Conductor sizing

[mike.oquinn]

Conductor installation conditions outside the limits/conditions listed in the ampacity tables 310.16 through 310.19 in the NEC?, such as higher ambient temperature, more than 3 current-carrying conductors, installing conductors in conduits on rooftops, require adding all derating factors together to determine the ampacity of the conductor in this condition: additional heat outside the conductor will require lowering the ampacity of the conductor to ensure the conductor insulation will not be damaged, and all additional heat source must be addressed.

But what about voltage drop ? the heating effect causing effective voltage limitation because of excessive conductor length. Is this derating additive to the other derating factors, or exclusive? In other words, if a poorly designed system has all possible derating factors to consider - high ambient temperature, more than 3 current-carrying conductors, installing conductors in conduits on rooftops, as well as long conductor runs ? does voltage drop derating also add to a larger conductor size, or just ensure a minimum conductor size as a starting point as required in 110.14(C)(1)? If voltage drop derating is additive, how is this done?

Also, if temperature derating increases the minimum conductor ampacity, requiring for instance a jump fro 52A to 156A, what do we use for voltage drop calculations - 52A or 156A?

 

[don_resqcapt19]

Voltage drop calculations are based on the actual load and the wire size used for the circuit. It has nothing to do with the code required derating.

 

[charlie b]

I agree with Don. I will add that if you have had to use a larger conductor because of one of the three factors you named, then there is a good chance that that selection will have solved any possible voltage drop issues you might have had with the smaller conductors.