Another Feeder Tap Question

[kevinware]

I had a contractor submit a drawing to install a standard 225A panel w/main breaker as follows: An existing 480V 400 Amp disconnect was going to be tapped with #3 THHN leading to the line side of a 100 amp disconnect fused at 90 amps Less then 25' away. The load side of the 100 amp (fused at 90A) disconnect is also supplying #3 to the high voltage side of a 480/208/120V 75kVA transformer. The low voltage side less then 25' was going to be 4/0 THHN to the 225 amp main breaker. My questions and concerns are these:

1. Shouldn't the conductors from the 400 amp disconnect to the line side of the 100 amp disconnect and be sized at 1/0 according to 240.21(B)(2)
400A / 3 = 133A Table 310.16 1/0 = 150A

2. Is the primary conductor considered to be the total length from the 400 amp disconnect to the lugs of the transformer. Or the length from the load side of the 100 amp disconnect to the lugs? Does 240.21(B)(3)3 apply here?

Thanks for your help,
Kevin

 

[augie47]

1. Yes 1/0
2. The transformer primary would be from the transf to the OCP devicd ahead of the transf. If I read correctly, that would be the 90 (100) amp fused disconnect.

 

[wasasparky]

1. 1/0... 90A is pretty light for a 75kVA transformer, might have inrush problems...

2. Doesn't matter since you have primary protection. If you wanted to eliminate the 90A disc, you would measure Pri&Sec from the tap to the main of your 208Y/120V board.