How it works
Fill % = (Σ wire area) / conduit area × 100. Limit: 1 wire — 53%, 2 wires — 31%, 3+ wires — 40%, ≤24″ nipple — 60%
Σ wire area sums the cross-section of every conductor (Table 5), the conduit internal area comes from Table 4 for the given type and trade size, and the percentage limit is set by Table 1. For a single gauge that is just n × wire area; for a mixed set each gauge is summed separately. The limit drops to 60% for a nipple ≤ 24 in between enclosures (Note 4), and rises through 53%/31%/40% by conductor count otherwise. 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 + nipple 60% exception NEC 2023, Chapter 9, Table 1 (Notes 4 and 7)
- Conduit internal areas NEC 2023, Chapter 9, Table 4 — EMT (Art. 358), IMC (Art. 342), RMC (Art. 344), PVC Sch 40 / 80
- Conductor dimensions NEC 2023, Chapter 9, Table 5 — THHN/THWN-2, XHHW/XHHW-2, RHH/RHW-2
- More than three current-carrying conductors NEC 2023, 310.15(C)(1) — ampacity adjustment factors
- Pre-computed fill tables NEC 2023, Informative Annex C, Tables C.1, C.4, C.8, C.10, 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 for conduit fill?
Yes. Every conductor inside the raceway occupies area, so count all of them for fill — hots, neutrals and equipment grounding conductors alike.
Do grounds count for ampacity derating?
No. The equipment grounding conductor does not count as a current-carrying conductor for derating under NEC 310.15(C)(1). A true neutral on a 3-phase 4-wire wye that carries only unbalance current may not count either — but on a 2-wire or most multiwire branch circuits the neutral does count. Enter only hots and counted neutrals in the "current-carrying conductors" field; the equipment ground is for fill, not derating.
What about mixed wire sizes in one conduit?
Switch the conductor selector to "Mixed sizes" and add a row per gauge — for example 3 × #4 hots, 1 × #4 neutral and 1 × #8 ground. The calculator sums the individual areas from Table 5 and compares the total against the allowed fill area. The percentage 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, Table 1, Note 4), and the conductors in it are not counted for ampacity derating. Toggle "Short nipple" on and the limit, allowed area and verdict update live.
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)(1) requires ampacity derating — so a heavily packed conduit usually triggers both checks. This tool warns you when that threshold is crossed; size the derated conductor with the [wire ampacity chart](/electrical/wire-ampacity-chart/).
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.