Conduit Fill Calculator
Conduit and wire areas to fill percent and verdict.
What it does
Too many wires in a pipe overheat, jam on pulling, and fail inspection. Fill math is one division: copper area over pipe area against a code cap that moves with wire count. Thirty seconds prevents a re-pull.
The tool above takes conduit inside area, wire count, and one wire area, then prints fill percent, the 53/31/40 cap for that count, and a plain verdict. It embeds no tables; look areas up in NEC Chapter 9 for your exact conduit and insulation.
NFPA publishes the fill caps in NEC Chapter 9 notes with areas in Tables 4 and 5, and UL conduit standards back the dimension side. Two wires cap lowest at 31 % because pairs wedge worst; singles breathe at 53 %.
Fill is only half the compliance. Ampacity derating for bundled current-carriers runs separately and bites earlier on long loaded runs. Pass both, pull with lube, and leave room for the next circuit.
How to use
- Enter conduit inside area in sq in.
- Enter whole wire count.
- Enter one wire area in sq in.
- Read fill percent and the wire-count limit.
- Read the within-or-over verdict.
- Look areas up in NEC Chapter 9 tables.
Formula
- conduit area
- Inside cross section of the conduit in sq in.
- wires
- Count of wires, 1 to 50 whole.
- wire area
- Cross section of one wire with insulation.
Max is 53 % for one wire, 31 % for two, 40 % for three or more. Areas come from NEC Chapter 9 tables by conduit type and wire insulation; this tool computes from your entered areas rather than embedding the tables.
Worked example
Example: 4 wires, 0.864 sq in
Four same-size wires in a 0.864 sq in conduit.
- Conduit inside area 0.864 sq in, 4 wires of 0.036 sq in.
- Copper: 4 x 0.036 = 0.144 sq in.
- Fill: 0.144 / 0.864 x 100 = 16.7 %.
- Limit for 4 wires is 40 %.
- Verdict reads Within fill.
Reference table
| Wires | Max fill | Why |
|---|---|---|
| 1 | 53 % | Single pulls freely |
| 2 | 31 % | Pairs wedge worst |
| 3 or more | 40 % | Bundle standard |
Limits
- Fill only. Derating, box fill, and supports are separate.
- Needs correct Table 4/5 areas for the exact types.
- New work math; old jams need eyes on site.
- Not an install permit or sign-off.
Common mistakes
- Using outside diameter for area.
- Mixing insulation types in one area.
- Forgetting the 31 % pair cap.
- Skipping derating on loaded pipes.
- Forcing an overfilled pull.
Try it yourself
Quick checks. Answers sit tucked under each question.
Fill for 2 wires of 0.02 in 0.304 conduit?
Copper is 2 x 0.02 = 0.04. Fill is 0.04 / 0.304 x 100 = 13.2 % against 31 %: Within fill.
Fill for 3 wires of 0.02 in 0.122 conduit?
Copper is 3 x 0.02 = 0.06. Fill is 0.06 / 0.122 x 100 = 49.2 % against 40 %: Overfilled, upsize conduit.
Questions
What are the fill limits?
53 % for one wire, 31 % for two, 40 % for three or more, from NEC Chapter 9. Singles move freely; pairs jam worst, hence the strictest cap.
Where do I find conduit area?
Inside area of the raceway from NEC Chapter 9 Table 4 by trade size and type. Measure or look it up; outside diameter overstates badly.
Where do I find wire area?
Cross section of one wire with insulation from Table 5 by gauge and type. Same gauge differs across THHN, XHHW, and UF, so match the letters.
Can I mix gauges in one pipe?
Yes, when jackets match the fill math and all wires share the rating. Mixed gauges sum fine: add each area, then divide by conduit. Keep high-voltage separation rules apart from fill.
Does fill cover derating too?
No. Derating (ampacity adjustment for 4-plus current-carriers) is separate math in 310.15. A pipe can pass fill yet fail derating, so run both.
What if the verdict reads overfilled?
Jammed pulls strip insulation and fail inspection. Upsize the pipe, split the run, or drop a circuit before forcing it.