# Power and resistance calculations

Discussion in 'Electricians' Talk' started by Bodgitandlegit, Apr 2, 2011.

1. ### BodgitandlegitMember

My best mate is an old school electrician, and I'm an old school vehicle technician. We were discussing power consumptions the other day, when he stated that when the voltage drops, say from 240 volts to 230 volts the current consumption increases. He is adamant that this is the case and was taught this way at college. His calculation goes something like this:- if you have a 9.8KW shower and it runs on 240 volts, you need a specific size of cable, if you run it on 230 volts then you need to increase the cable size as it will draw more current. The rule he uses is watts = volts X amps. So at 240 volts a 9.8 KW shower draws 40.83 amps. Using this same formula, at 230 volts a 9.8KW shower draws 42.65 amps. I calculated the resistance of a 9.8KW shower (V = I X R) is 5.87 ohms. This doesn't change whatever voltage you run it on. So at 230 volts the current flow will be 39.18 amps. So multiplying it up, your 9.8KW shower, run at 230 volts becomes a 9KW shower. My mate is not convinced by these figures. Can anyone back me up, or is he right?