Conduit Fill Correction of Ampacity
Where a conductor is installed in a raceway with other conductors, there may
be a reduction in the table ampacity for a specific conductor based on how many
conductors fill the raceway.
Keep in mind that this correction does not apply to nipples 24 inches long
or shorter. And gutters have separate rules about ampacity correction. To wit if
there are 30 or fewer conductors at any cross section of a gutter, then a
reduction in ampacity is not required.
Back to the raceway calculation. Without regard for insulation type or
conductor size, when there are more than three current carrying conductors in a
raceway, then there must be a reduction in ampacity to accommodate the buildup
of heat that will occur. (See table 310.15B2a)
When there are 4 or more current carrying conductors in a raceway, then
various percentages come into play depending on the number of current carrying
conductors present. See the above NEC reference for the rules which apply when
counting the conductors.
Here are the percentages which apply for various numbers of conductors:
46 80%
79 70%
1020 50%
2130 45%
3140 40%
41 and above 35%
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Note: The NEC presents a small twist to ampacity correction. When a
continuous load is supplied by a conductor installed in a raceway which at any
point in its travel is included with conductors which are subject to ampacity
correction for conduit fill, then only one correction is needed.
So here's the deal. If the correction factor for conduit fill is more
restrictive, producing a lower corrected ampacity than would result from a
continuous load correction, then the correction for fill is made and the
correction for continuous load is not made.
The thought here is that this pair of correction factors are both aimed at
reduction of heat around conductors so only the most severe is needed to correct
for heat.
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