Conduit fill rules

How full a conduit may be is set by Chapter 9 of the NEC. The rules are short; this is all of them that matter for building wire.

The percentages (Table 1)

Two wires get less room than one because two round wires of similar size can jam against each other across the pipe as they're pulled.

What counts

• Every conductor: hots, neutrals, travelers and the equipment ground. A bare ground uses its bare area from Table 8.
• Area is with insulation, from Table 5 (Table 5A for compact conductors).
• A cable in conduit counts as one conductor with the area of its outside size; a flat cable uses its major diameter.

Rounding up (Note 7)

When every wire is the same size, a count that ends in 0.8 or more is rounded up to the next whole number. That is why 3/4" EMT takes 22 #14 THHN even though the math gives 21.98. With mixed sizes, the total area simply has to stay under the limit.

Annex C

Annex C's tables are this math done for you, one table per raceway type, for wires all of the same size. Our fill charts are built the same way from Tables 1, 4 and 5, and match Annex C.

Jam ratio

Three wires of the same size can jam when the conduit's inside diameter is 2.8 to 3.2 times the wire's diameter. It isn't a code limit, but pulling tension and bends make it worse. Change the conduit size or the pull if you're in that range.