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The equation you are looking for is amperage = kva x 1000/1.73 x voltage.

150 x 1000 = 150000/1.73 x 480 = 150000/830 = 180 amps for primary side of transformer.

150 x 1000 = 150000/1.73 x 240 = 150000/415 = 361 amps for the secondary side of the transformer.

The electrical code requires Transformers and motors conductors to be sized at 125% of the rated amperage.

So 180 x 125% = 225 amps, and 361 x 125% = 451 amps.


A 3/0 copper conductor with an insulation factor of 90 degrees C is rated at 225 amps.


A 600 MCM copper conductor with an insulation factor of 90 degrees C is rated at 475 amps.


Parallel 3/0 copper conductors with an insulation factor of 90 degrees C will give a capacity of 450 amps.

There is no 2 phase 240 volt secondary.


Any two legs of a three phase system are classified as single phase. Single phase 240 volts can be obtained from any of the following transformer tap connections, A-B, B-C, C-A.

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8y ago
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12y ago

The ground wire is based on the current carrying capacity of the feeder conductors. To calculate the feeder conductor size the transformer amperage must be calculated. To calculate the transformer amperage the primary and secondary voltages must be stated.

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12y ago

For a single phase connection a 400 MCM wire will be needed. For a three phase connection a 3/0 wire will be needed.

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Q: What size wire should you use to feed a 150 KVA transformer 480 3 phase primary - 240 volt 2 phase secondary?
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