A question for an electrician

[Canetoad]

I was wondering what the power rating was for the electrical cable to my electric shower. The shower is 7.5Kw Mira sport and the cable has 7 conductors (I think, my eyesight isn't as good as it used to be) for the live and neutral feeds. They look like .4 or .5mm width for each strand.

Does anybody know? :scratch:

[Martinh]

Hi Bernie,

Generally speaking, you should put in a 10 mm2 twin and earth cable for an electric shower. For a 7.5 kW unit, you may be OK with a 6 mm2, but you may not be able to upgrade to a more powerful unit in the future.

However, there are factors that will derate the cable, such as mounting method, ambient temp, cable length etc. that will affect the final decision.

Only a qualified electrician will be able to advise you, once he's seen the installation and done the maths.

To identify the cable you have, what's the size across the outer sheath? Does it have a stranded or solid earth wire?

Hope this helps,

[Ali Tait]

Yes, better to use 10mm. A lot of sparks will fit 6mm because it's cheaper, it suffices but it's close to it's max rating, so much better to go for the 10mm.

[Ali Tait]

To clarify, P=IV, which means power (watts) equals I (amps) times V(voltage)

So, for a 7.5Kw shower, that's 7500 watts. You know the voltage, 240v. So that means a 7.5Kw shower will use 31.25 amps. Now, 6mm cable will carry 47 amps at best, which sounds fine, but as has been said, you need to allow for derating for insulation and cable length etc. which can mean you have to derate by as much as 50%. Not looking quite so good now is it?

You also need to find out what the shower manufacturer recommends in regards to RCD protection, and whether your earth bonding arrangements are suitable.

Depending on your particular circumstances, the 6mm may well be fine, but to reuse it, you really need to have the cable Meggered to check insulation resistance and between live neutral and earth.

Hope this helps.

[The Grand Wazoo]

My advice would be that if you have to ask such a question, you should stand well back and allow someone qualified to do it.

[Canetoad]

I'm not installing the shower. It's what was installed when the house was built. I was just curious as to the amp rating if I decide to put in an 8.5-9.0 kW version.

[Covenant]

I know this is not relevant to the question asked but I want to remove our 10kw shower because the electricity bill shot up after it was installed. I hope the gas combi can cope with two showers though it's unlikely that both would be used at the same time.

[Martinh]

I know this is not relevant to the question asked but I want to remove our 10kw shower because the electricity bill shot up after it was installed. I hope the gas combi can cope with two showers though it's unlikely that both would be used at the same time.

If you know your electricity tariff, you can easily calculate the cost of taking a shower.

At 12p per KWh, the cost will be £1.20 per hour (10 KW at 12p). If you shower for 5 mins, it's going to cost a whopping 10p. Doesn't sound too expensive to me.

[Martinh]

I'm not installing the shower. It's what was installed when the house was built. I was just curious as to the amp rating if I decide to put in an 8.5-9.0 kW version.

HI Bernie,

If it is a 10 mm2 cable, you should be able to upgrade to a more powerful unit, subject to the deratings that we discussed above. You will need to review the MCB rating at the same time too and fit an RCD or RCBO.

An electrician will be needed, as he will have the tools required to measure the values for cable insulation, RCD trip time and trip point, loop impedence, check the earth bonding and CPC continuity, polarity etc.

I'd not risk it without a full test and certificate.

Cheers,

[Canetoad]

Cheers Martin, I'll get a professional to check it all if I decide to upgrade. :)

[Covenant]

If you know your electricity tariff, you can easily calculate the cost of taking a shower.

At 12p per KWh, the cost will be £1.20 per hour (10 KW at 12p). If you shower for 5 mins, it's going to cost a whopping 10p. Doesn't sound too expensive to me.

Do you have an 18 year old daughter with hair down to her arse Martin? Believe me you need to multiply the time by 4! :lol:

[Martinh]

Do you have an 18 year old daughter with hair down to her arse Martin? Believe me you need to multiply the time by 4! :lol:

:lol:

My daughter is only 6, but I already have an idea of what you mean :rolleyes: