Continuous Load Correction of Ampacity
Where a conductor is selected to provide power to a load which is
continuous, expected to continue for three hours or more, then the conductor
table ampacity must be corrected (reduced) to determine the correct ampacity for
that specific use.
The table value ampacity is multiplied by 80% to find corrected ampacity for
a continuous load.
There are many loads which are considered continuous. Here is a partial
list:
all motors except those which can not operate for three
hours by design
parking lot lighting
store lighting
office lighting
signs
Other loads specifically mentioned in the NEC which are considered
continuous for conductor selection are:
electric heating
refrigeration equipment
air conditioning compressor motor
water heater
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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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