transformer conductor calculations

[thenoob]

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

 

[dpc]

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

http://www.cvoes.com