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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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    ...Pretty well any manufacturer providing BS EN 61439 compliant switchgear - although I think we are at cross purposes - I was pointing out that the switchboard terminals at full load may already be at 90C - and that impacts the cabling. I think you are suggesting the opposite ie the cable is at 90C and influencing the terminals


    ... many manufacturers struggle to comply with the requirements (and thus limit the cable temp to 70C, to give the switchgear some headroom




    I had not considered it from that angle, i.e. the extra heat-transfer to the switchgear might actually push the switchgear over 90C, rather than just it could not safely operate between 70C and 90C.  Albeit this does still leave us in the situation where we should assume 70C unless we can confirm that  higher conductor operating temperatures will be sustainable.

     

    Thanks OMS.

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    ...Pretty well any manufacturer providing BS EN 61439 compliant switchgear - although I think we are at cross purposes - I was pointing out that the switchboard terminals at full load may already be at 90C - and that impacts the cabling. I think you are suggesting the opposite ie the cable is at 90C and influencing the terminals


    ... many manufacturers struggle to comply with the requirements (and thus limit the cable temp to 70C, to give the switchgear some headroom




    I had not considered it from that angle, i.e. the extra heat-transfer to the switchgear might actually push the switchgear over 90C, rather than just it could not safely operate between 70C and 90C.  Albeit this does still leave us in the situation where we should assume 70C unless we can confirm that  higher conductor operating temperatures will be sustainable.

     

    Thanks OMS.

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