Re: Shower electrics

[BigWallop]

Hello,

> > We need to replace our electric shower. It's currently 9kW and we were > thinking of upgrading to 10.8kW. Its connected directly to the main > fuse box on a 30amp trip. A quick calculation shows that 30amp isnt > really enough for the current shower: 9000/240 = 37.5 amp. I'm quite > surprised it has never tripped. > > So it looks like we would need to upgrade to a 40amp or 45amp fuse. > However, our main circuit board is only rated at 60 amp total. We have > a gas cooker so i doubt we'd ever draw 60amp, but I guess we would > need to upgrade to a 100amp to be on the safe side? > > I also need to check the rating of the cable. Is there anywhere which > tells me roughly how the external dimensions of cable correspond to > the core sizes? > > Thanks. > > David.

The first thing I'm going to ask is, "Is the cable large enough to take this size of heating element ?"

You will have to reinforce the cable to at least a 10mm Two Core and Earth if it already isn't.

The reason the fuse hasn't blown is more than likely to be that it is a rewireable fuse with the wrong size fuse wire in it. An MCB (miniature circuit breaker) would not have been able to take the current load and would have tripped off at the first switch on of the shower.

You consumer unit (fuse box) is rated at 60 amps on the main switch and BUS bar and you should probably never reach this load even with everything in the house switched on.

More information about the whole installation as it is now, would help us make a better diagnosis of any problems you may get if you go ahead with the your proposed changes.

[David Ward]

Thanks for the reply. I'll try and give some more details.

We consumer unit is WYLEX brand. From right to left we have:

1. Main ON/OFF switch.
2. 30 amp fuse with trip. It has a push button which pops out when tripped. An there is a test button. This feeds the kitchen
3. 20 amp fuse with trip. Looks the same a 2. This feeds a single ring main (upstairs and downstairs).
4. 10 amp fuse with trip. Otherwise same as 2. This feeds the light circuit (also ring).
5. 30 amp "mini-trip" TYPE 2 - M3. It looks different to the other trips. This feeds the shower.

I estimate that the system is circa 1980's.

Looking at the shower cable, I think it is 6mm twin and earth (7 core). There is about 5 metres of cable from the consumer unit to isolation switch and < 20cm on to the shower.

I'd be quite happy to settle for another 9kW unit, but still concerned that our currect setup is not up to the job.

I hope this helps.

David.

[Christian McArdle]

Unlikely. A Type B MCB would have required a minimum of between 90A and 150A to trip as soon as it turns on (I can't remember the Type 2 characteristic). To trip on 37.5A, it would require many minutes to trip, if it bothers at all.

Christian.