Shower help

Matt Milner · 26 Feb 2005

Hi I recently replaced my old shower with a new Mira 10.8 KW new one. The instructions recommended that I could use 6mm cable up wards. As I live in a renovated barn it would have been a massive job to re-cable so I left the original 6mm cable in. After a week the doulble pole switch burnt out as I forgot to change it to a new 45amp one. Sorted that out but I did notice that for the last 10 years the showers MCB is not on the earth breaker side of the box. So for added protection I thought about putting one of these in http://www.screwfix.com/app/sfd/cat..../cat/pro.jsp?id=90026&slotName=null&homeRef= any suggestions?

andy · 26 Feb 2005

put shouwer on RCD side of CU.

how longis the 6mm cable? i would recommend you re wire it to 10mm.

Matt Milner · 26 Feb 2005

There isnt any more space to move it thats why i wondered wheter that devise from Screwfix would do the same thing. The distance betwwen the shower and the board is about 15 meters. It would be a massive job to rewire. Do you see a problem?

andy · 26 Feb 2005

Matt Milner said:
Do you see a problem?

yes. 6mm is no good for your shower. it should be 10. unless you want a fire to start. 6mm may be ok for short lengths (5M) but not 15. do you have an RCD in the CU? do you have a spare space on the RCD side? if notm then go for the shower CU. however, youll have to split the tails, which is only a DIYers job IF there is a isolator between meter and CU. if not youll be best of gettin someone to do that for you

Matt Milner · 26 Feb 2005

Blimey. It said in the install manual I could you 6 mm. Regarding an Isolator there isnt one I would have to pull the fuse in the metre box outside.

Thanks for your help ou have me worried now.

andy · 26 Feb 2005

Matt Milner said:
Blimey. It said in the install manual I could you 6 mm. Regarding an Isolator there isnt one I would have to pull the fuse in the metre box outside.

Thanks for your help ou have me worried now.

the longer the cable run, the thicker the cable. as a general rule, anything over 10KW should have a 10mm cable

as for the service fuse, i would recommend gettin the local REC to put an isolator in for you

Matt Milner · 26 Feb 2005

I think i will have to put a weekend to one side then and try and get all the boards up in the house. Will it really set on fire??

andy · 26 Feb 2005

possibly

theshogun · 26 Feb 2005

You dont give the size of mcb you have protecting the shower at the moment?R.C.D. protection is a very good idea.

ban-all-sheds · 26 Feb 2005

Matt Milner said:
I think i will have to put a weekend to one side then and try and get all the boards up in the house. Will it really set on fire??

6mm² is very borderline for that load. In free air it's OK, but anything that reduces the cable's ability to shed heat, like being bunched with other cables, or being in trunking or conduit or insulation means that it will not be OK.

felix · 26 Feb 2005

If 5m of 6mm cable is OK firewise then so is 15m - or 150m for that matter. The power dissipation PER METRE is the same in each case. Actually it goes down a little as the length goes up due to the real reason for using thicker cable on a long run. It's all about voltage drop.

According to my catalogue, 6mm cable is rated at 51 amps in free air but only 37 amps in conduit. Yours is in free air. I haven't got a calculator handy right now (thanks a bunch Microsoft) but some rough and ready mental arithmetic says that 15m of it is going to lose you about 5 volts. That's acceptable.

Matt Milner · 26 Feb 2005

I can confirm its running free. It isnt bunched together nor runs through any installtion either. I have re mesashered and its actually about 18 meters from shower to box. It is currently protected by the old 32 MCB but I have ordered a new 45amp one. I also want to add a r.C.D thats why I asked about the Screwfix one.

So is it still going to burn the house down?

felix · 26 Feb 2005

No more so than any other cable might. To eliminate hot spots, make sure all your joints are tight but don't get paranoid. You might just manage to start a fire with a bad joint inside a piece of choc block wrapped in PVC tape lying in a pile of old wood shavings - but who would be that stupid?

The answer is a cowboy electrician who rewired a flat for a penny pinching landlord who rented it to us thirty years ago. That was in my student days. The flat is still standing and I'd be very surprized if anybody else has found that choc block!

theshogun · 27 Feb 2005

you don't always have to uprate the mcb, a 32amp mcb should take 1.5times its rated current for a period of time before tripping normally upto 4 hours ,most people only shower for 10 minutes or so,the mcb should provide over current protection for the cable,as well as short circuit protection. If you uprate the mcb you should consider uprating the cable as well. The current rating of the cable depends also on the type of fuse /MCB used to protect the circuit as well as the length of the circuit .Installing a R.C.D is always a good idea considering the location.Make sure you have adequate supplementary earth bonding of the pipework if in copper

Matt Milner · 27 Feb 2005

I must admit it has not tripped the MCB as yet and I would rather have a lower amp han a higher one. There is one problem running a 32 Amp one is that when the shower is on the MCB buzz's a bit and get warm. It has burned the double pole switch out as well but thats because it was only a standard 30amp one.

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Shower help

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