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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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    the 90c thing is most use for a mixed route - so the ends may be better cooled than the middle for example.

    Also although the cores may be at 90 in the centre of the bundle, once the cable is split to singles, they are much better cooled, so in practice the terminal temperature has more to do with the environment. This is especially significant for thinner cables that have more surface area relative to cross-section.
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  • 0
    the 90c thing is most use for a mixed route - so the ends may be better cooled than the middle for example.

    Also although the cores may be at 90 in the centre of the bundle, once the cable is split to singles, they are much better cooled, so in practice the terminal temperature has more to do with the environment. This is especially significant for thinner cables that have more surface area relative to cross-section.
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