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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.


Parents
  • 0
    in general installation, the concern is mechanical. The break down voltage of undamaged insulation is far higher than the working voltage.

    If you want a feel for what minimum insulation thickness could be if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

    At higher voltages (many kV) then the insulation needs to be re-inforced accordingly. Even so at DC a cm of polythene will easily hold off 150kV,  but no cable maker would dare sell that as a working voltage for genera installation.

      (you need to be  more careful than a simple breakdown figure for AC because of displacement current causing corrosive corona at the air to insulation interface, so there is an additional bend radius to consider. There are tricks such as grading insulation as a series of cylinders with equipotential metal foils in between to force a regular voltage profile and smooth over any bubbles. Corona dicharge in any defect or  bubble in the insulation will lead to early failure.)
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  • 0
    in general installation, the concern is mechanical. The break down voltage of undamaged insulation is far higher than the working voltage.

    If you want a feel for what minimum insulation thickness could be if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

    At higher voltages (many kV) then the insulation needs to be re-inforced accordingly. Even so at DC a cm of polythene will easily hold off 150kV,  but no cable maker would dare sell that as a working voltage for genera installation.

      (you need to be  more careful than a simple breakdown figure for AC because of displacement current causing corrosive corona at the air to insulation interface, so there is an additional bend radius to consider. There are tricks such as grading insulation as a series of cylinders with equipotential metal foils in between to force a regular voltage profile and smooth over any bubbles. Corona dicharge in any defect or  bubble in the insulation will lead to early failure.)
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