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Electrical

8.5kw shower on existing 6mm installation

Anonymous user 09/03/2024 - 3.21 PM

I'm getting a new bathroom and was going to replace the existing 8.5kw shower with a newer one of the same power. The bathroom fitter is hesitant about using the existing 6ml cable (40a mcb and on a split board with RCD protection). He wanted to put in 10ml cable until he came round and we didn't see an easy route that won't make a lot of mess. Now he's suggesting a mixer shower. The cable run is quite long, maybe 15m and does go through loft insulation for the last couple of meters. Am I just lucky that I've had no problems yet? Or should I be OK with the existing cable?

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4 Answers

KV Smart Homes Ltd

Rating: 5 out of 5

Horsham

You’re lucky! As a standard I’d use one size above the wattage, I.e. 8.5kW is 10mm2. Various voltage drop calculators show that over 15m you’d need a 10mm2, especially when it runs through an unspecified amount of insulation.

Answered26 March 2021

Willow Electrical Services Limited

Rating: 5 out of 5

Shoreham By Sea

8.5kw will pull around 36.956amps so if you take diversity into account @80% of ohms law total your fine fella. Yeah a 10mm would be best but why should this existing circuit and MCB trip out now if it hasn’t ever done before?? Don’t blame the bathroom fitter for being cautious as he’s not a sparky and also I have to agree with him that a mixer is such a better idea compared to an electric shower if it is possible simply because of the ease of temperature control and especially the water pressure as electric showers simply can’t compete against a mixer!

Answered28 March 2021

Evergreen Electrical Ltd

Rating: 5 out of 5

Swanley

I agree with Willow Electrical Services Page 409 of BS7671 shows that 6mm cable is adequate up to 47 amps if "clipped direct". If covered by thermal insulation not exceeding 100mm, 34 amps. Exceeding 100mm then it drops to 27 amps. If that's the case, then upgrade to 10mm Whatever you decide, it must be RCD protected

Answered9 April 2021

AJ Dixon Electrical’s

Rating: 5 out of 5

Darlington

Iv just installed a new shower because the old one (8.5kW) was wired in 6mm with a 32A MCB, for 2 years the fuses was blowing at the consumer unit. Until the fuse holder melted in to the board. Moral of the story is use 10mm on anything bigger than 7kW and 16mm on 10-10.5 kW as standard practice.

Answered1 April 2021