Conductor Sizing

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wdingman

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Hi,

I'm sure that this has been overdone, but any clarification is appreciated. To size conductors, I assume that three calculations/factors need to be considered:
  1. The temperature rating of the terminals cannot be exceeded (110.14(C)).
  2. The nature of the load must be considered; that is, continuous vs. non-continuous (210.19(A)(1)(a)).
  3. The conditions of use (ambient temperature adjustment and correction factors for the number of current-carrying conductors---viz. what used to be called derating) (210.19(A)(1)(b))
If I understand the process correctly, these are independent calculations that could result is different wire sizes. If the result is different wire sizes, the largest wire size must be used. Is this correct?

Also, if I understand this correctly, the 125% multiplier does not apply to numbers 1 and 3, but only to number 2. Is this correct?

Am I missing any other major consideration?
 
wdingman said:
I get that, but that's not my current question.
Click to expand...
That's a major consideration in conductor sizing.

For example, let's say you have a 100A continuous load. Terminations are 75c.
100A * 1.25 = 125A. #1awg has a 75C ampacity of 130, so it meets the minimum size for continuous load. The OCPD is also size at 125A for continuous load.

The conditions of use give a 0.728 derate. the #1 THHN has an ampacity of 145A*0.728=106A.

The #1 awg conductor would meet your conditions of 1, 2 and 3...however, they would not be properly protected byt the 125A c/b because 110A is the next standard size up.
 
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