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transformer conductor calculations

transformer conductor calculations

transformer conductor calculations

(OP)
If I have a 3 phase transformer rated at 30kVA, 600V primary, 120/208V secondary:

Ip=[30x1000]/[600x1.73]=28.9A (then multiply by 1.25 for code) and the cable size would be #8AWG  
Is=[30x1000]/[208x1.73]=83.3A (then multiply by 1.25 for code) and the cable size would be #2AWG   

And for a 1 phase transformer rated at 30kVA, 600V primary, 120/240V secondary:  

Ip=[30x1000]/[600]=50A (then multiply by 1.25 for code) and the cable size would be #6AWG  
Is=[30x1000]/[240]=125A (then multiply by 1.25 for code) and the cable size would be #1/0AWG

I'm going by Table 2 in the CEC for cable size and 90 deg. rated.  Is this correct? I'm out of my element on this one.  Just wondering if there is other factors I missed.  The breaker on the primary would be set to trip to protect the cable (125% primary rated current).

Thanks,

thenoob

RE: transformer conductor calculations

Determine the necessary overcurrent protection needed for the transformer and then size the feeder according the breaker size.  

For a primary breaker, it is also necessary to account for the transformer inrush current.  If you size the breaker strictly per 125% of the transformer FLA, a molded case breaker can trip on inrush.  I'm guessing you'll want at least a 40 A primary breaker and the conductor must be sized for a 40 A breaker.  If it was me, I'd put in a 50A breaker.  But I'm an NEC guy.  For 480 V primary breakers on 30 kVA transformers, I use a 60 A breaker.  NEC allows primary protection sized up to 250% if secondary protection is provided.   

David Castor
www.cvoes.com

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