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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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    ... if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

     




    Good point. 


    Doing my best to not continue the thread-hijack too far... but..


    my question was brought to mind when reading one of the [factory] test methods for AFDDs.  There are few tests but the gist of them is to cause damage to a sample cable, connect it to the subject AFDD and prove the AFDD picks up any arcing.  One test uses 7kV+ to "condition" an already damaged sample, to create an arc-track between cores... and in theory an arcing fault for the AFDD to detect. 


    I obviously queried to myself whether the tests are at all realistic and wondered if the one I mention tries to mimic a pinched cable that later via some current flow between cores (L-N) develops a fault that then becomes partially carbonised...


    Based on the dielectric strength of even thin insulators, the above looks doubtful.


    Thanks,

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  • 0

    ... if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

     




    Good point. 


    Doing my best to not continue the thread-hijack too far... but..


    my question was brought to mind when reading one of the [factory] test methods for AFDDs.  There are few tests but the gist of them is to cause damage to a sample cable, connect it to the subject AFDD and prove the AFDD picks up any arcing.  One test uses 7kV+ to "condition" an already damaged sample, to create an arc-track between cores... and in theory an arcing fault for the AFDD to detect. 


    I obviously queried to myself whether the tests are at all realistic and wondered if the one I mention tries to mimic a pinched cable that later via some current flow between cores (L-N) develops a fault that then becomes partially carbonised...


    Based on the dielectric strength of even thin insulators, the above looks doubtful.


    Thanks,

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