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In this series, we looked at which NEC requirements affect circuit calculations. Notice, we never said the NEC determines what size your conductors should be. It won’t do your engineering for you and explain how to optimally size conductors. All it does is does is provide the framework for determining the minimum size conductors must be.
Today, the larger engineering firms large design houses use software that incorporates the NEC minimums in some way or another and lets the engineer focus on optimal design. For example, the drawing will show a 1200 foot run with a 70A load. The software won’t let the engineer specify a conductor size that’s below NEC minimums. Or if it does allow that, it provides a warning.
But if you don’t have this software, you’ll need to do the calculations twice. First, you do the NEC calculations. Then, using the same data you can decide to increase the conductor size.
For example, you might do further calculations for voltage drop and select a larger conductor that reduces this number so you get the desired efficiency. The NEC does not require you to optimize for efficiency [90.1(B)]. Failure to do this can have a noticeably adverse effect on operating costs. For this and other reasons, doing just enough to “meet code” is irresponsible of the electrical designer.
It’s not just voltage drop you need to account for. You might derate conductors and equipment for expected high harmonic content and other issues.