Conduit fill: forty percent of the pipe is the most you may fill

Wires need room to be pulled and to shed heat, so with three or more conductors in a conduit their total cross-section may fill no more than 40 percent of the conduit's inside area; two conductors get 31 percent and one gets 53 percent.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

Six wires in a half-inch pipe pull fine on the bench and jam at the second bend on site, and the insulation scraped off in the jam is a fault that appears a year later. The inspector counts the wires in the pipe; so should you, before it goes in.

How to think about it

Get the conduit's total inside area from the table for its type and size, take 40 percent, and divide by the area of one wire from the wire table. Round down: a fraction of a wire is a wire too many. Or read the number straight from the ready-made tables, which do this arithmetic for you.

Worked example

3/4 inch EMT: inside area 0.533 sq in

From the conduit table. EMT, IMC, PVC and flex all have their own rows.

Usable: 0.533 × 0.40 = 0.213 sq in

Three or more wires, so 40 percent.

10 AWG THHN: 0.0211 sq in each

From the wire table. THHN is thinner than THW for the same copper.

0.213 / 0.0211 = 10.1, so 10 wires

Round down. Ten 10 AWG THHN in 3/4 inch EMT, which is what the ready-made table also says.

Your turn

1-1/4 inch EMT has 1.496 sq in inside. Write the usable area for three or more wires.

1.496 × 0.40 =  sq in

The trap

Counting the ground wire as free. An equipment grounding conductor takes space like any other wire and counts toward the fill. A bare wire is a little smaller than an insulated one, and it still counts.

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