Electric Showers

[andy Mullin]

i have a 10.5 kw showerand need to run a cable some 25 meters one electrician says 10 mm Twin & earth will suffice but others say it should be 16mm. i am led to believe that 10mm goes up to 9.5kw. any advice would be welcome

[supersparks]

at 25 meters your right on the edge...litrally at 26 meters it wont comply, id install 16mm personally

[ban-all-sheds]

I get it going non-compliant at 22.1m, assuming 0.8 ohms Ze and a B50 - where am I going wrong?

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[Big_Spark]

Not sure of when the non-compliance kicks in, but manually calculating at 25 m and using my cbale calc software the cable must be 16mm² for compliance.

So kick the 10mm² into touch and go with 16mm²

[ban-all-sheds]

Big_Spark wrote:

Not sure of when the non-compliance kicks in, but manually calculating at 25 m and using my cbale calc software the cable must be 16mm² for compliance.

Did it say why?

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[arctic]

where's it being run?

[ban-all-sheds]

If it's distance related it's not an installation method or other de-rating issue...

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[smokeyJoe]

Which rule(s) are we using to determine compliance here?
voltage drop (525-01-02)?
disconnection time assuming a TN system (413-02-08)?
impedance of the protective conductor to the main equipotential bond with a guess at the type of protective device (413-02-13)?
Please point me at the relevant chapter.
Is there really enough information here to make a sound decision?

[ban-all-sheds]

smokeyJoe wrote:

Which rule(s) are we using to determine compliance here? voltage drop (525-01-02)?

It shouldn't be hard to establish that it isn't that....

Quote:

disconnection time assuming a TN system (413-02-08)? impedance of the protective conductor to the main equipotential bond with a guess at the type of protective device (413-02-13)?

Those two are effectively the same, and assuming TN-S & B50 was how I got to 22.1m

Can't remember the figure for TN-C-S but it was a lot more....

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[Steve]

10mm² XLPE SWA? anyone?

[ban-all-sheds]

XLPE SWA is only rated as high as it is because the insulation can tolerate higher temperatures.

Run conductors at 90°C into normal switches and shower terminal blocks and expect trouble....

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[securespark]

ban-all-sheds wrote:

Can't remember the figure for TN-C-S but it was a lot more....

Mmmmm. PME has Ze of 0.35Ohm. It will be much higher.

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[Big_Spark]

ban-all-sheds wrote:

XLPE SWA is only rated as high as it is because the insulation can tolerate higher temperatures. Run conductors at 90°C into normal switches and shower terminal blocks and expect trouble....

BAS..the reason SWA XLPE or PVC/PVC has a higher current rating is actually due to the copper the cable is made from.

T&E is soft drawn annealed copper that is only 98% pure for compliance with the relevent BS, however SWA XLPE or PVC/PVC is hard-drawn annealed and 99.1% pure as a minimum..typically most seem to be about 99.4% pure copper..

The annealing process has an effect on the current carrying capacity of the cable, the insulation has a minor effect, but not to the extent that exists...

I do agree with the 90°C comment however..but if you ever find that in your home I think you'll already be dead..

[ban-all-sheds]

securespark wrote:

ban-all-sheds wrote: Can't remember the figure for TN-C-S but it was a lot more.... Mmmmm. PME has Ze of 0.35Ohm. It will be much higher.

That's what I said. I can't remember how much higher, and I can't be **sed to work it out again. I have a vague memory that volt-drop kicked in at a lower figure anyway.

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[ban-all-sheds]

Big_Spark wrote:

BAS..the reason SWA XLPE or PVC/PVC has a higher current rating is actually due to the copper the cable is made from. T&E is soft drawn annealed copper that is only 98% pure for compliance with the relevent BS, however SWA XLPE or PVC/PVC is hard-drawn annealed and 99.1% pure as a minimum..typically most seem to be about 99.4% pure copper.. The annealing process has an effect on the current carrying capacity of the cable, the insulation has a minor effect, but not to the extent that exists...

10mm² 2-core cables, single-phase, Reference Method 1:

4D2A 70°C Thermoplastic 63A
4D5A 70°C Thermoplastic (T/E) 64A
4D4A 70°C Thermoplastic SWA 67A
4E4A 90°C Thermosetting SWA 85A, together with a note to say that if you need the conductors to operate at 70°C you should use the 4D4A figure....

So I'll have to accept that the conductor material makes a slight difference (approx 5 or 6%) to the capacity.

And you'll have to accept that "the insulation has a minor effect, but not to the extent that exists" is wrong, as the ability to operate at 90°C adds another 27% to the capacity (67A -> 85A).

Quote:

I do agree with the 90°C comment however..but if you ever find that in your home I think you'll already be dead..

Meaningless.

You'd be dead if the ambient temperature got to 70°C too.

If you ran 85A through your 10mm² XLPE SWA, the conductor temperature would reach 90°C. You wouldn't die from that, but you might get some unfortunate consequences (probably not including fire, at least not immediately) if that conductor was screwed into something not designed to operate at 90°C.

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