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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
    OK - perhaps this will help:

    Built-in components 

     

    In accordance with the relevant product standard requirements for the individual components or, in accordance with the component manufacturer’s instructions , taking into consideration the temperature in the assembly.


    So if the external insulated cabling connects directly to (in this case) the circuit breaker output terminals the switchboard manufacturer states the limit of the connected conductor (often 70C as directed by the circuit breaker supplier) - this would be in COTS switchgear with minimal form of separation


    However:

    Terminals for external insulated conductors  - 70K


    This would be the case in say a Form 4 Type 6 Switchboard, where the circuit breaker output terminals are extended by the switchgear manufacturer to the glanding/termination enclosure for connection to the external insulated enclosure. In this case, the temperature rise on the terminals could be 70C (from a 20C ambient) so would put at risk the 70C cable insulation, but would allow you to make full use of the 90C capability of XLPE


    You can play safe and use XLPE insulated cabling constrained in operation to 70C for the design current and that way, you can comply (probably) with no further knowledge of the switchboard design criteria - but you don't have to if you know what the switchboard is designed to do


    Regards


    OMS




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  • Former Community Member
    0 Former Community Member
    OK - perhaps this will help:

    Built-in components 

     

    In accordance with the relevant product standard requirements for the individual components or, in accordance with the component manufacturer’s instructions , taking into consideration the temperature in the assembly.


    So if the external insulated cabling connects directly to (in this case) the circuit breaker output terminals the switchboard manufacturer states the limit of the connected conductor (often 70C as directed by the circuit breaker supplier) - this would be in COTS switchgear with minimal form of separation


    However:

    Terminals for external insulated conductors  - 70K


    This would be the case in say a Form 4 Type 6 Switchboard, where the circuit breaker output terminals are extended by the switchgear manufacturer to the glanding/termination enclosure for connection to the external insulated enclosure. In this case, the temperature rise on the terminals could be 70C (from a 20C ambient) so would put at risk the 70C cable insulation, but would allow you to make full use of the 90C capability of XLPE


    You can play safe and use XLPE insulated cabling constrained in operation to 70C for the design current and that way, you can comply (probably) with no further knowledge of the switchboard design criteria - but you don't have to if you know what the switchboard is designed to do


    Regards


    OMS




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