HV and LV difference on distribution boards

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Discussion Overview

The discussion revolves around the differences between high voltage (HV) and low voltage (LV) systems in distribution boards, particularly in the context of electrical motors for cooling pumps. Participants explore the implications of using different voltages and their effects on current and cable sizing, referencing Ohm's Law.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant describes the challenge of understanding the implications of using a 415V motor instead of a 3.3kV motor for a larger cooling pump, questioning whether this would require more current and larger cables.
  • Another participant confirms the initial understanding and suggests calculating the exact required current at 415V to demonstrate knowledge of Ohm's Law.
  • A participant later calculates the required current at 415V, initially estimating 477 amps, then revising it to 860 amps, indicating a concern about the practicality of using lower voltage for high power applications.
  • One participant notes that very high voltages are used for the national grid, implying a rationale for HV usage.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between voltage, current, and cable sizing, but there is no consensus on the exact current calculations or the implications of using different voltages.

Contextual Notes

The discussion includes calculations that may depend on specific assumptions about efficiency and power factor, which are not explicitly stated. The implications of using different voltages on cable sizing and safety are also not fully resolved.

kdcoops
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. 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
kdcoops
 
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kdcoops said:
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??

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

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