Teaching electricians how to size conductors for continuous loads is a challenge. Especially since I have to relearn the topic myself prior to each class.
Mike Holt, Fredrick Hartwell, and others use a four step method to size the OCPD and the conductor.
I have a three step method.
Step 1
OCPD = continuous load x 125% (next higher if applicable)
Step 2
Conductor size = continuous load x 125% based on the column in Table 310.16 that coincides with the temperature rating of the terminals, CB, etc. (Even if the temperature rating of the conductor exceeds the temperature rating of the terminal)
If there are no adjustment or correction factors to contend with, the problem is solved. Get out of Dodge.
Step 3
If there are adjustment factors, correction factors, or both, do this.
Find the next lower OCPD - from the one determined in Step 1 - and add 1 amp.
Divide this value by the product of adjustment factor x correction factor.
Scroll down the column in Table 310.16, that coincides with the temperature rating of the conductor being used, until you arrive at a value that is equal to, or greater than, the value determined by the above calculation.
That's it!
Mike Holt, Fredrick Hartwell, and others use a four step method to size the OCPD and the conductor.
I have a three step method.
Step 1
OCPD = continuous load x 125% (next higher if applicable)
Step 2
Conductor size = continuous load x 125% based on the column in Table 310.16 that coincides with the temperature rating of the terminals, CB, etc. (Even if the temperature rating of the conductor exceeds the temperature rating of the terminal)
If there are no adjustment or correction factors to contend with, the problem is solved. Get out of Dodge.
Step 3
If there are adjustment factors, correction factors, or both, do this.
Find the next lower OCPD - from the one determined in Step 1 - and add 1 amp.
Divide this value by the product of adjustment factor x correction factor.
Scroll down the column in Table 310.16, that coincides with the temperature rating of the conductor being used, until you arrive at a value that is equal to, or greater than, the value determined by the above calculation.
That's it!
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