240.21(C)(6)

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augie47

augie47

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The "ratio" wording always confuses me.
If I have a 75 kva 480 to 208/120 transformer protected by a primary 100 amp breaker, what is my minimum size secondary conductor ?
 
augie47 said:
The "ratio" wording always confuses me.
If I have a 75 kva 480 to 208/120 transformer protected by a primary 100 amp breaker, what is my minimum size secondary conductor ?
Click to expand...

I'm probably going to get "schooled" but I'll give it a shot. Assuming using 75 deg conductors, I get #6 AWG CU
 
Voltage ratio gives you multiplier of either 2.3 or .43 depending on which way you are going.

don't let the 75 kVA transformer primary rated amps of 90 fool you, you said you have 100 amp overcurrent protection - which is what you must go by here. 100x 2.3 = 230. Then it says you must have conductor ampacity of at least one third of that which is 76.7. 6 AWG at 75C is only 65 amp, #4 is good for 85 amps.
 
My thinking (probably wrong) would be that you need a conductor with an ampacity of 1/3 of OCP X the ratio (480:208 or ,43) or a 15 amp conductor, (100 X 1/3 X .43)
 
augie47 said:
My thinking (probably wrong) would be that you need a conductor with an ampacity of 1/3 of OCP X the ratio (480:208 or ,43) or a 15 amp conductor, (100 X 1/3 X .43)
Click to expand...
On right track, but wrong way with the ratio - that would be correct if stepping up instead of down with the transformer. If primary has a 100 amp OCPD, then the secondary current is 230 amps (before it gets into trip range of the 100 amp primary device) and 1/3 of that is 76.7.
 
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