Several NEC sections contain the very important requirement to size conductors and overcurrent devices at 100% of the noncontinuous loads plus 125% of the continuous loads. This takes into account heat buildup resulting from the current flowing through the conductors and overcurrent devices for an extended period of time.
These requirements are found in NEC 210.19(A)(1) for branch circuits, 210.20(A) for branch circuit overcurrent devices, 215.2(A)(1) for feeder conductors, 215.3 for a feeder overcurrent protection, 230.42(A)(1) for service conductors, 409.20 and 409.21 for industrial control panels, and 625.21 for electric vehicle charging system overcurrent protection.
It is not necessary to apply the 125% factor for grounded (often neutral) conductors that are not connected to an overcurrent device, NEC 210.19(A) (1) Exception No. 2, 215.2(A)(1) Exception No. 2, and 230.42(A)(2).
The logic of this Code exception is that the heat developed in the neutral conductor
will not contribute to the possible nuisance tripping of a circuit breaker or the opening of a fuse because
the neutral conductor is terminated on a neutral bus, not to the terminal of an overcurrent device.
A second exception provides that if both the overcurrent device and its assembly are listed for operation at 100% of their rating, it is not necessary to increase the ampacity of the conductors and overcurrent device by 25%. Be very cautious with this exception.
Most electrical equipment in the 600 volt class is not rated for continuous operation at 100% of its rating.
Store lighting is an example of continuous loads, whereas receptacle outlets typically are not considered
continuous loads.
There are two ways to compensate for continuous loads. One way is to apply a 125% factor to the
continuous load plus 100% of the noncontinuous load, and this becomes the minimum rating of the
conductor and the overcurrent device.
The second method is to limit the continuous load on the circuit to not more than 80% of the rating of the overcurrent device and the conductor.
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