Conduit Fill Calculator

Mixed THHN or XHHW fills in EMT, IMC, RMC, or PVC — checked against the NEC Chapter 9 limits: 53% for one conductor, 31% for two, 40% for three or more.

Calculator

Check a raceway fill

Conduit type

Conduit size (trade)

Insulation type

Number of cable types

One row per cable type · same insulation for all

Cable typeSize (AWG)Qty

Areas per NEC Chapter 9 Tables 4 and 5 (2023 basis). Compact-stranded conductors are smaller than tabulated. Ampacity adjustment (310.15) is a separate check. Nipples 24" or shorter are permitted 60% fill (Table 1) — not modeled here. Quick-reference only — a licensed engineer dictates final values and confirms against the adopted NEC.

How it works

The tables behind the result

Fill limits (Ch.9, Table 1)

1 conductor ≤ 53% · 2 conductors ≤ 31% · 3+ conductors ≤ 40% · nipples ≤ 24": 60%

Conductor areas

NEC Chapter 9, Table 5 — area in in² per conductor for the insulation type. Copper and aluminum share the same tabulated area at a given size and insulation.

Raceway areas

NEC Chapter 9, Table 4 — total cross-sectional area (100%) for the raceway type and trade size.

Formula

fill % = (sum of conductor areas ÷ conduit area) × 100

Not checked here

Ampacity adjustment for >3 current-carrying conductors (310.15), derating for ambient temperature, and pulling-tension practicality

FAQ

Common questions

What are the NEC conduit fill limits?

NEC Chapter 9, Table 1: 53% fill for one conductor, 31% for two, and 40% for three or more conductors. Nipples 24" or shorter between boxes or enclosures are permitted 60%.

How is conduit fill calculated?

Add the approximate cross-sectional area of each conductor (NEC Chapter 9, Table 5, for the insulation type) and divide by the total cross-sectional area of the conduit (Table 4, for the raceway type and trade size). This calculator covers THHN/THWN-2 and XHHW/XHHW-2 in EMT, IMC, RMC, PVC Schedule 40, and PVC Schedule 80. Areas are insulation-specific — copper and aluminum of the same size and insulation have the same fill area.

Does conductor size mean the same thing for compact stranding?

No — compact-stranded conductors have a smaller overall diameter than the Table 5 values. If your conductors are compact stranded, the areas here are conservative; verify with the manufacturer’s data.

What about derating for more than three current-carrying conductors?

Fill and ampacity are separate checks. More than three current-carrying conductors in a single raceway triggers the NEC 310.15(C)(1) adjustment factors — this calculator does not check ampacity.

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