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70C PVC thermoplastic data in BS 7671 instead of Acutal 90C XLPE SWA thermosetting cable

Former Community Member
Former Community Member
if I am designing a circuit where, due to installation conditions, I was required to use SWA 90C XLPE thermosetting cable. The cable is de rated to 70C as it will be bunched with other circuits on a cable tray and then buried in duct in another part of the run. My question is, if I am derating the XLPE to 70C, do I use the data given for 70C PVC thermoplastic cables in the absence of manufacturers data? like treating the XLPE as same as the PVC cable ??


here is the design question


" You are required to calculate the supply circuit for a 41 kW 400 V three-phase injection-moulding

machine with a power factor of 0.87. The 36 m circuit will be run from a distribution board at the

origin of the installation using three-core 600/1000 V 90 ˚C thermosetting steel-wire armoured

cable with circular conductors. Due to the logistics of the installation, the circuit will be run on

existing cable tray. The circuit will run next to seven other loaded circuits, some of which have a

maximum operating temperature of 70 ˚C. All the cables will be run in one flat layer, but without

gaps between.

"
  • 0
    You do. If your 90C cable is at a maximum load temperature of 90C then it will apply extra heat to the 70C cables overheating them. Take care with this question, think carefully about the installation method. The manufacturer's data will closely match BS7671 figures.


    Regards

    David
  • 0
    I agree - the simplest approach is just to use the 70-degree (PVC) tables.


    An alternative might be to use the 90-degree XLPE tables and apply a de-rating factor to bring the conductor temperature down to 70-degrees (there's a formula somewhere) - which might give you a slightly higher rating (as XLPE has a somewhat better thermal conductivity than PVC) - although BS 7671 does seem to encourage us to use the 70-degree tables approach.

       - Andy.
  • Former Community Member
    0 Former Community Member
    I was thinking the other way round, I will use the 90C SWA XLPE cable and derate it to 70C as this approach is better because the other 70C circuits are already installed.
  • 0
    Azam:

    I was thinking the other way round, I will use the 90C SWA XLPE cable and derate it to 70C as this approach is better because the other 70C circuits are already installed.


    Is a new PVCI cable rated for 70°C available and cheaper?

    Jaymack


  • Former Community Member
    0 Former Community Member
    yes, but some part of the cable run will be buried underground in a duct. This cable will be subject to heat and mechanical factors. so a 70C PVCI does not seem to meet the requirements.

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