3 Phase Transformer feeding Single Phase

[BPoindexter]

I have an installation i was asked to look at. It is for some temporary job trailers. Two trailers rated for a 200A service each are being supplied from a single 3 phase delta-wye transformer rated at 150 KVA. One supply is connected to A Phase, B Phase and the XO while the other is connected to B Phase, C Phase, and the XO. Each one has 200A fuses. So in essence they have 1-200A fues on A, 2-200 fuses on B, and 1- 200A fuse on C. This does not appear to be correct to me. Am I missing something?

 

[iwire]

It may be fine as far as the NEC.

112.5 kva at 208 volts is about 312 amps. We can bring that up 125% which gets us to 390 amps. We are then allowed to roll up to the next standard size which would be a 400. When using multiple over current devices the total of those cannot exceed the ampacity of a single device.

So two 200s seems OK.

 

[iwire]

This assumes the connected / calculated load is under 312 amps.

 

[BPoindexter]

Thanks we figured it out. The rated load of the transformer is actually 416A so we are well within it's rating.

 

[GoldDigger]

Also, ignoring power factor for the moment, two independent 200A line to line resistive loads will only generate a current of 200 x 1.73 amps in the common line conductor.

 

[iwire]

Thanks we figured it out. The rated load of the transformer is actually 416A so we are well within it's rating.

How is a 112.5 kva rated 416? :?

 

[GoldDigger]

How is a 112.5 kva rated 416? :?

The OP listed 150kVA. You just went to a standard size.

 

[iwire]

The OP listed 150kVA. You just went to a standard size.

Doh!

Thanks. :ashamed1: