10mm cable for shower - Why?

[Handyman]

I just installed a 9.5kw elecric shower. Regs required me to use 10mmT&E. But what's the point, when the internal wiring in the shower (to the heating element) is 6mm multi-strand on the live, and two bits of 2.5 on the neutral?

Is this another example of IEE regs being written for the benefit of lawyers, and not for best practice?

Why the need for 10mm cable?

 

[ChrisRogers]

I just installed a 9.5kw elecric shower. Regs required me to use 10mmT&E. But what's the point, when the internal wiring in the shower (to the heating element) is 6mm multi-strand on the live, and two bits of 2.5 on the neutral?

Is this another example of IEE regs being written for the benefit of lawyers, and not for best practice?

Why the need for 10mm cable?

if you are asking questions like this you should call in an electrician! A 9.5kw shower draws alot of current! if you wired it with small cable it would be overloaded, heat up and possibly cause a fire.

 

[Handyman]

if you are asking questions like this you should call in an electrician!

Now, that's really helpful, isn't it?

A 9.5kw shower draws alot of current! if you wired it with small cable it would be overloaded, heat up and possibly cause a fire.

This is obvious, and is why I asked the question. The wiring inside the unit is much less than 10mm, and from what I could tell (certainly on the neutral), less than 6mm. Therefore, this internal wire will be overloaded. Providing the capacity of the supply cable exceeds that of the internal wiring, why specify 10mm cable, when 6mm will suffice?

 

[paulh53]

Your shower will require about 40 amps, 6mm2 cable will take a current of 40 - 50 amps, (Depending on installation methods), The wiring within the shower unit is very short (About 6 - 8 inches) So 6mm2 cable will suffice
But the wiring to your shower is much longer (Maybe 15+ metres) which is prone to volt drop, ambient temp's, grouping, and thermal insulation factors, Which "downrate" the cable capacity, So requiring a larger size of cable to compensate for these discrepancies ,

 

[Handyman]

Ah, now that's a good answer. Well explained, Paulh. Fully understood, now. Thank you.

 

[ban-all-sheds]

It's also possible that the wiring within the shower has higher temperature rated insulation. Copper itself doesn't melt until over 1000C, so if you use something other than the normal PVC, you could run wires at a hotter temperature than normal.

 

[Handyman]

something other than the normal PVC

This, indeed, could well be the case, as the internal wires run very close to the hot parts of the little boiler.

 

[sterose]

I think this is on the same principle as the "fuse".

Fuses have a diameter much smaller than the cables connected to them, but function without burning up, except in the case where they are supposed to. However, the small length of wire in the fuse offers little resistance under normal conditions.

The 6sqmm cable inside your electric shower could be considered a weak link in the circuit. However, the shower's components are likely to be of high[er]-quality than just standard electric cable thrown in from 10ft away.

6sqmm cable is acceptable for up to about 40amps I believe. It can of course take much more than this - cables are not rated to their melting point.

The problem is that safer is better than sorry. A whole length of 6sqmm cable would offer a bit more resistance than just a small length in the shower - and possibly cause a fire hazard too.

As I have said before, just use the bigger stuff. It's safer. The shower's internal cables have been tested by the manufacturer to be safe under load; your installation hasn't.

PS. the regs include a margin of error. Although sometimes these rules have to be streched by knowledgeable individuals, they should not be blatantly broken for the sake of ease or for a few ££'s.

The key here is that you have no idea what load that cable can take, and under what conditions.
The majority of people don't - (some people can make informed and educated guesses, and a very small number of qualified electricians actually know the maths to work out cable tolerances) - and that is why the regs should be followed by people who have no clue.

 

[securespark]

with you 100% there sterose. Well said.

 

[ban-all-sheds]

...that is why the regs should be followed by people who have no clue.

Is this the same Sterose that advises people to repair damaged cables (even shower ones) with junction boxes and then plaster over them because "its not always convenient to dig a cable out of a wall just to satisfy regs"?

Who says that you should earth plastic pipes, and radiators fed by plastic pipes, and will brook no arguments because he knows he's right?

Who's so in favour of regulations being followed even if they don't understand them that he thinks that Part P is b*11*cks and has no intention of using electricians because of it?

Who advises people to move their electricity meter themselves if the supplier asks £too much?

Who thinks that AC means that the neutral conductor is at 240V 50 times a second, and when people tell him he's wrong he accuses them of being mad?


Surely not....

 

[Handyman]

Goodness, Ban, that's a bit strong. Though I haven't seen all this previous "sound " advice. Presumably, (if it is the same person), we are safe to assume that he/she would not fit an RCD in Zone 0 because the water would leak out of the mounting holes?

 

[ban-all-sheds]

That's one good reason, I suppose....

 

[joseph77]

This reminds me of when i used to install AV equipment, the sales people would insist on oxygen removed speaker cables etc..... When you took the back off the speakers it was actually conected using bell wire. Although it wasnt a matter of burning your house down with speakers.

Joseph

 

[AdamW]

Just a thought: what if you want to go for a more powerful shower at a later date? There have been posts from people pulling out 7-8KW showers and replacing them with 11kW showers. Who knows, in the future they might bring out 15kW showers for greater water flow during winter?

Joseph raises an interesting point. The reason being that this way the speakers are cheaper to make, and the use of bellwire means you need to spend £10 a metre on cables just to limit the damage to sound quality!

 

[securespark]

Adamw

As far as i know, the 50A limit (12kW @ 240V) will not be exceeded by showr manufacturers, as this is a practical limit.