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Cable sizing for three phase heaters

nElectric

Please bear with me as I am mostly dealing with single phase and am not used to designing 3ph circuits, and getting my head in a spin about three phase theory.

Am quoting to fit two ceiling mounted infra red heaters which are rated at 7.8kW each and can be either connected as single phase (7.8kW) or 3PH as 3x 2.6kW - which I calculate at 11.3A per phase. These are Hershel IR-7800-L heaters and the wording on the manufacturer's spec sheet is...

"7.8kW: 220-240V – Wired as 3x 2.6KW, line to neutral circuits, suitable for single-phase or 3-phase supply".

The supply board is 3ph so I am considering connecting as 3ph using a 4C+E cable with csa 1.5mm - cable ideally to be FP200 (which I have in stock) and is suitable for the environment which is a church.

Looking in BS7671 table 4D2A ref method C for 1 four core cable three phase 1.5mm csa is rated (before correction factors) at 17.5A. My question is this - is the 17.5A rating per phase i.e. per separate conductor within the 4-core cable?  And will this cable be ok to use for this circuit? It makes sense to me that each line core will be carrying a load of 11.3A but does this work for the neutral cable? Getting my head in a spin here regarding phase rotation for voltage but what about current - should current in the neutral conductor be considered as cumulative at 3 x 11.3 = 33.9A or can that be considered as rotating as well and therefore only 11.3A at any one split second?

Be gentle please - am trying to increase my knowledge and understanding.

Nick

  • +1

    four core cable ratings do indeed assume that at least 3 of the cores are fully loaded, so your 1,5mm will be fine for your 7.5kW, with each core taking 2.5kW (a touch over 10A each, 1mm2 wont quite do.) When all 3 elements are on, and perfectly identical,one on each phase the neutral carries nothing, in practice it carries very little.

    as one sinewave is going up, another is at peak and one is going down - they sum to zero...

    M.

  • +1

    In normal operation, neutral current will be almost zero and can be disregarded for cable rating purposes. If one element fails, then the neutral current will be the same as the phase current, but still only 3 cores loaded as the core connected to the failed element will be unloaded.

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