Cables with thermosetting insulation (XLPE cables)

[PLW]

I understand that XLPE cables operate at a maximum temperature of 90c compared with 70c for PVC insulation. What I don't understand is why the increased temperature permits a reduction in conductor size if XLPE insulated cables are used in preference to cables having PVC insulation.

If the conductor size is reduced the resistance would increase if XLPE cable was used. I understand that the cable has special insulation to withstand the high temperatures, but why is it necessary to reduce the conductor size?
I was under the impression that high resistance and high temperature could potentially be a safety hazard due to the increased risk of overheating as well as having a negative impact on the performance of the cable.

Please can someone explain this. Thanks.

 

[Leesparkykent]

Unless the switchgear is also rated to 90degrees then you should use the CCC table for 70 degree.

 

[telectrix]

xple cable has a higher current capacity so the conductor size can be reduced wrt 70deg. cable. however, as lee pointed out, you must consider the temp rating of any switchgear connected to the cable.

 

[westward10]

The current carrying capacity of a conductor can be dictated by the applied insulation. The temperature of a conductor under load is transferred to the insulation and this must not reach a temperature which can degrade this insulation. Thermoplastic (PVC) will degrade at a lower temperature than cross layered polythene (XLPE) and this dictates its current rating.
Conductors with PTFE insulation will happily operate at 200°C.

 

[ImpededLoop]

Also bear in mind that the impedance of the conductors increases with temperature, so running at 90degC will increase volt drop, R1,Rn and R2.

 

[PLW]

xple cable has a higher current capacity so the conductor size can be reduced wrt 70deg. cable. however, as lee pointed out, you must consider the temp rating of any switchgear connected to the cable.

why would a higher current carrying capacity mean you'd reduce the size of the conductor ? I thought that for a higher current carrying capacity you'd want a thicker cable to reduce resistance ?

 

[Richard Burns]

why would a higher current carrying capacity mean you'd reduce the size of the conductor ? I thought that for a higher current carrying capacity you'd want a thicker cable to reduce resistance ?

As the XPLE cable can take more current for a given cross sectional area then if you have a specific current you need to carry the conductor size could be smaller for the XPLE insulated cable compared to the PVC insulated cable.

 

[Ricard]

The current carrying capacity of a cable is effectively limited by its capacity to dissipate the heat generated in the cable. If the temperature of the cable is higher, then more heat will flow away from it through the insulation and surrounding environment. If more heat can be dissipated (due to the higher operating temperature) then the cable can carry a greater current.
Hence a current carrying requirement that needs say a 6mm cable at 70deg c (pvc) may only need a 4mm cable if it can operate at 90 deg c (XPLE).

 

[Simonslimline]

Also bear in mind that the impedance of the conductors increases with temperature, so running at 90degC will increase volt drop, R1,Rn and R2.

The volt drop figures in 7671 are based on the cable being at its maximum operating temperature are they not!

 

[westward10]

The current carrying capacity of a cable is effectively limited by its capacity to dissipate the heat generated in the cable. If the temperature of the cable is higher, then more heat will flow away from it through the insulation and surrounding environment. If more heat can be dissipated (due to the higher operating temperature) then the cable can carry a greater current.
Hence a current carrying requirement that needs say a 6mm cable at 70deg c (pvc) may only need a 4mm cable if it can operate at 90 deg c (XPLE).

Much is to do with conductor temperature rise and its effect on the insulation. PVC will deteriorate at lower temperatures than XLPE hence the difference in ccc.