# Required current rating of 3-phase power system

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1. Apr 24, 2015

### kyle pender

My background is mechanical engineering so not great with the electrical side. I plan on buying a air-preheater (resistive wire heated) which has a power rating of 30kW, supplied by 3-phase. I need to determine whether my 3-phase supply (415V 32A) has a max current that can cope with this. My calculation is as follows:

since purely resistive, power factor =1 hence, kW=kVA=30kW
line to neutral voltage=line to line voltage/3^1/2=230V
3-phase power is 30kW, therefore single phase power = 10kW

Current=VA/line to neutral voltage=10000/230=43.478 amps

As far as I am aware this is the current rating from my supply i would need to achieve 30kW from 400V. Do my calculations appear correct? If so I will need to find a power supply rated above this current.

2. Apr 24, 2015

### psparky

To find the amps in 3 phase, we typically leave it in line to line voltage.

30KW=415*1.73*I
I=41.78 amps per phase

Therefore, your 32A power supply falls a bit short.

A 60 amp three phase breaker should be ideal assuming your load requires 415 volts.
NEC code would suggest 3 #6 wires with #10 ground. If your load is more than 400 feet away, you may have to upsize your wire size for voltage drop.

Your heater specs may list the ideal over current breaker size. They may list the fuse size for a disconnect as well. Some disconnects get no fuse. They may list the recommended wire size as well. They may not.

Last edited: Apr 24, 2015