1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

2 x 15kw boilers on single phase ??

Discussion in 'Electrics UK' started by Maciek, 22 Nov 2019.

  1. Maciek

    Maciek

    Joined:
    7 Oct 2018
    Messages:
    26
    Thanks Received:
    0
    Country:
    United Kingdom
    I know that all calculations say no but this is the situation and I'm looking for a solution.

    There is the house that has and electric 15kw heating boiler plus the other household equipment, like cooker, kettle etc. All run from single phase 16mm^2 main cable and protected by 80A fuse in cut out. Main cable to house cut out is 16mm^2 as well. This is all good, now the problem..

    There is an outbuilding where my friend want to install same 15kw boiler plus other workshop equipment, like welder, air compressor etc... no commercial use. There is a second meter and cut out there connected by 16mm^2 cable and protected by 80A main fuse. The problem is that main cut out cable is looped from main house cut out so there is a 16mm^2 cable coming to property and is split to 2 different cut outs/meters so eventually he would be running 30kw in boilers through 16mm^2 underground cable plus all the other equipment.

    In theory all 2 circuits will be protected and fine on their own but the problem is in main cable. I'm afraid it will get fried. I contacted local DNO and they said that they can upgrade it to 3 phase but it will cost a lot.

    Any ideas??
     
  2. Sponsored Links
  3. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    42,451
    Thanks Received:
    2,695
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I would have thought that if the DNO are providing two "80A supplies" (assuming 80A fuses in both cutouts), it would be their problem (and their cost, if change was required) to ensure that the cable feeding them was adequate for 2 x 80A 'supplies'.

    Kind Regards, John
     
  4. Maciek

    Maciek

    Joined:
    7 Oct 2018
    Messages:
    26
    Thanks Received:
    0
    Country:
    United Kingdom
    I'm dealing with DNO on a daily basics due to my work, it is hard, it is very hard, sometimes impassible to get a suitable solution.

    16mm swa 2 core cable can handle about 130A. I advised my friend to put 9kw boiler to the outbuilding which will be sufficient enough but he is stubborn and already bought 15kw one. If he will melt a main cable it will be "fun" :)

    Unfortunately DNO can't provide any advice apart from upgrading to 3phase (around £8k) :(

    Only solution I see is to replace both 80A fuses with 60A fuses and hope for the best.

    Any other ideas ??
     
  5. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    42,451
    Thanks Received:
    2,695
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Well, that, in itself, is as near as damnit to being enough for 2 x 15 kw loads (but nothing else in addition :) ) - which I suppose is a start!
    As you'd probably agree, the chances of anything melting are pretty low - not the least because it's probably not likely that both boilers would be drawing 15kW simultaneously for appreciable periods of time (aka 'diversity') - and, as before, if the cable upstream of the DNOs cutouts did melt, that would surely be their problem, wouldn't it?.
    I don't really get that. As I understand, you're talking about two separate DNO supplies, neither of which needs to be 3-phase (nor to be more than 80A).
    Well, for a start, that would obviously be down to the DNO to decide and do. However, as I've said, if they have put 80A fuses in both of their cutouts, that surely implies that they are obliged to provide 2 x 80A supplies (and feed them with a cable adequate to provide that), doesn't it?

    Kind Regards, John
     
  6. John D v2.0

    John D v2.0

    Joined:
    26 Aug 2016
    Messages:
    6,165
    Thanks Received:
    854
    Country:
    United Kingdom
    Who've pays the bills is going to get a shock if the heatloss is such that you need a 15kw boiler not only in the house but another in the outbuilding, especially if it's electric! You're talking 2-3 pounds an hour during the heat up period where most people would spend that in a day if the house isn't so well insulated.
    I think the bank account will melt before the cable does!
     
    • Like Like x 2
  7. Maciek

    Maciek

    Joined:
    7 Oct 2018
    Messages:
    26
    Thanks Received:
    0
    Country:
    United Kingdom

    That's irrelevant to my question....
     
  8. John D v2.0

    John D v2.0

    Joined:
    26 Aug 2016
    Messages:
    6,165
    Thanks Received:
    854
    Country:
    United Kingdom
    It isn't quite but maybe i was being obtuse. My point is the electric boilers are going to use far less on average then 2x 15kw, so you'll be more likely to be alright.
     
  9. flameport

    flameport

    Joined:
    10 Mar 2007
    Messages:
    8,393
    Thanks Received:
    1,552
    Location:
    Poole, Dorset
    Country:
    United Kingdom
    You cannot run 2x 15kW loads on a looped supply, regardless of what fuses may or may not be installed.

    A new service cable will be required for the second building.
     
  10. Sponsored Links
  11. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,383
    Thanks Received:
    554
    Location:
    Cumbria
    Country:
    United Kingdom
    I can see a couple of options ...

    One is to just go ahead and if the common supply cable gets hot, tell the DNO that your supply equipment is getting hot and you're worried. Then see what they say. I could imagine them wanting paying to upgrade the supply - but then you point out that you aren't overloading your supply so why should you be paying to upgrade it when it's someone else's load that's overloading your supply cable.

    The other way I see is to use a load shedding relay to shed some load when the total exceeds whatever you consider to be a "safe" level for the common supply cable. The tricky bit here is doing it safely when the load is split across 2 separate supplies. In effect, interlocking the two boilers (and perhaps other loads) so that you can't run them both at the same time.
    Just interlocking the boilers (without any load monitoring) might be enough.
     
  12. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    42,451
    Thanks Received:
    2,695
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Indeed so. Although I realise that flameport suggested that the rating of the DNO fuse was not relevant, I would have thought that if they provided two supplies, each with an 80A fuse in the cutout, the implication would be that they were 'contracting' to provide two 80A supplies - and that it would be their problem to ensure that what was upstream of the cutouts was adequate to service those supplies.
    Given that the boilers would easily represent the lion's share of the potential total load, I would think that the latter would be adequate.

    However, as I previously wrote, I strongly suspect that 'diversity' would probably do this adequately for us - i.e. it is probably relatively unlikely that the two boilers would be drawing 15kW simultaneously for long periods of time, so that the average current over a reasonable period time would not be all that 'frightening' ... and, as a further pragmatic point, I don't think that, in reality, 130A or so would get anywhere near 'melting' 16mm² conductors!

    Kind Regards, John
     
  13. SimonH2

    SimonH2

    Joined:
    4 Nov 2010
    Messages:
    5,383
    Thanks Received:
    554
    Location:
    Cumbria
    Country:
    United Kingdom
    No, I don't think that is the implication - as we know, it's a lot more complicated than that.
    They will be providing two off "typical household supplies" to feed "typical household loads" with typical diversity profiles, and AIUI you are required to notify them if you are going to be connecting "abnormal" loads. Unless it's an area without mains gas, then I would suggest that a 60A electric boiler would not be a "normal" load.

    A quick look suggests that 16mm² copper/PVC concentric is rated for around 100A clipped direct or in free air. As you say, unless you managed to get both loads running together for some time, it's probably going to be within the safety margin of the cable.

    There is the view which you sort of state - if they use a cable to provide two supplies, each fused at 80A, then that cable ought to be cable of carrying 160A (times whatever factors for the fusing characteristics) continuously. But I think we're mostly agreed that the DNOs work to different assumptions - that 16mm² cable certainly won't be protected by the 300+A fuses at the substation.
     
  14. jelliottelectrical

    jelliottelectrical

    Joined:
    14 Nov 2013
    Messages:
    51
    Thanks Received:
    9
    Location:
    Warwickshire
    Country:
    United Kingdom
    Something to consider here, just because you have an 80 amp fuse does not give you free reign to load the supply up to 80amps. When the supply was put in, it would have had an agreed capacity. The 80amp fuse is there just to protect the DNO equipment. If your additional load causes problems on the local network, and you are adding considerable load, then the DNO has the legal power to disconnect you from the network.

    Taking into account a job i did last year, full rewire of a property, all electric, I also installed a 12kw electric boiler. Two 3kw immersions heating hot water, 9.5kw electric shower (customers request) and electric range cooker. Now whilst the boiler does modulate it does also draw 12kw for long periods of time. I considered it too much of a risk running all this load off the single phase 80amp supply, don’t forget the effects that constant heating and cooling of terminals and cutout fuses long term. So we went with a new 80amp 3 phase supply. Overkill really for the actual load, but I can rest easy at night knowing that the occupants aren’t going to be plunged into darkness when the boiler is on, someone is cooking and the kids are in the shower! And that I’m not going to be called out to a burnt out conductor or Mcb!
     
    • Thanks Thanks x 1
  15. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    42,451
    Thanks Received:
    2,695
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Yes, maybe - but that, in itself, would be an argument for them not 'allowing' the OP to install his boiler in the first place, even if there was not someone else who wanted to do the same thing on a different supply which 'happened' to be looped off the supply to his installation.
    Indeed, and furthermore ....
    I obviously don't know for sure, but I doubt that it would 'melt' (or even necessarily come to any particular harm) at 300A, any more than I would expect 2.5mm² conductors to 'melt' at around 81A.
    Yes, that was certainly a concept underlying what I've been saying. As you imply, wiring within the scope of BS7671 would not be allowed to be done like that - one could not rely on an argument that, although the fuses were 80A, one was not 'expecting' (but, in reality, 'could not rule out') average currents more than a small fraction of that!

    Kind Regards, John
     
  16. plugwash

    plugwash

    Joined:
    28 Mar 2004
    Messages:
    9,060
    Thanks Received:
    288
    Country:
    United Kingdom
    it's not the conductors you have to worry about, it's the insulators.

    Remember heat generation and hence temperature rise over ambiant is determined by power dissipated in the cable which is (to a first approximation) proportional to the square of current.

    So if we assume under normal max load a cable has a temperature rise of 40C over ambient then at 3x overload we would expect it to have a temperature rise of 360C over ambient. That is plenty to melt PVC.
     
  17. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    42,451
    Thanks Received:
    2,695
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I realise that, and when I was talking about 'cables melting', I was talking about the insulation, noit the conductor, melting.
    I cannot disagree with your arithmetic. I suppose I was subconsciously thinking of a situation (which, in reality, would be very common) in which a 3 x max current was not going to flow for a 'long period', such that the theoretical peak temp would probably never be even approached.

    However, if one wanted to quibble, I think there are at least two factors which probably make the situation somewhat 'less bad' than you figures suggest. Firstly, whilst heat generated in the conductor will be essentially proportional to I², the same will also be approximately true of heat loss from the conductor (since that is roughly proportional to temp difference, which itself is roughly proportional to I²) - thereby presumably meaning that the equilibrium temp would be appreciably (maybe even considerably) lower than your calculation suggests.

    Secondly, I suspect that the cable 'ratings' are very conservative. There tends to be an assumption that if a "70°" or "90°" cable is loaded with its 'maximum rated current, its conductor temp will rise to 70° or 90°, but I rather doubt that such is even remotely true. It's hard to know what temp one would expect on the outside of the insulation/sheathing, but I'm sure I'm come across some 'loaded to max' cables in my time, and I have never come across one that feels 'seriously hot' (and would have worried if I had!) - remembering that even 70 (if one could touch the conductor' is 'far too hot to touch'. Don't forget that implicit in the published 'ratings' is the fact that a cable 'rated at', say, 100A is deemed to be able to carry 145A for an hour without coming to any harm (and, I suspect, probably not even coming to its 'rated temp').

    Kind Regards, John
     
Sponsored Links
Loading...

Share This Page