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NEC 310.15(A)(2) Exception, 10% or 10 feet or less for sizing conductors


Q:

I don't quite understand the Week 1, Assignments 2, Code-Compliant Conductor Sizing - Part 1 when you talk about the "10 feet or 10%" . I wonder if you can explain it more or give me an example.
Thanks!

A:

To put this in context, 310.14(A)(2) tells us:

When more than one ampacity applies to a given circuit length, the lowest value shall be used.

Then the NEC 310.15(A)(2) Exception states:

Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 10 feet or 10% of the circuit length figured at the higher ampacity, whichever is greater.

What this is telling us is that if there are different ampacities applied to different parts of a circuit, which could be the circuit is installed under different conditions of use throughout the length of the circuit.

Example: 

A circuit is on a rooftop in sunlight and then goes in a cool building. If the entire circuit was a total of 50 feet and only 4 feet was in sunlight on the roof, then we can design the circuit with the ampacity of the conditions inside the building.

If that same circuit had 6 feet outside in sunlight out of 50 feet, then more than 10% of the circuit would have an ampacity that is higher in the sunlight, then we would have to size the entire circuit conductor for the outside conditions.

The reason that we can have short runs outside in the 4 foot example, is that the heat from the wire outside can dissipate inside of the house where it is cooler. The wire can act as a heat sink.

Also, to be clear about the 10% or 10 feet whatever is less part. If the wire in the hot place is ever over 10 feet, then we will have to oversize the wire in the cool place. What ends up happening is that any circuit that is over 100ft, we are going to have to have 10 feet or less wire in the hot place in order to apply this exception. If our circuit is less than 100ft, the we are going to have to limit our run in the hotter place to 10% of the circuit length, in order to apply the 310.15(A)(2) exception.

Thanks,
Sean White


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