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HV and LV difference on distribution boards

  1. Feb 13, 2014 #1
    . Hello, I have recently been given a course at work as someone left and they had already paid for it, unfortunately I am a bit green to electrical systems ( I only work in stores but want to learn) And my course helper, our onsite electrical man is off on long term sick!
    In my course I have been asked:

    " Demonstrate a clear understanding on Ohms Law by showing why different voltages are used on certain distribution boards on site"

    Now I have chosen to try and explain why we have two different cooling pumps on site. A large one with a motor at 3.3kv and a smaller one at 415v. Using Ohms Law I have worked out that the larger one which runs at 60amp is 198,000 watts and the small one at 228amps is 94,620watts.

    What I am struggling with is, if we used a 415v instead of 3.3kv motor on the bigger pump would we need more amps meaning a lot bigger cables to get the power to run the big pump??

    I understand it says on site you don't like to give direct answers and I don't expect that but would appreciate any help what so ever and if you could try and keep it pretty simple.

    Kind Regards
  2. jcsd
  3. Feb 13, 2014 #2


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    What you said is correct. It seems as though you have a good handle on this problem!

    if you really want to demonstrate knowledge of ohms law, I would calculate what the exact required current would be if the voltage dropped down to 415 V. then use some reference material too look up exactly what size wire you would need

    Keep up the good work!
  4. Feb 13, 2014 #3
    Hi donpacino,
    Thanks for the reply, good to know I am on the right track. I have had a go a working out the current needed and came up with 477amps does this sound about right?? Hopefully it is, then I just need to try find out what cable size would be need to carry that.
  5. Feb 13, 2014 #4
    Sorry scratch that last one, now I have had another go I have come up with 860amps!! This seems a bit more likely to why we would use a higher voltage?
    Thanks Kev
  6. Feb 15, 2014 #5


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    This is exactly why very high voltages are used for the national grid.
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