60kW 3 x 3phase+N heater unit - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

60kW 3 x 3phase+N heater unit


I've a question on the best method of connecting to a 60kW heating unit (picture of terminal box below plus a cartoon of my circuit). I was led to believe this heater was a 60kW star configuration with neutral return and so had anticipated connecting via one supply cable. However, it is in fact 3 separate 20kW heaters (4-wire star configuration), the manufacturer has issued the unit with 12AWG (4mm2) heat-resistant wires from each heater bank and has advised to 'gang' (parrallel) them together.

Anyone have any comments?

I have calculated full load line current at approx 88 Amps. I've selected a 125Amp type B MCB and am using a 35mm2 SWA supply cable. (all checks out ok with BS7671, volt drop, Zs, derating factors eg grouping, ambient etc) I will be installing an isolator close to the heater units and from there (due to this wiring configuration) either use 3 x 5core silicon sheathed (180°C) cables to connect to each of the elements or continue with 4 x 35mm2 as singles in conduit (or 12 x 10mm2 singles) and parallel all 3 heater elements together in its junction box (the lead wires you see aren't long enough to terminate outside of the box and i'm just wondering whether to replace them alltogether with longer singles (180

Im told this is standard practice in the heating industry but in my mind i would have thought the 4mm2 lead wires albeit fairly short (and not big enough in my opinion) aren't adequately protected by the 125Amp breaker anymore, or does this fall under parrallel cabling rules, do they actually need their own branch circuit protection or is it ok to have 3 smaller cables in paralell like this?? 

ANy advice greatly appreciated.

  • I too am a little confused by the photo - I suspect I'm being mislead by the assumption that the capped terminals are line and the uncapped ones N - if it were the case that L1 was uncapped and L2, L3 and N capped it almost makes sense - but without knowing for use where the ends of each element appear, it's difficult to be sure.

    If we can assume that the 4mm² only need fault protection (i.e. the individual elements can't overload, or short to case in a way that'll be detected and disconnected - e.g. by upstream RCD) then it's just a matter of checking the cable withstand against the let-through of the protective device. The devil will be in the detail for that one (possibly unusual values for 'k' as well as needing manufacturer's data for the circuit breaker) but it may well be OK.

    The upstream SCRs will likely alter the earth fault loop impedance - so it might be worth double checking how you're providing shock protection downstream of them. (In practice it may well be by disconnection by magic smoke escape from the electronics) but a more formal proof might be needed.

       - Andy.

  • Yes it took me a while to study the photo as I was also very confused! Oddly they have ceramic caps on L1, L2 and N but not on the L3s …?? … Maybe it is a space constraint thing. 

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