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Current carrying capacity of XLPE/SWA cables

M Gilmore
Hi, I keep coming across the same issue in regards to the current carrying capacity of a XLPE/SWA cable. I always use the de-rated current carrying capacity when I am not sure if all associated equipment is rated at  90°c, which is all the time. So as an example, a 4 core 150.0mm² XLPE/SWA cable clipped direct is rated at 386 Amps from table 4E4A column 3 in BS7671:2018. A standard 4 core 150.0mm²  PVC/SWA is rated at 306 Amps from table 4D4A column 3. This is a big difference! If this was protected by a 355 Amp BS 88 I would flag the conductor as being undersized.


There is a note by table 4E4A that points out Regulation 512.1.5. This is regarding compatibility and says equipment should not be connected to conductors intended to operate at a temperature exceeding 70°c unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions. 99% of the time I wouldn't have the available data on site. Most circuit breakers I have checked are rated at 70°c.


Any thoughts? I just want to make sure I'm providing the correct information. I don't want to provide an observation if the conductor is ok to be rated at full capacity. Cheers in advance.


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    I think that this matter encapsulates the conflict between, on the one hand, being strictly compliant, and therefore conservative; and on the other hand using engineering judgement.


    I don't know what terminals are rated to 90 deg C, but I suppose that an industrial oven might be. I assume that in industrial installations, cables may run near their max temperature (so don't touch them else you will be off to A&E) but in domestic, commercial, and light industrial (i.e. small units in an industrial estate) I doubt that cables get nowhere near 70 deg C.


    As Mike has pointed out, cables bunched in conduit or trunking might well get to 90 deg C, but even a few inches of freedom in a panel would allow them to cool down.


    So if you err on the safe side, the worst thing that you have done is wasted a little copper and money.


    ETA: if this is in the context of an EICR, then I think that you do have to make a judgement call (C2 or C3 - I don't think that a non-compliance can be ignored) but back it up with reasoning in the Observations section.
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    I think that this matter encapsulates the conflict between, on the one hand, being strictly compliant, and therefore conservative; and on the other hand using engineering judgement.


    I don't know what terminals are rated to 90 deg C, but I suppose that an industrial oven might be. I assume that in industrial installations, cables may run near their max temperature (so don't touch them else you will be off to A&E) but in domestic, commercial, and light industrial (i.e. small units in an industrial estate) I doubt that cables get nowhere near 70 deg C.


    As Mike has pointed out, cables bunched in conduit or trunking might well get to 90 deg C, but even a few inches of freedom in a panel would allow them to cool down.


    So if you err on the safe side, the worst thing that you have done is wasted a little copper and money.


    ETA: if this is in the context of an EICR, then I think that you do have to make a judgement call (C2 or C3 - I don't think that a non-compliance can be ignored) but back it up with reasoning in the Observations section.
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