The NEC allows a nipple to be filled to 60% of the cross sectional area. If
all the conductors are the same size, and small, this can be a problem. Just
jamming in more conductors will exceed the 60% limitation for sure. But if the
conductors are large, the nipple fill allowance is usually more than will
comfortably fit for any given raceway. An increase in size might be sensible.
Since a nipple can not be longer than 24 inches, there is often not a problem
unless an offset or 90 degree bend is included in the 24 inches.
The calculation for nipple fill is basically one of addition. Add the square
inches of each conductor. Determine what 60% of the raceway in area is for the
type you will use. Then subtract the conductor area from the 60% value. If there
is space remaining, then the conductors will fit. If you get a number for the
conductors which is larger than the 60% number, a larger conduit or fewer
conductors will be needed.
There was a time in the NEC history when the type raceway did not matter.
All calculations were based on trade size, period. No more. Now each raceway
type has a separate table.
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