I like to verify

[Ponchik]

I don't do much 3 phase transformers and I like to verify this with you guys.

Primary = 240V
Load Voltage = 480V

Existing load is 19A adding max of 50A more (So total secondary current of 70A).
I have attached the calculation that I did and I like to know if I am correct or screwed it up.

Thanks
View attachment 17189

Sorry I can not get the PDF file to upload.

 

[Smart $]

That'd be 180A rated primary current.

Going with a 75kVA transformer with only a 70A secondary load (continuous? noncontinuous? %'s?) leaves you a few options. You only have to match 140A (same continuous, noncontinuous %'s) on the primary side.

450.3(B) permits up to 125% of rated primary current for the ocpd rating using the primary-only transformer protection schema. But if you put ocpd on the secondary side, which will likely be required to protect the secondary conductors, and the ocpd is limited to 125% rated secondary current (90A), the primary can then go as high as 250%.

Wire sizes, primary and secondary, will be based on the protection schema albeit the ocpd rating chosen and all must be sufficient for the load.

Your chosen primary and secondary wire sizes are reversed. Either that or your primary and secondary voltages are reversed.

 

[augie47]

what he said..

75 kva 240v current for a 75kva would be 180 amps.. using 125% protection would be a 225 amp breaker.. your conductor on the 240v side would need to be sized to match the breaker (225 or less). Secondary protection & conductors would depend on your load requirements.

 

[Ponchik]

Thanks guys.

I did have my wire sizes reversed.

I am planning on 225A primary OCPD with 4/0 & 100A secondary with #3.

The loads are punching and laser machines. May or may not run at the same time. If they do run at the same time they are not continuous anyway.

 

[Smart $]

You are good to go... :happyyes: