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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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  • Former Community Member
    0 Former Community Member
    Take a look at table 6 in BS EN 61439 - that should tell you that the allowable temperature rise on terminals is 70K (usually the ambient is 20C) - so compliant switchgear could easily be operating at 90C terminal temperature when at full load - which will clearly melt PVC.


    XLPE insulated copper connected to compliant switchgear can happily run at 90C - you would expect the external surface temperature of the cable to be around 80C


    The problem usually arises at the load end - and often it can be shown that the actual connected load is usually less then the protective device setting (often significantly) and therefore usually well below the 90C rating of the conductor - even if the load ratio is much closer, loads often also don't operate continuously - so the cable time constant factor also becomes important (basically the point at where a change in load causes change in temperature (we usually work it out to a 66% change in temperature following a change in load ie you are working out the change from Time, T0 to T1 following the load step change)


    For the vast majority of cases the system is neither fully loaded nor constantly loaded - but it is a common mistake made by designers and then picked up by testers - where it becomes a shitstorm of acrimony and argument based entirely on "What Iffery" that has no bearing on reality


    Regards


    OMS


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  • Former Community Member
    0 Former Community Member
    Take a look at table 6 in BS EN 61439 - that should tell you that the allowable temperature rise on terminals is 70K (usually the ambient is 20C) - so compliant switchgear could easily be operating at 90C terminal temperature when at full load - which will clearly melt PVC.


    XLPE insulated copper connected to compliant switchgear can happily run at 90C - you would expect the external surface temperature of the cable to be around 80C


    The problem usually arises at the load end - and often it can be shown that the actual connected load is usually less then the protective device setting (often significantly) and therefore usually well below the 90C rating of the conductor - even if the load ratio is much closer, loads often also don't operate continuously - so the cable time constant factor also becomes important (basically the point at where a change in load causes change in temperature (we usually work it out to a 66% change in temperature following a change in load ie you are working out the change from Time, T0 to T1 following the load step change)


    For the vast majority of cases the system is neither fully loaded nor constantly loaded - but it is a common mistake made by designers and then picked up by testers - where it becomes a shitstorm of acrimony and argument based entirely on "What Iffery" that has no bearing on reality


    Regards


    OMS


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