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Looking at a potential job. Agricultural buildings being divided into 10 mixed use units of around 1000 sq ft each.
Present supply is a 100A TPN incomer.
Future use of units is unknown at this stage, but planning is for B1 B2 so uses could be anything from a warehouse with a bit of lighting and single phase power, to a machine shop full of cnc gear...
I don't know where to start calculating max demand for this, given the usage is so unknown.
The landlord would like to keep the single intake, and put units on sub meters, but this will clearly limit the power available to each unit.
I'm guessing the start point is to work it backwards, and rather than calculate the actual load, you give each unit a limit, if that makes sense?
Any pointers welcome, ideally I'd like to sub out the design side, I feel a bit out of my comfort zone on this!
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The Ghost

100A TPN incomer.
The landlord would like to keep the single intake
These two items seem to be at variance. I would say the Landlord has high hopes if it is just single phase. Probably offices or storage would be about it. But if it is TPN incomer and you can get 200A fuses in there then you could be in a more sensible position for the load of other types of uses. If he does not want to invest in that then it is dead in the water already. For instance we have TPN 200A incomer supplying 66 units (offices) through three TPN DB then off to a DB in each unit supplying sockets and lighting and the LL DB of course. However I would caution this will involve a smart meter install and a requirement for identification of Kva demand throughout. Failure to get this correct may result in penalty charges for being under or over the calculted Kva demand. To put this in perspective ours is 18Kva per month (average measured per HH meter i.e. Half Hour) if I remember correctly. It is a new thing DNO/supplier are doing and came in in April 2018 I think. The good news is you can monitor usage through the meter and get software on the reception or Landlord computer to keep tabs on it all.
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  • #3
Thanks, what I meant was that he wants to use 1 TPN intake, as opposed to getting DNO to run in individual supplies to each unit. Yes, a 200A per phase supply is starting to sound more realistic. Existing supply fed from a small pole transformer though...
Diversity always seems to end up on the side of being able to run a huge amount more than what you actually think!
For what it's worth, my gut feel would be that a 200A per phase incomer, coupled with a panelboard giving 50A per phase supply to a 6 way TPN board in each unit would be a good starting position.
Don't fancy pricing and installing on that basis though!

The Ghost

A six way may be too generous. I might price on a two way and an alternative (4 Way). What is key in this is what will the units be used for? And what will planning/building regs allow, has that process been started? I might be inclined to balance the phases in sequence to each office from a main 3ph board at origin with a DB and meter at each unit din mounted. Or one meter over the Three phase and divide by 11. Even 100a per phase gives you enough to do that. Starting at one ring final one lighting and of course the LL DB for communal. Maybe a heavier load on that with water heaters and hand dryers but wont be that much with just 10 offices. We have an MCCB cabinet off to the meters for each unit then SWA to each office with four way DB. Some are three phase DB from the main DB. There are a lot of ways this can be done but it depends on the budget and intentions of the Landlord.

The Ghost

I think @Leesparkykent would be a good one to bounce this off of. In fact the whole forum is but it seems a bit quiet here. I must say that when faced with a larger project it seems good form to get a firm specialising in design. The reason being is the liability for getting anything wrong rests upon them. At the same time one has to watch just what they are putting out as they do get it wrong and put stuff in the wrong place that just wont work in the real world. You could also get software to design yourself. It is just working out the loads and protection and bonding etc. But you have to have the intended purpose first.
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Lets see, 100A per phase, three phase and ten units,
So each unit getts 10A per phase
potentually 30A power per unit
That should be just barely enough for small units / offices.
If the tennants are aware that power is limited
then it should not be a problem.
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  • #8
I don't think there will be provision for serious space heating.
Each unit will have a small kitchenette and WC, so a 10l undersink would likely satisfy hot water requirements.
If part of the unit gets partitioned off to form an office then you could imagine typical setup of 2kW oil fired rad/panel heater I guess.


I'd be inclined to get a 2nd 3 phase supply installed; then you can distribute to the units, maybe have 2/3 units suitable for 3 phase heavier loads, 3/4 lighter 3 phase, and 3/4 single phase split across the lighter loaded 3 phase supply. as said before though, might be advisable to get a more specialist designer on board.
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