wire size question

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4thgensparky

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i have a design/build with some issues. its a 800a/3p breaker feeding a 166kva 480/208v, 3p xfmr. the xfmr feeds a 1600a breaker. beside the xfmr being severly undersized the E.E. in the office said to size the conductors by the xfmr ratings not the ocpd. i dont know if i agree with this statment, any advice.
 
I would refer the EE to article 450. In particular article 450.3(B). As I see it the transformer is not protected as required.

Also direct the EE to article 240 for overcurrent protection of conductors.

Pete
 
4thgensparky said:
i have a design/build with some issues. its a 800a/3p breaker feeding a 166kva 480/208v, 3p xfmr. the xfmr feeds a 1600a breaker. beside the xfmr being severly undersized the E.E. in the office said to size the conductors by the xfmr ratings not the ocpd. i dont know if i agree with this statment, any advice.
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Are you sure its not an 800A breaker feeding three 166kva 480-120V single phase transformers that are connected in a delta-wye configuration, then feeding a 1600A breaker? That makes more sense for the breaker sizes you listed.

The conductor sizes should be per the breaker sizes. For instance, two sets of 500mcm on the primary (with an ampacity of 760) and four sets of 600mcm on the secondary (with and ampacity of 1680.)
 
david luchini said:
Are you sure its not an 800A breaker feeding three 166kva 480-120V single phase transformers that are connected in a delta-wye configuration, then feeding a 1600A breaker? That makes more sense for the breaker sizes you listed.

The conductor sizes should be per the breaker sizes. For instance, two sets of 500mcm on the primary (with an ampacity of 760) and four sets of 600mcm on the secondary (with and ampacity of 1680.)
Click to expand...

no its just one xfmr. thanks for the input, i thought it was sized of the ocpd.
 
4thgensparky said:
no its just one xfmr. thanks for the input, i thought it was sized of the ocpd.
Click to expand...

I asked because 167kVA doesn't seem to be a standard three phase transformer rating, but it is a standard single phase transformer rating.

Connecting (3) 167kva single phase (480-120) transformers in a delta-wye would give you (electrically) one three phase transformer rated 500kVA, 480-208/120. The primary rated current would be 601A and the secondary rated current would be 1388A. So the 800A primary c/b would be 133% and the 1600A secondary c/b would be 115%. These OCPD's are within the protection requirements of 450.3.

I suppose a non-standard 3 phase transformer could be ordered, but why not just use a standard 150kVA or 225kVA 480-208/120 transformer?
 
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