I need 80 dedicated 208V/15 amp circuits. How many amps of 480V/3Ph service are required?

Thanks for the help.

I will assume this is commercial application, where the load will likely run 3 hours or more. Each circuit needs to be derated by 80%, so the total max load will be 12 amps per circuit.

To answer your specific question, the amp load at 480 V 3 Ph will be 416 amps.

You will need a transformer to handle at least 345.6 KVA of load plus 25%, or 432 KVA. The 480 volt feeder to the xfer also needs to be increased by 125% of the max load.

The next standard size transformer is a 500 KVA. This is a very large transformer, about 6 feet wide, 6 feet high, and 5 feet deep, and there are certain requirements for installing this size transformer not to mention the weight to deal with.

If this is for an IT equipment, use a K rated transformer.