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

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 is allowed the highest percentage

A single conductor is the easiest to pull, so Chapter 9, Table 1 allows the highest fill, 53%. Two conductors are the jammed case and are held to 31%. Once there are three or more, the allowance settles at 40%. The single-conductor percentage is higher because it needs the least spare room, not the most.

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.

Related calculators

• Conduit Fill Chart
• Conduit Fill Calculator
• Quick reference (PDF)