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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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    OMS:

    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


     




    Have you asked many switch gear manufacturer's if they would warrant such a situation?  How many would?  


    My experience is that perhaps only Schneider, on certain devices, might concede to allow conductor temperatures rise to 90C. 


    As far as a nay saying a"what if" argument, it is understandable that the conductor once arriving at the terminal would have had some unspecified opportunity to lose some heat.   However, as far as commercial risk goes, if the design (and therefore the designer) were already under some sort of scrutiny, it would be easy for someone to point to "incompatible" temperatures as a deficiency, unless of course you can prove by calculation that the conductor temperature will not significantly riser above 70C or that the switch gear manufacturer agreed that a rise to 90C is acceptable..

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  • 0

    OMS:

    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


     




    Have you asked many switch gear manufacturer's if they would warrant such a situation?  How many would?  


    My experience is that perhaps only Schneider, on certain devices, might concede to allow conductor temperatures rise to 90C. 


    As far as a nay saying a"what if" argument, it is understandable that the conductor once arriving at the terminal would have had some unspecified opportunity to lose some heat.   However, as far as commercial risk goes, if the design (and therefore the designer) were already under some sort of scrutiny, it would be easy for someone to point to "incompatible" temperatures as a deficiency, unless of course you can prove by calculation that the conductor temperature will not significantly riser above 70C or that the switch gear manufacturer agreed that a rise to 90C is acceptable..

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