x formers

[wyatt]

Transformer conductors sized to amp of transfer or OCPD ? IE 75kva 480-208/120 90 pri and 208 sec or say 125 ocpd pri and 225 Sec (and lets just say sec load not continuous duty)

 

[jim dungar]

Re: x formers

Don't get the cart before the horse.

1. Conductors are sized based on the load per 310.

2. Overcurrent protection is sized based on the conductors per 240.

3. The transformer is sized any way you want.

4. Transformer protection is sized per 450, the overcurrent protective device chosen in step 2, may be used to meet the requirements of 450.

 

[wyatt]

Re: x formers

So start with the load you whant to feed and work your way back to the sorce pick a trans to fit the load and Make sure all ocpds are in keeping with 450. Soud right?
Ie I need 100amp 120/208 i a have 480v
100 amp panel fed with 3awg cu to trans pick a 37.5kva 480 208 trans. feed trans with 8awg cu from a 45 to 50 amp braker in the 480 panel
I could feed the 30kva trans with a 90 amp braker with 3awg cu
(this is not a real case just trying to get the math right)

[ February 14, 2006, 10:01 PM: Message edited by: wyatt ]