Conduit Fill Calculator
Check a raceway against NEC Chapter 9 fill limits before pull day — mixed conductor sizes, any conduit type, straight to a pass/fail and a recommended trade size.
Based on NEC Chapter 9, Table 1 fill limits and Table 5 conductor areas for THHN/THWN-2. Does not account for conduit bodies, sweeps under 180°, or jacketed/multi-conductor cable — confirm with a licensed electrician for final raceway design.
How the fill percentage is set
Each conductor's cross-sectional area (jacket included) comes from NEC Table 5. Those areas are summed and divided by the conduit's internal cross-sectional area from Table 4. NEC caps how much of that area conductors are allowed to occupy — the max allowed drops as conductor count goes up, since more conductors need more room to pull without damaging insulation.
| Conductors in Raceway | Max Fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
Per NEC Chapter 9, Table 1. This is a hard code limit, not a recommendation — exceeding it is a pull-day problem (or an inspection failure) either way.
Common questions
Why does the max fill percentage change with 1, 2, or 3+ conductors?
With only one conductor, there's no bundling friction, so NEC allows up to 53% fill. Two conductors get the tightest limit — 31% — because they tend to lie side-by-side and bind against each other during a pull. Three or more conductors settle into a more circular bundle, which is why the limit relaxes slightly to 40%.
Does conduit fill affect voltage drop?
Not directly, but they're solved together on the same run in practice — fill percentage governs how much heat a bundle of conductors can shed (affecting ampacity derating), while voltage drop is a function of conductor length, size, and load current. Check both on any long industrial run — see our companion voltage drop calculator below.
Does this account for conduit bends and pull points?
No. This estimate assumes a straight or lightly-bent run within normal NEC bend radius allowances. Runs with multiple 90° bends, long offsets, or several pull points may need a larger conduit than the fill math alone suggests, purely for pullability.
Is PVC Schedule 80 always a tighter fit than Schedule 40?
Yes, at the same trade size — Schedule 80 has thicker walls for higher pressure/impact ratings, which reduces the internal area available for conductors. If a Schedule 40 run is already close to its fill limit, switching to Schedule 80 conduit of the same trade size will very likely push it over.
Electrical & Instrumentation Tools
Raceway design is easy on paper. It's the pull that finds the mistakes.
PLC's Electrical & Instrumentation team designs and installs raceway systems, control panels, and power distribution for oil & gas, midstream, and industrial facilities across Texas, the Permian Basin, and Colorado — sized right the first time.
- Raceway and conduit design reviewed by licensed engineers
- In-house control panel fabrication and conductor termination
- Field-proven sizing for long industrial and midstream runs