How it works
Fill % = (n × wire area) / conduit area × 100. Limit: 1 wire — 53%, 2 wires — 31%, 3+ wires — 40%
n is the number of conductors, wire area comes from NEC Chapter 9, Table 5 (THHN/THWN-2), and the conduit internal area from Table 4 for the given type and trade size. The percentage limit is set by Table 1. When computing the maximum count, Note 7 allows rounding up if the decimal is 0.8 or greater — that is why 26 × 12 AWG THHN fit in 1" EMT even though the raw math gives 25.98. On long runs, size the conductors for voltage drop on the circuit first — an upsized wire takes more of the conduit area.
Code references
- Percent fill limits NEC 2023, Chapter 9, Table 1 (and Note 7 on rounding)
- Conduit internal areas NEC 2023, Chapter 9, Table 4 — EMT, PVC Sch 40 / Sch 80
- Conductor dimensions NEC 2023, Chapter 9, Table 5 — THHN/THWN-2
- Pre-computed tables NEC 2023, Informative Annex C, Tables C.1, C.11, C.12
FAQ
Why is the limit 40% and not 100%?
Wires need free space to be pulled in without scraping the insulation and to shed heat in service. NEC Chapter 9, Table 1 caps fill at 53% for a single conductor, 31% for two and 40% for three or more — a single wire can hug the wall, while several wires cross and bind.
Do equipment grounds and neutrals count?
Yes. Every conductor inside the raceway occupies area, so count all of them — hots, neutrals and equipment grounding conductors alike.
What about mixed wire sizes in one conduit?
This calculator assumes all conductors are the same size. For a mix, add up the individual conductor areas from Table 5 and compare the total against the allowed fill area shown here — the limit itself does not change.
Is there an exception for short conduit nipples?
Yes. A raceway not longer than 24 inches between enclosures may be filled to 60% (Chapter 9, Note 4), and the conductors in it are not counted for ampacity derating.
Does conduit fill affect ampacity?
Indirectly. Fill itself is a mechanical limit, but once more than three current-carrying conductors share a raceway, NEC 310.15(C) requires ampacity derating — so a heavily packed conduit usually triggers both checks.
This calculator is provided for estimation purposes. Always verify results against the current NEC edition and local amendments with a licensed electrician or electrical engineer before pulling conductors.