Supply over 3 phase 100A for commercial unit

Discussion in 'Electricians' Talk' started by omega1, Aug 15, 2022.

  1. omega1

    omega1 New Member

    Hi all,

    I am looking at a commercial unit which already has installed a 3 phase supply, each fuse is labelled as 100A so I am assuming that this is a typical 100A 3 phase installation. I will be needing more than this and will be contacting the network supplier, but in the meantime I have a few questions...

    1) Am I correct in assuming that this is 100A over the 3 phases (rather than having 100A for each phase, so 300A)? If I am mainly using single phase devices, is this still that case or can I load each phase with 100A?

    2) If I needed a larger supply, what would be the next 'tier'? I'm not sure how it works in terms of whether there are typical tiers of supply after this (250A, 300A, etc), or do you just go to the network and tell them how much you want and they set up a project for it?

    3) At what stage/level of power would you be required to have your own transformer? I assume there will be a stage whereby it is not feasible to supply over LV feeds (due to cable cross section sizes) and you'd need your own transformer?

    Any feedback on this would be appreciated.

    Thanks in advance.
     
  2. CraigMcK

    CraigMcK Screwfix Select

    1) Each fuse is 100A, so you can draw that from each phase
    2) They go in kVA, but in reality it will be based around fuse sizes, so 125, 160, 200...
    3) Don't go there if your DNO can supply what you want at LV What rating do you think you would need?
     
    omega1 likes this.
  3. omega1

    omega1 New Member

    Hi CraigMcK,

    Many thanks for replying, much appreciated.
    1) Got it, thanks.
    2) Thanks for that info.
    3) I'm hoping not to. I'm looking at around 500-600A in total consumption. I'm not familiar with what can be supplied over LV, is that doable?

    Thanks again.
     
  4. Bazza

    Bazza Screwfix Select

  5. Bazza-spark

    Bazza-spark Screwfix Select

    @omega1 you may get more help if you gave a little more info as to what you intend to do and use in this unit.

    Edit: and a much livelier row lol :D
     
    chesterw likes this.
  6. Philip Hyde

    Philip Hyde Screwfix Select

    What you planning on running? 500amps is a big supply. And will cost a massive amount as it will probably mean new cable from sub station to unit.
     
  7. The Happy Builder

    The Happy Builder Screwfix Select

    The only real solution is to look for premises that already has a supply that size.
     
  8. omega1

    omega1 New Member

    Thank you to everyone that has provided useful information on this.

    I always find it fascinating on forums the amount of people asking the OP 'why' they need what they are asking for... I was hoping to have been as specific as possible with my questions in my original but was happy to clarify further if any more relevant information was required.
     
  9. MGW

    MGW Screwfix Select

    The step down transformers tend to be 315, 400, and 500 kVA, so it would depend on what supplied the unit to start with, if it is simply the DNO needs to change a fuse from 100 amp to 300 amp then no real problem, but if they need to install new cable or transformer, then the cost can spiral.

    Some times the DNO charges don't seem to make sense, where I work, they bought a building, which already had three supplies, the cost of standing charge v cost of supply removal is around 25 years, so they simply sit there. What will likely happen is they will get more EV charging points installed.

    But until you ask you simply don't know, my son wanted a supply moved, and he was quoted a really low price, he thought he was very lucky, but it took years to move, I have seen where I worked before, we paid for a supply to be moved, and 15 years latter still waiting. The firm changed their plans so were not pushing for the move, but time scale can be a problem.

    1935 the UK's national grid was born, I think the last hamlet to be connected was 2007, that's some wait for power to be connected.
     
  10. Bazza-spark

    Bazza-spark Screwfix Select

    The reason I asked why is so that we can verify your figures. It may be that you have just added all your loads together and arrived at a figure, when diversity can be applied which would reduce your effective load.

    Hey, if you don't want to supply the info, that's fine, you won't get an accurate response.
     
    Philip Hyde likes this.
  11. The Happy Builder

    The Happy Builder Screwfix Select

    How do we know if you have even asked the right questions?

    If you don’t know what the answers are, how do you know what the questions are?

    If you don’t say what you intend to use nobody can actually tell if you have asked the right questions to be able to give accurate answers.​
     
  12. The Happy Builder

    The Happy Builder Screwfix Select

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