Power and resistance

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

  1. 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?
  2. Sorry about three posts all the same, but the forum keeps throwing me out when I try to post
  3. J.P.

    J.P. New Member

    Hmmmm..could the above be classed as a variable?..reason -  variable ambient temps.
  4. removed 7

    removed 7 New Member

    The usual waffle from JP ^.

  5. J.P.

    J.P. New Member

  6. unphased

    unphased Screwfix Select

    No your friend is incorrect. Voltage is the FORCE driving the current. If there is less voltage there is less current. V=IZ. Since Z is a constant if you reduce the force driving the current, ie if you reduce V then the current falls with it else the equation could not work. If you look at shower manufacturers data they give two power ratings, one for 230V and one for 240V. You should see that 230V is less powerful than 240V.

    P = VI

    Your friends argument is flawed because he is saying that at 240V a 9.8kW shower draws 40.83A but at 230V it draws more current, thats runbbish because at 230V it is no longer a 9.8kW shower its power reduces due to the voltage reduction.

    You are correct.
  7. Moses

    Moses Member

    I think your friend is correct when he is talking of inductive motors operating under constant load.

    in that situation, when the supply volts drop, the motor draws extra current, to provide the constant load output.


    For resistive loads its V=IR, as explained in the above posts.

    Message was edited by: Moses
  8. removed 7

    removed 7 New Member

    This thread could do with a comment from Handy-Really, not that it would help - but it would make me chuckle at least.......

  9. Thanks Moses, I knew there was one situation where there would be an exception. Motors have inductances etc that would account for this behaviour.
  10. removed 7

    removed 7 New Member

    Such nonsense!
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I already did in one of his 'duplicate' threads(and I think I was right in what I said!!!!)

    Mr. HandyAndy - Really
  12. Well I'm not answering again...

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