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60kW 3 x 3phase+N heater unit

Udtwmc

Hello,

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.

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    To be honest I'm struggling a bit to reconcile your sketch where 9 ends got to neutral and 3 ends go to each phase with what appears in  the photo,  I presume you have used meter and convinced yourself it is OK.  Obviously the neutral is not needed unless during the operation of the SCR controls, one phase may be cut off while the other 2 remain on - but for proportional controls where there needs to be a part way on as well as a fully on and an off,  this is quite possible so the neutral may well be needed.

    However, to set your mind at rest, 4mm2 copper will be good for a lot more current than normal 37A or so that is the PVC covered rating, when insulated like this. The down side is that what it connects to may not be that keen on the heat . Ceramic choc bloc may be your friend here where you transition to larger cable but in something less heat resistant, though that example I linked to may not be quite enough.

    You are in the right ball park with 80-88 amps per phase when running, so you need  16mm or more cable for the feeder and 100A or higher breaker. I presume you have gone up a bit for length and voltage drop or routing environment  reasons.

    How hot is this wiring box expected to get during operation ? - I assume they have not used high temperature wire for fun, so you need to do your connections either in something equivalent, or use those tails and have a cooler junction box nearby but safely  out of the worst of the heat.

    More questions than answers I think but keep us updated.

    Mike

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  • 0

    To be honest I'm struggling a bit to reconcile your sketch where 9 ends got to neutral and 3 ends go to each phase with what appears in  the photo,  I presume you have used meter and convinced yourself it is OK.  Obviously the neutral is not needed unless during the operation of the SCR controls, one phase may be cut off while the other 2 remain on - but for proportional controls where there needs to be a part way on as well as a fully on and an off,  this is quite possible so the neutral may well be needed.

    However, to set your mind at rest, 4mm2 copper will be good for a lot more current than normal 37A or so that is the PVC covered rating, when insulated like this. The down side is that what it connects to may not be that keen on the heat . Ceramic choc bloc may be your friend here where you transition to larger cable but in something less heat resistant, though that example I linked to may not be quite enough.

    You are in the right ball park with 80-88 amps per phase when running, so you need  16mm or more cable for the feeder and 100A or higher breaker. I presume you have gone up a bit for length and voltage drop or routing environment  reasons.

    How hot is this wiring box expected to get during operation ? - I assume they have not used high temperature wire for fun, so you need to do your connections either in something equivalent, or use those tails and have a cooler junction box nearby but safely  out of the worst of the heat.

    More questions than answers I think but keep us updated.

    Mike

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