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Domestic upgrade to 3 phase...

D4G

Hi All 

I have a customer that is upgrading his supply to 3 phase. Not entirely sure why but wants a 22kW EVCP.

He currently has a single phase supply feeding 2 consumer units which are under the stairs.

He is looking to get the 3 phase supply to the side of the property and requires connection to the existing wiring.

I do a lot of industrial 3 phase work but not domestic!

My idea is to run a suitable cable from the new supply to under the stairs and fit a 3 phase cu to pick up the existing wiring. 

Should I be making any efforts to balance out the load on the supply?

Could the 3 phase supply just be split and feed the existing consumer units??

Thanks in advance Grinning

  • 0

    As you have 2 consumer units, one per phase may be the easier way to do this. The load on the street cable will not be any more or less unbalanced than it was yesterday, and this is not a factory with hundreds of amps per phase that dominates the substation.

    Now, if you were doing a full re-wire then to fit 3 single phase CUs or one 3 phase one and balance up the loading would be the preferred, but for now that is not essential. I do suggest a 4 pole isolator (example REC4 ) before any consumer units, so that there is still a very clear single point of isolation.

    If the CUs  have to come out anyway for some other reason then maybe rethink into 3 groups.

    Mike.

  • 0

    Agree that the simplest approach is to retain the two existing consumer units and connect one to each phase, Make certain that the new cable is suitable for ALL THREE phases and not just for any two phases. This will facilitate the later fitting of a third consumer unit for any significant extra loads, such as a heat pump, electric shower, or a single phase EV charger if required in addition to the three phase one.

  • 0
    D4G said:
    My idea is to run a suitable cable from the new supply to under the stairs and fit a 3 phase cu to pick up the existing wiring. 

    You haven't said anything about the route: it could be straight through the wall, or it could be quite lengthy. Will the EVCP be adjacent to the new intake or elsewhere?

    Squeezing in a 3-phase board under the stairs might be quite a struggle and it seems rather a waste for one 3-phase plus 2 single-phase distribution circuits.

    At the moment, I don't feel that we have sufficient information to provide useful suggestions.

  • 0

    Thank you for your thoughts.

    Some further information:

    The new intake will be one room (along one wall) away from the existing equipment under the stairs, approximately 12 meters.

    The EVCP is currently installed close to where the new intake will be, the charge point is 3 phase but currently running 1 phase 7kW (was bought with the upgrade in mind).

    Thinking about it, it may be best to fit a 3 phase CU at the new supply, reconnect the EVCP to this CU and feed either 1 or 2 phases through to the existing CUs.

    Is there any safety issues with CUs on different phases with 400v potential shock for example the cooker is fed from DB1 and the kitchen sockets from DB2

  • 0 in reply to D4G

    No issue, as we hope that in normal operation bare metal at the two phase voltages will not be simultaneously accessible..

    or indeed, accessible at all.

    Years ago if you brought more than one phase within 6 feet you needed a warning label, and in the same box, an insulating barrier. Modern plastics and earthed construction, have caused that requirement to be removed about 50 years ago...

    Mike

  • 0 in reply to D4G

    What size M.C.Bs would you use for the two existing consumer units from the new proposed three phase board?

    Z.

  • 0

    What size M.C.Bs would you use for the two existing consumer units from the new proposed three phase board?

    Or preferably not an MCB at all, but something that might discriminate with the downstream devices in the CUs - HBC fuse or certain types of MCCB perhaps.

       - Andy.

  • 0 in reply to AJJewsbury

    Wouldn't say, a C63 M.C.B. suit, installed in the new three phase board to supply a single phase consumer unit?

    Z.

  • 0

    it would but if there was a dead short on the ring say, then the odds are good that the 32A and 63A would fire more or less equal chances of either, or both. This should be so rare as to not matter.

    Mike.

  • 0 in reply to D4G
    D4G said:

    The new intake will be one room (along one wall) away from the existing equipment under the stairs, approximately 12 meters.

    The EVCP is currently installed close to where the new intake will be, the charge point is 3 phase but currently running 1 phase 7kW (was bought with the upgrade in mind).

    Thinking about it, it may be best to fit a 3 phase CU at the new supply, reconnect the EVCP to this CU and feed either 1 or 2 phases through to the existing CUs.

    So you are going to need protection for the distribution circuits. A TPN board at the origin would be fine, but you could also split the supply and use two switch-fuses (one at 32 A for the EVCP, and one at 63 A to 100 A for the house). If you use a board, 63 A MCBs will be fine. A fault that will trip one should be a very rare event, and I doubt that more than 63 A x 2 will ever be drawn for any length of time - think of the cost!

    If you need RCD protection for the distribution circuits, it begins to look messy.

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