What is the 40% conduit fill rule?

The 40 percent conduit fill rule says that when a conduit carries three or more wires, their combined cross-sectional area may take up no more than 40 percent of the conduit's interior area. It comes from NEC Chapter 9, Table 1.

Fewer wires get different limits: one wire may fill 53 percent, two wires 31 percent. The lower two-wire number exists because a pair tends to jam against the walls when pulled.

The rule protects the pull and the insulation. Overfilled conduit means scraped insulation, stuck pulls, and trapped heat that derates every conductor inside.

There is no 60 percent rule in the NEC; that figure usually comes from nipple allowances, which permit 60 percent only on runs of 24 inches or less.

The NEC conduit fill calculator applies the right percentage automatically and cites the table it used.