# How to calculate R for an AC load on a 3-ph system

I'm trying to calculate the resistors in an AC dummy load for a 3-ph system.

A simplified schematic can be found below. The idea is to switch (with regular AC 1-ph contactors) some resistors on each phase, the middle point being the neutral point.

The generator has a line-to-line Voltage of 280V and we're trying to dissipate a maximum of 60kW.

I'm using the following formula : I = P/(sqrt(3)*U) = 60000/(sqrt(3)*280) = 123A

==> I'm trying to have 123A per phase in order to dissipate 60kW.

R=U/I = 2,276 Ohms ==> Am I correct in assuming I have to have a 20kW resistor of this value on each phase? (of course divided between several resistors in order to be able to reach intermediate power values via the contactors).

Thanks for the input!

• Yes, your math is correct. Feb 28, 2019 at 16:30
• Your diagram shows you have grounded all 3 phases on both sides of the load. This is not how you create a wye point.
– K H
Mar 1, 2019 at 5:40
• Hi KH7, yes indeed, bad schematic on my side. I guess you got the idea behind it which is that the middle point of the wye is the return path for the resistor loads. Will correct the schematic ASAP. Mar 1, 2019 at 13:21
• @K H : Updated the schematic, is that more like it? Mar 1, 2019 at 17:21
• Yes, the schematic is better. I'm assuming three phase contactors. I'd recommend spreading loads out and adding some form of cooling. Mar 1, 2019 at 18:16