Three phase distribution board

Discussion in 'Electricians' Talk' started by rogerk101, Nov 27, 2020.

  1. rogerk101

    rogerk101 Screwfix Select

    I'm trying to wrap my head around the way in which to implement a three phase distribution board that suits my requirements ... requirements which are by no means unusual.

    I need three phases only for the pump of a heat pump. The whole of the rest of the system can be single phase.

    From the meter, a four core cable comes into a 4 pole isolator. That's fine. I don't want a three phase RCD there, as I don't want the whole house to go down just because the heat pump motor trips it or if any one of the phases used as monophase trips it.

    I therefore need a three phase RCBO for the heat pump motor.
    I also need EITHER three single phase RCDs (one for each of the balanced loads going to the rest of the house) OR a single phase RCBO for each of the circuits in the rest of the house.
    So, nothing complicated about all that.

    My current plan for the physical implementation in the three phase distribution box is as follows:
    • The 4 pole mains isolator sits at the bottom, and it feeds the two vertical columns.
    • On the left column I will mount a 3 phase RCBO for the heat pump motor.
    • On the right column I will mount RCBOs on each of the phases in a balanced way to feed each of the existing circuits in the house. If there are too many single phase circuits for the right column, I can use any remaining single circuits on the left column, and blak the rest.
    I assume this is how things are done nowadays (now that RCBOs are cheaper than they used to be).
    How would it be done if I wanted three single phase RCDs?

    I have been looking at Hager products. Does anyone have any thoughts on whether they're a good choice or not? Are there any other vendors I should be looking at? (Schneider? Legrand?)

    FWIW this is all going to be done in France, so please don't let's start the usual finger wagging exercise of telling me off because all this is notifiable. It's in France, so it's not. The electricity supplier upgraded the property to three phase yesterday and they didn't even bother to check what they were connecting to.
  2. ElecCEng

    ElecCEng Screwfix Select

    As long as you are balancing the loads across the three phases you should be fine. RCBOs in the 3 phase board will be far easier and allow you to have things fed off non-RCD protected breakers if needed. RCDs would need to be in a separate DB fed from a sub main off your 3ph board.

    Might also want to consider interlock between your loads. If you have a heat pump, high kW shower, high kW cooking appliances, car charger etc, you don’t really want all that on or starting at the same time.

    Also watch your MCB types for inrush current etc.
  3. Comlec

    Comlec Screwfix Select

  4. Bobby Dazzler

    Bobby Dazzler Active Member

  5. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Out of interest, why do you need RCD protection at your heat pump?
  6. Pete Jones

    Pete Jones Active Member

    Teki likes this.
  7. rogerk101

    rogerk101 Screwfix Select

    I'm thinking of having no RCDs at all.
    A three phase RCBO for the heat pump motor.
    Single phase RCBOs for each of the existing circuits.
    If I were to have any RCDs, why do you say they would need to be in a separate DB fed from a sub main off the 3ph board?

    The hot water cylinder is only about 2kW and will be on its own phase. The oven is also only about 2kW, so will be on a different phase from the oven. The clothes washer and the dishwasher will probably be on their own phase too. There are no other power hungry devices.
    FWIW, French houses commonly use something called a délesteur, which prioritises power hungry devices so that they don't run at the same time.

    When I eventually decide what heat pump I'll be using, I'll check if it needs anything special in the way of MCB curve types.

    Thanks for the input!
  8. rogerk101

    rogerk101 Screwfix Select

    I had in mind something like this (
    It's the configuration many 3phase systems use, and it gives huge amounts more space than the typical single phase domestic consumer units where everything seems more difficult then it should be because of lack of space.

    Because it's in France, it's probably best to go for the likes of Hager, Legrand, Schneider, etc., as there will be many spares and additional units available if ever I need anything.
  9. rogerk101

    rogerk101 Screwfix Select

    I can't answer that other than to ask why wouldn't I?
    I have the choice of 3 phase MCB without no earth leakage protection, three phase MCB + three phase RCD, or three phase RCBO. I'm currently steering towards the RCBO option, but am open minded if someone convinces me that earth leakage protection is unnecessary. It is after all an exterior circuit and could potentially be wet.
  10. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    It's your choice obviously, but as I understand you're fine to omit RCD protection in this case
  11. BiancoTheGiraffe

    BiancoTheGiraffe Screwfix Select

    Does your heat pump need the neutral?
  12. rogerk101

    rogerk101 Screwfix Select

    I don't know yet. I'm due to be getting the quotes in early next week. I don't even know yet what power rating the different installers are recommending.
  13. sinewave

    sinewave Screwfix Select

    When you say 'Pump' needs three phase, I assume you mean the Refrigerant Compressor and not the circulation pump?

    I doubt very much you'd get a 3 phase pump on a domestic ASHP or GSHP unit?
  14. rogerk101

    rogerk101 Screwfix Select

    I was informed that I need an air source heat pump of about 8kW in order to heat the building.
    Because the house is up in the mountains of the French Alps and right at the end of a small rural line, the maximum I can get out of a single phase is 9kVA (which is pretty much 9kW for normal loads).
    If the pump is running flat out and someone turns on a 2kW kettle, the electricity suppliers fuse or trip switch or whatever protection they use will trip. It's on their side of the meter so I wouldn't be able to fix it, so would have to wait for them to fix it.
    Bottom line is that there is not enough power available for the heat pump in monophase but more than enough on 3 phase ... hence the change.

    3 phase motors come is a wide variety of sizes and power ratings. What makes you think that I wouldn't get a 3 phase pump for a domestic heat pump? The heating engineers who are busy making up their quotes have all said I need a 3 phase system, and I very much doubt that they would say that if there none available ... not even if they're all French! Mon dieu!
  15. sinewave

    sinewave Screwfix Select

    Well if you answered the question it would help?

    Don't know why you're even asking on here if you and your 'electricians' already have the answers! :rolleyes:
  16. rogerk101

    rogerk101 Screwfix Select

    I would have thought my answer mentioning the 8kW requirement was pretty obviously for the pump for the refrigerant compressor, as no other pump in the system would need to come close to 8kW.
    The circulation pump for circulating the water around the underfloor heating exists already, is single phase and is not much more than 100W. That's been in place for the last 30 years and I see no reason to change it.
    If any other pumps are needed, they would also be of the order of 100W, so again, absolutely no need for 3 phase. This leaves just the motor for the compressor.

    My original question has been answered sufficiently for me to proceed.
    The only part of it that I haven't yet exhausted is whether it's worth getting two more RCDs for the two additional phases so that I can reuse all my old MCBs, or should I just buy 10 new RCBOs ... one for each of the existing circuits?
    I'll probably go the RCBO route, as that brings the house into the 2020 era electrically even though it was actually built in 1818.

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