AmpSizer

Free calculator

Will it all fit in the pipe?

Build your conductor set, pick a conduit, and see the fill against the permitted percentage — plus the smallest trade size that takes the whole run without a fight.

Conductors

What's going in the pipe

3 × 12 AWG
THHN / THWN-2

Raceway

Fill check

13.1 %Within 40%
  • Conductors3
  • Conductor area0.0399 in²
  • Conduit internal area0.304 in²
  • Permitted fill40% (0.1216 in²)
  • Actual fill13.1%

Fill percentages assume a straight run of the same conductor type. Nipples 24 inches and shorter, and raceways with bends or pull points, carry their own rules this calculator does not model.

1/2" EMT capacity table

Source: NFPA 70, National Electrical Code, Chapter 9, Table 1; NFPA 70, National Electrical Code, Chapter 9, Table 4; NFPA 70, National Electrical Code, Chapter 9, Table 5. Published by NFPA.NEC 2023

Why fill is limited at all

Two reasons, and neither is about running out of room. The first is heat: conductors packed tightly cannot shed heat into the surrounding air, which is the same physics behind the bundling adjustment applied to ampacity. The second is mechanical — a crowded raceway is difficult to pull without damaging insulation, and damaged insulation in a metal pipe is a fault waiting to happen.

The three fill percentages

  • 53% for a single conductor — one conductor centred in a pipe has room around it and pulls easily.
  • 31% for exactly two conductors — the worst case for jamming, where two conductors can wedge against the pipe wall on a bend.
  • 40% for more than two — the number most installations use.

The counter-intuitive one is two conductors getting a lower limit than three. That is the jam ratio: three or more conductors bundle and roll through a bend, while exactly two can lock.

What this calculator does not cover

Nipples 24 inches and shorter, which are permitted a much higher fill. Raceways with multiple bends between pull points, where pulling tension rather than fill is the constraint. Conduit bodies, wireways, and cable trays, which have their own rules. Equipment grounding conductors are counted for fill but not for the ampacity bundling adjustment — a distinction worth keeping straight.

Also see the full conduit fill tables and the ampacity lookup, where conductor count drives derating.