Shower - can I replace an 8.5kw with a 9kw, and keep 6mm wiring?

buyitall

Hi,
Could anyone confirm that it would be OK to replace an existing 8.5kw shower, with a 9kw one (specifically, the Mira Sport Air Max http://www.screwfix.com/p/mira-sport-max-with-airboost-manual-electric-shower-white-9kw/9694f ), without replacing the existing 6mm wiring?
Reviews on Screwfix and Amazon say that purchasers are running it successfully on 6mm wiring, but I just wanted to double check?
We have no problems with the current shower, but are replacing it as part of a bathroom refit (for various reasons we have to have an electric shower, I know they're not the best - just wondering if this particular one is OK for the 6mm wiring ).

Thanks for any advice

paulpud

The theoretical current draw is just over 39 amps and, depending on how it is installed, 6mm T+E can handle up to around 47 amps so based on that data it would be fine. In less than ideal situations (long run of cable, cable run in insulation etc.) the cable rating is reduced and there is a possibility that the cable could get ever-so-slightly warm but I wouldn't be overly concerned, especially taking into account the relatively short time a shower is operational in one session. You'll need to ensure that there's a 40A mcb protecting the circuit and in the interests of safety it would also be sensible to have RCD protection as well.

jakemillspaek

May I ask why are you replacing the 8.5kw shower if it is fine?

buyitall

jakemillspaek wrote: »

May I ask why are you replacing the 8.5kw shower if it is fine?

Because it's very old (over 15 years), so we are replacing it at the same time as retiling the walls.

buyitall

paulpud wrote: »

The theoretical current draw is just over 39 amps and, depending on how it is installed, 6mm T+E can handle up to around 47 amps so based on that data it would be fine. In less than ideal situations (long run of cable, cable run in insulation etc.) the cable rating is reduced and there is a possibility that the cable could get ever-so-slightly warm but I wouldn't be overly concerned, especially taking into account the relatively short time a shower is operational in one session. You'll need to ensure that there's a 40A mcb protecting the circuit and in the interests of safety it would also be sensible to have RCD protection as well.

Thanks for the detailed reply

cyclonebri1

jakemillspaek wrote: »

May I ask why are you replacing the 8.5kw shower if it is fine?

It's called a "pre-emptive strike", :rotfl::rotfl::A

jk0

This is going to sound a bit mad, but have you considered keeping the 15 year old shower?

I have a number of rental flats, and find showers last 3 years max nowadays, no matter how much I spend on them. Older models seem to have been built to last.

macman

If you are retiling anyway and effectively gutting the bathroom, I'd take the opportunity to upgrade it to 10mm, as the next time you change it you'll probably put in a more powerful shower. Future proof it now. As mentioned above, an RCD on a shower circuit is not a legal requirement, but not putting one in is a really false economy.

Risteard

macman wrote: »

As mentioned above, an RCD on a shower circuit is not a legal requirement

It is a requirement of the Wiring Regulations, however.

macman

But not retrospectively on an existing circuit. If the cabling were uprated to 10mm, it would still not be a 'new' circuit.

albert_s

OK, wiring regs BS7671 state that manufacturers instructions must be followed,
I'll bet you a pound to a penny the instructions for the shower say that a 30mA RCD is required.
the wiring regs are NOT statutory, but part P of the building regs are, and they state that BS7671 should be adhered to, unless you have sufficient knowledge to deviate from them you could have massive problems in the future.
replacing 6mm cable with 10mm cable IS a new circuit, you are not changing like for like,
to use a 40amp MCB on a 6mm cable is extremely 'dodgy' to say the least, how much insulation does the 6mm cable pass through? its not unknown for even a 10kW shower to run quite happily on a 32amp MCB, it wont 'blow' instantly at 33amps, in fact it will happily hold at 40amps for the length of time it takes the average shower,
the MCB is NOT there for the shower, its there to protect the cable, and in some situations its not unknown for 6mm cable to have to be downrated to 25amps or less, so to put a 40amp on a 6mm cable without any testing or prior knowledge is madness at best, or a burnt down house , or worse.
I cant post pictures, but I have a few of badly installed showers, and the devastation it can cause,
remember, you cant see, hear, or smell electricity, its too late when it goes wrong.