Power and resistance calculation

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. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Not Answered.[/b]

    ....and probably never will be.

    Mr. HandyAndy - Really
  3. unphased

    unphased Screwfix Select

    Oh yes it is and it has. :)
  4. wally

    wally Screwfix Select

    Page 9 should help.   *

    Message was edited by: Screwfix Moderator
  5. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Message was edited by: Screwfix Moderator
  6. seneca

    seneca Screwfix Select

    Page 9 of what?
  7. Bogit, you are right and have worked it out correctly. And your mate is confused. Your mate doesn't seem to realise that the kw rating of the shower is completely variable, and the stated 9.8kw rating is BASED on a 240V supply. Reduce that voltage, and the current will fall accordingly - and the kw output of the shower will fall in line too.

    It's a bit alarming the way his brain works, tho'. To think that when you reduce the voltage you should increase cable thickness... That's such a basic...
  8. Lectrician

    Lectrician Screwfix Select

    Most showers will show two ratings on the box - one at 240v and one at 230v.

    For example, a Mira states 10.8kw @ 240v, 9.9kw @ 230v.
  9. sinewave

    sinewave Screwfix Select


    Better put a bigger cable in ours then as our supply voltage rarely drops below 251V! 
  10. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Well this message is in reply to my original reply to the Op. For some reason, my original reply has NO text in it.

    It doesn't say that it has been edited by the idiots, and it contained nothing contentious in any way.

    Or is it just ME that cannot see any content ?

    It should say,

    Not Answered.[/b]
    [/b]....and probably never will be.

    Can anyone else see it ? it's really hacking me off.

    Screwfix: You can get friggin' FREE software to produce a better set-up than this!!!

    Mr. HandyAndy - Really
  11. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    Click 'Add Reply'.

    Message says, 'An error occured while trying to submit your post. Please try again.'

    Click 'Cancel' and it returns to page as if cancelled.

    Refresh page and it shows the reply entered.


    Mr Not Impressed - Really
  12. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    And Screwfix: You can get FREE software that does a better job than the stuff you are using for web-hosting.

    Mr. HandyAndy - Really
  13. Eye Squared R

    Eye Squared R Active Member

    The one constant is the Resistance of the shower heating element(s).  Power = Volts x Amps and/or [Amps]^2 x Resistance and/or [Volts]^2 / Resistance.  Taking DA's 9.8kW @ 240V, the elemet(s) resistance is [240]^2 /.9.8kW which equals 5.88 Ohms.  If you change the supply voltage and the element(s) resistance remains the same then at 230V less current flows and the power falls to [Volts]^2 x 5.88 Ohms = 9.0kW.

    Hopefully being your best friend you can forgive him his ignorance.
  14. chippie244

    chippie244 Super Member

    If you have an android based phone I found an app today called Electrodroid which has all sorts of useful calculators and tables and is free.

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