Does the equipment grounding conductor count toward fill?

Yes. It occupies space in the raceway, so it counts toward the fill calculation.

Does the 40% limit change with conductor size?

No. The percentage is fixed by conductor count; the size determines how many actually fit.

What about conduit nipples?

Short raceways between enclosures have a specific allowance that differs from the general rule. That case is not covered by this calculator.

Conduit Fill Percentages: 53%, 31% and 40%

The allowable fill depends only on how many conductors are in the raceway: 53% for one, 31% for two, and 40% for three or more.

The three allowances

The limits are set so conductors can be pulled without damage and heat can dissipate.

One conductor: up to 53% of the raceway internal area.
Two conductors: up to 31%.
Three or more: up to 40%.

Why one conductor gets the most room

A single conductor needs the most clearance for pulling and for heat to escape; two conductors jam against each other and need proportionally more free space. Once you reach three, the practical limit settles at 40%.

Doing the arithmetic

Add up the cross-sectional area of every conductor, divide by the internal area of the raceway, and compare with the allowance for that conductor count.

Remember that the count that matters is the number of conductors you are actually installing, including any bare or insulated equipment grounding conductor. The calculator does this and also tells you the maximum count that fits.

Frequently asked

Does the equipment grounding conductor count toward fill?: Yes. It occupies space in the raceway, so it counts toward the fill calculation.
Does the 40% limit change with conductor size?: No. The percentage is fixed by conductor count; the size determines how many actually fit.
What about conduit nipples?: Short raceways between enclosures have a specific allowance that differs from the general rule. That case is not covered by this calculator.

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Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.