Conduit Fill Calculator
Conduit (raceway) fill is the fraction of a conduit's internal cross-sectional area taken up by the conductors inside it.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the conduit's internal cross-sectional area, the number of conductors, and the combined cross-sectional area of all conductors — use the SAME units (mm² or in²) for both areas.
- Read the actual fill %, the NEC maximum allowed fill %, and whether the pull is within the limit.
- If it shows 'Over limit', increase the conduit size or reduce the number/size of conductors, then recalculate.
How it works
Conduit (raceway) fill is the fraction of a conduit's internal cross-sectional area taken up by the conductors inside it. The tool computes the actual fill as fill% = (total conductor area ÷ conduit internal area) × 100, then compares it to the code limit.
Under NEC Chapter 9, Table 1 the maximum allowable fill depends on how many conductors are in the raceway: 53% for a single conductor, 31% for exactly two conductors, and 40% for three or more. The allowed conductor area is conduit area × max %, and 'spare area to limit' is that allowed area minus the conductors' area. Fill limits exist to leave room for heat dissipation and to allow the wires to be pulled without damaging the insulation.
Worked example
Three THHN conductors in trade-size 3/4" EMT. A 3/4" EMT conduit has an internal cross-sectional area of about 222 mm². You are pulling 3 conductors with a combined cross-sectional area of 78 mm². With 3 conductors the NEC allowable fill is 40%, so the allowed area is 222 × 40% = 88.8 mm². The actual fill is 78 ÷ 222 × 100 = 35.14%, giving a spare area of 10.8 mm². Result: Within limit — the three conductors fit.
Common mistakes
- Mixing units — entering the conduit area in in² but conductor area in mm² (or vice versa). Both areas must be in the same units.
- Using the conduit's trade size or nominal diameter instead of its true internal cross-sectional area. Look up the actual internal area from the manufacturer or the NEC conduit dimension tables.
- Applying the wrong fill percentage — remember 2 conductors is 31% (not 40%); the 40% figure only applies to three or more conductors.
Frequently asked questions
What conduit fill percentage does the NEC allow?
NEC Chapter 9, Table 1 sets the maximum fill at 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. This tool selects the correct percentage automatically from the conductor count you enter.
Where do I find the conduit's internal area and each conductor's area?
Conduit internal cross-sectional areas are listed in NEC Chapter 9, Table 4 (by conduit type and trade size), and individual conductor areas are in Chapter 9, Table 5 (by insulation type and wire size). Add up the areas of all conductors for the total, and use the same units for both the conduit and conductor areas.
Does this replace a licensed electrician or the code book?
No. This calculator is an estimating aid. Conduit fill, derating, bend counts and other requirements vary by jurisdiction and installation. Always verify against the current edition of your local electrical code and have a licensed professional confirm the design.
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