Area

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Allowed Fill

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Actual Fill

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Minimum Conduit Size (this type)

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Conduit Fill Calculator

Guide

Why Fill Limits Exist

Derating Reminder

Practical Tips

Results

Conduit Fill Calculator Report

Summary