Conduit Fill Calculator
Checks a conductor group against Chapter 9's fill-percentage limits — 53% for a single conductor, 31% for two, 40% for three or more — using Table 5 THHN/THWN-2 conductor areas.
What it calculates
- Total conductor cross-sectional area for the given conductor sizes and count
- The applicable Table 1 fill percentage for that conductor count
- The minimum trade size conduit that keeps the fill under that percentage
Typical inputs
- Conductor sizes and quantity
- Insulation type (THHN/THWN-2 areas)
- Conduit type, if comparing trade sizes across types
Frequently asked
Why does the fill percentage change with conductor count?
Chapter 9 Table 1 sets tighter limits as conductor count increases specifically to leave room for heat dissipation and pulling clearance — a 2-conductor raceway is allowed 31% fill, while 3 or more drops to 40% only because Table 1 itself treats 1 and 2 conductors as special cases, not because more conductors get MORE room.
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