# Transformers



## TheDuke (Jan 12, 2013)

What would the secondary max size to min size ocpd be for a 30 kva 3 phase 480D to 120/208W dry type transformer in a commercial building be?


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## parnellelectric (Dec 23, 2011)

Welcome "The Duke"


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## HARRY304E (Sep 15, 2010)

TheDuke said:


> What would the secondary max size to min size ocpd be for a 30 kva 3 phase 480D to 120/208W dry type transformer in a commercial building be?


70 Amps.....Welcome to ET.:thumbsup:


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## Tom45acp (Sep 6, 2011)

Full load capacity on the secondary is 83 amps, so I'd say you could use 110 amps (or less if you want) if you're doing primary and secondary protection. If you're doing primary protection only, then the OCPD on the secondary side can be as big as you can afford to make it. This is based on 450.3(B) and does not take into consideration any conductors.


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## TheDuke (Jan 12, 2013)

The way I think I understand it is for the secondary it would be the amps x 125% which for a 30 kva would be 83.3 x 125 % which would be 103 amps. The city stamped 1 line shows running number 3 to a 100 amp ocpd at the pnl its feeding. My boss wants to use a 150 amp breaker because he already has one and size the feeder wire to the 150. It seems to me you'd be over-amping the transformer if you did this so I'm telling him to stick to the one line or they might make him resubmit the drawings. Not real clear on this. Thoughts?


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## TheDuke (Jan 12, 2013)

There is primary protection. Not sure how big. I'm thinking they went 250% would have to check it was provide by the mall electricians and they ran number 3s


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## Tom45acp (Sep 6, 2011)

If the plan review indicates #3 wire and 100 amps, you should stick to it unless you want to discuss this with the AHJ and get it approved prior to installation. If you use primary and secondary protection , you could go to a 110 amp breaker with #2 copper.

The only way to use the larger conductors & breaker on the secondary side is to go with primary protection only which means the primary side OCPD needs to be 125% of full load (36 x 1.25) = 45 amps. Again, you should run this past the AHJ since they have already reviewed the plans.


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