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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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  • 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.



    That's helpful. So if the manufacturers' data are not available, get a copy of the relevant standard.

    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.

    In other words, allow for diversity. I think that the OP was concerned (as I would be) that the difficulty that many (most?) testers would have is verifying the design assumptions and calculations if the records are absent.


    It may be relatively easy to get to grips with the design assumptions. Let's take the example of a small joinery business, initially with a sole trader who has half a dozen machines plus chip extraction. Clearly working hours are limited, only one machine will be in use at any one time, use is intermittent, motors go on and off load as planks are fed in and spat out again, etc. It may be necessary to allow for future developments - e.g. the joiner takes on an assistant.


    It may well be possible to plug all the assumptions into the right software, but at the lighter end of the scale, it is easy to see why testers (and designers) find it easiest to ignore diversity.
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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.



    That's helpful. So if the manufacturers' data are not available, get a copy of the relevant standard.

    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.

    In other words, allow for diversity. I think that the OP was concerned (as I would be) that the difficulty that many (most?) testers would have is verifying the design assumptions and calculations if the records are absent.


    It may be relatively easy to get to grips with the design assumptions. Let's take the example of a small joinery business, initially with a sole trader who has half a dozen machines plus chip extraction. Clearly working hours are limited, only one machine will be in use at any one time, use is intermittent, motors go on and off load as planks are fed in and spat out again, etc. It may be necessary to allow for future developments - e.g. the joiner takes on an assistant.


    It may well be possible to plug all the assumptions into the right software, but at the lighter end of the scale, it is easy to see why testers (and designers) find it easiest to ignore diversity.
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