Electric Showers

[Big_Spark]

BAS..just because the insulation of a cable can withstand a higher ambient operating temperature does not mean that the cable automatically can carry X amount of more current.

Of course there is a slight improvement, however it is nothing like the 27% you have considered...

The tables in the regs book give the operational current carrying capacities of different cables, however you also need to look at the quality of the copper conductors, how the conductor is constructed and what that cable is designed to be typically used for.

These all have a bearing..I know that most people think that copper is copper is copper....however the quality and annealing process is critical, as it the construction of the cores...

The type of insulation will have a bearing..but not significantly..

 

[smokeyJoe]

So, andy Mullin, It seems that your question is answered. The things to consider are:
Cost & ease of installation -
which would lead you to prefer 10mm2.

Thermal effects -
10mm2 will run hotter than 16mm2 which will shorten its lifetime. 10mm2 is likely to offer a reasonable lifetime when carrying the 46(ish) amps required for a 10.5kW shower, provided that you don't route it through a vacuum or some other thermally inert media.

Operation of protection device (fuse / mcb) under fault conditions -
In the nightmare scenario of your shower heating element vapourising and turning into a short circuit, you'd want a very high current to flow so that your fuse/mcb operated quickly. The terrifying arcing noises would last a bit longer if you install 10mm2 cable.
To determine whether 10mm2 cable will give an acceptable breaking time, more information about your installation would be required. BAS has used reasonable assumptions about the type and characteristics of your installation and calculated that 10mm2 would not be sufficient! Big_Sparks software recommends 16mm2 as well.

 

[ban-all-sheds]

BAS..just because the insulation of a cable can withstand a higher ambient operating temperature does not mean that the cable automatically can carry X amount of more current.

Yes it does. The more current it carries the hotter it will run. 70°C/90°C is nowhere near the melting point of any type of copper - the difference is what can be safely tolerated given the materials used for insulation. For thermoplastic that is 70°C and for thermosetting it is 90°C.

Of course there is a slight improvement, however it is nothing like the 27% you have considered...

With operating temperature limited to 70°C the cable can carry 67A.

With operating temperature limited to 90°C the cable can carry 85A.

85 ÷ 67 = 1.269

FYI - with bare pyro not exposed to touch and not in contact with combustible materials, where the sheath is allowed to reach 105°C, 10mm² has a capacity of 96A.

For rubber insulated flexible cable with conductors allowed to reach 85°C the capacity of 10mm² is 73A.

Do you not see that the higher you can allow the temperature of the conductors to rise, the more current you can carry?

There will be slight differences in the resistivity of different types/grades of copper, I'm sure, which will lead to different heating effects, and therefore slight differences in the capacity of conductors at the same temperature limit, but by far and away the biggest factor is the ability to run at higher temperatures.

Unless you think that a copper conductor itself can be damaged by being run at 90°C instead of 70°C, then the insulation is the only thing that can make a difference.

The type of insulation will have a bearing..but not significantly..

What other differences are there between PVC SWA and XLPE SWA that allow the latter to be run at 90°C and the former at no more than 70°C?

 

[andy Mullin]

thanks for the advice, i think i will go for the 16mm as i don't want the cable to get too warm especially where it goes through the loft and the cheapest i've found 16mm is 43 + vat. if theres any one out there near gnosall in stafford that would give me a price to connect it to the mains after i've run it through the house please give me a quote