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Wire sizing is based on the amperage. The formula you need to find amperage from HP for single phase is

, I = HP x 746/E x %eff x pf. For three phase I = HP x 746/1.73 x E x %eff x pf.

A standard motor's efficiency between 5 to 100 HP is .84 to .91.
A standard motor's power factor between 10 to 100 HP is .86 to .92.

I = Amperage, E = Voltage.

When you have calculated the amperage ask a electrician to look up the wire size for that specific amperage. He should have a code book and there is a list of wire sizes and amperages. The quickest way of finding the amperage is by looking on the motor's nameplate. There you will find all of the information that you need.

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Q: How is cable size calculated when given the HP rating?
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