# Sizing Secondary of Transformer?



## wildleg (Apr 12, 2009)

don't know what code cycle you are on, but take a read through article 450. it answers all your questions other than conductor sizing. you can fuse per the table either just primary, or both (table 450.3) make sure you read note 1 because you can go next higher fuse.

as to disconnect: (2011)


> 450.14 Disconnecting Means. Transformers, other than
> Class 2 or Class 3 transformers, shall have a disconnecting
> means located either in sight of the transformer or in
> a remote location. Where located in a remote location,
> ...


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## Dennis Alwon (May 9, 2009)

Not enough Info. You need to look at 240.21(C) for the secondary side conductors


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## varmit (Apr 19, 2009)

If the MCC is lockable (It probably is.) this could serve as the line side disconnect - if there are markings stating where the line side disconnect is located.

You must have over current protection, on the secondary within 25 feet of the transformer.


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## frenchelectrican (Mar 15, 2007)

travis13 said:


> Let’s say I have a 112kva 480/208 delta/wye transformer. It is fed from a motor control center and will be a continuous load. The secondary equipment load is a 185A (according to nameplate) UPS 150’ away.
> 
> Question 1:
> Will I need a fused disconnect at the transformer on the primary side?
> Or is the MCC the only means of disconnect I need?



I am sure most MCC do have lockable disconnect so that will serve as disconnecting means as long both this MCC disconnect and transformer stated where it located. 

Yes you will need fused or breaker OCPD in there. ( primary side )



travis13 said:


> Question 2:
> Since I am only using 185A (231A continuous) on the secondary side of the transformer, does there need to be a 250A fused disconnect 150’ away at the UPS?


Yes you will need fused disconnect or main subpanel breaker in there 



travis13 said:


> Question 3:
> Wire size and conduit on primary would be 3/0 CU in 2” EMT with a 175A breaker?
> Wire size and conduit on secondary would be 300kcmil in 2.5”?
> 
> Thanks in advance for all your help


 what length for this question related to ?


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