45kva transformer

[PaulEd]

Hello

I have a 45kva transformer. Delta to wye. Sizing the primary and secondary conductors the engineer is calling for:
Primary: 4awg and 70amp ocpd
Secondary: 1/0awg and 150 ocpd

Shouldn’t the secondary conductors be 2/0 awd if:
45000/208x1.73

Furthermore does derating at 80% come into play with a 4 wire secondary?

Thanks,
Ronnie

 

[augie47]

Using a 150 amp breaker on the secondary, a 1/0 would be sufficient.

I don't know that the 80% would come into play.

 

[infinity]

Primary: 4awg and 70amp ocpd
Secondary: 1/0awg and 150 ocpd

Those numbers seem fine. As Augie stated since the secondary feeds a 150 amp OCPD you only need 150 amp conductors. What size conductors for the EGC, GEC, SBJ, and SSBJ's are being specified?

 

[petersonra]

Hello

I have a 45kva transformer. Delta to wye. Sizing the primary and secondary conductors the engineer is calling for:
Primary: 4awg and 70amp ocpd
Secondary: 1/0awg and 150 ocpd

Shouldn’t the secondary conductors be 2/0 awd if:
45000/208x1.73

Furthermore does derating at 80% come into play with a 4 wire secondary?

Thanks,
Ronnie

There is no such thing as derating for 80%. There are maximum rating of over current protection devices and minimum ampacity of conductors required, and as long as you meet those requirements you are good. You can use less than the maximum over current protection device rating. You just can't use more.

 

[JoeStillman]

...Furthermore does derating at 80% come into play with a 4 wire secondary?

Transformers are 100% rated. The derating would apply to the conductors. 150A @ 80% = 120A. At 208, 3Ø, that is 43.2 kVA of continuous load that can be fed from your secondary and the transformer won't be overloaded.

From another perspective, the values in the "Currents of 9A or More" column of table 450.3(B) are maximums. So (45,000/208/1.732)*1.25 = 156A and a 150A is less, so it complies. You could go to the next size up (450.3(B), Note 1) with 175A cb, 2/0 AWG, but you don't have to.