Transformer Conductors Size - Nameplate Vs. Load

[TheElectrician]

Hey Guys,

I have this transformer setup at a solar installation which is basically used to convert 480/277V (Primary - To the Utility) to 208/120V (Secondary - From string inverters). There are 5 inverters (5 x 79.4 x 1.25) with 496.25A as the total maximum output current, and I used 3 sets of #3/0 (w/roof top temperature derate) to feed the transformer secondary.

Using inverters FLA (79.4 x 5 = 397A) & 480V, I calculated the transformer rating to be 330 KVA so I am using a 500 KVA transformer

Now the real question is should the transformer's primary conductors be sized by the name plate detail (500 KVA) or the transformers actual load (330 KVA)?

Because if I use the load, I am good with 4 set of #350 CU and 1200A primary OCPD. But if I go by the name plate, there is a significant increase the wire size and the OCPD which is 2000A.

Does NEC give details on this? I refered to 215, 450 and could not get a proper interpretation of the code. Any advice is much appreciated

 

[Dennis Alwon]

Sounds like you are asking if you can use a larger trany than needy and feed it at a lower amperage for the load. There is no reason, afaik, that the trany has to be fed for the max it can carry but if someone wants to add more load later on then they would have to change all the work you have done.

 

[TheElectrician]

Sounds like you are asking if you can use a larger trany than needy and feed it at a lower amperage for the load. There is no reason, afaik, that the trany has to be fed for the max it can carry but if someone wants to add more load later on then they would have to change all the work you have done.

Thank you,I get your reason. But is there any NEC code that calls out for this?

 

[Dennis Alwon]

Thank you,I get your reason. But is there any NEC code that calls out for this?

I doubt that there is a code that states this but there isn't one to disallow it so IMO, this is fine. How many times have I installed a 200 amp breaker or main lug panel and fed it with 100amps. As long as the conductor is protected properly and the load is covered by the conductor and ocpd there isn't an issue.

 

[electrofelon]

Thank you,I get your reason. But is there any NEC code that calls out for this?

See 215.2(B)(1)