Hi all,
Will be helping my Dad fit a new electric shower in a few weeks. My Mum has made him go for the most powerful one she could find which is
10.5Kw. For this we will need to add a 45A breaker to the CU which is an oldish Wylex unit with MCBs marked as NB05, NB15, etc. My question is, is it possible to get 45 amp rated MCBs to fit this board and if so, where? I've seen a few NB40 and NB50 ones on eBay, but no luck on the 45A front.And help muchly appreciated.
Thanks, Richard.
On Mon, 28 Apr 2008 13:16:56 +0100 someone who may be Richard Conway wrote this:-
I make that 45.7A @ 230V. I would fit a 50A one if that is available, but see below.
According to places like
and it appears they are only available new up to 40A. The latter mention that Proteus models can be fitted, so you might just get a higher rated one.
Amongst the things to consider before installing either is diversity. Will this be the only high power circuit, or is there a cooker or something else rated at more than 32A? Are they likely to be in use at the same time, if so for how long? Is the consumer unit (and the cables feeding it) going to be able to cope with the new load?
The next thing to consider is the cable size. If fitting a 50A MCB it may be necessary to go up a cable size. There is also the question of cable size with regard to the installation methods. For instance, will the cable run through insulation or be in a conduit?
Lastly, if you can't find a suitable MCB you may find a suitable fuse. However, the carrier will be two modules wide.
Except its probably 10.5kW at 240V. The manual will make clear. These numbers are not accidental but by design.
This is all too marginal for easy advice to a non-expert. Don't forget the installation instructions may *recommend* an RCD.
If the OP cares about his mum he should chose a shower rating which works with 32A 6sqmm. Or get it done properly.
And hey isnt't it Part P territory for a reason?
Jim A
Hi Richard,
I've just gone through this exercise for a friend. There is no 45A MCB for the Wylex only 40A or 50A in the MCB. Wylex do make a 45A fuse but there is no holder to put it into the existing fuse box. Not wanting to risk a 50A MCB I brought an MK IP65 Shower Unit. This comes with a 30mA RCD switch and and 50A MCB but you can buy 45A MCB for it. I got it from Screwfix, cost £42.99 for the unit and £4.57 for the 45A MCB.
Luckily there was a secondary fuse box connected to nothing so I was able to replace this for yourself it could mean a complete fuse box replacement which isn't nice!!
I hope it helps a bit but sorry its not what you want.
Take care, Ryan
On Mon, 28 Apr 2008 14:42:51 +0100 someone who may be "Jim Alexander" wrote this:-
32A in thermal insulation. 38A in conduit and 46A clipped direct.
I think you may be out of luck with this board. You could use a 50A breaker if you run 10mm^2 cable and a reasonably simple cable run without any need to de-rate the cable. Check the earth loop impedance carefully if you are not using a RCD.
On Mon, 28 Apr 2008 14:49:18 +0100 someone who may be Ryan wrote this:-
The 45A fuse is the same size as the 35A (and the 40A if they make one), though I wouldn't be rash enough to assume that a 35A carrier has as much metal inside it as a 45A one. 45A carriers are certainly available for some "modern" consumer units, there is one a few metres away from me as I type.
Thanks for the replies everyone.
Just to clarify, the instructions do call for a 45A protective device along with recommending an RCD (the current CU is split load so would obviously out on RCD side). The cable run is fairly straightforward as the bathroom is pretty much directly above the CU. The cable will run in trunking from the CU to the downstairs ceiling then under the floor a bit followed by a short run through the uninsulated stud wall that the shower will be fitted to. Probably not much more than five meters in total. I was planning on using 10mm^2 anyway to be on the safe side.
It does look as though I'm pretty buggered here which is a shame. Looks like I'll be replacing the CU then!
One option not mentioned which I assume would be okay would be to have
45A breaker in a seperate enclosure wired back to a 50A breaker in the existing CU. Although this would seem a bit overkill considering a new kit box with more than enough MCBs probably wouldn't cost much more. Only benefit to this that I can see is it wouldn't require me to break the seal on the main supply fuse - but I don't have too many quarms over that having done my own not too long ago.Thanks again, Richard
It would probably be simpler to leave the CU as is, and split the tails feeding it via a Henley. Then you can have the new mini CU with RCD completely independent of the existing one. Saves adding more circuits to the main RCD as well.
It might be worth noting that my 10.5kW@ 240V shower has worked fine from a
40A MCB for the last 6 years. Changing the MCB is on the things to do list.AdAM
Interesting. There is a spare 40A MCB in this unit already, so if that did work it could save me a job. Can anyone think of any reason NOT to do this?
Yes, it's a 10.5kW shower.
It's a pity you didn't reign in the choice to 9.5kW
Jim A
Jim A
Chances are it will work just fine, but you would be designing the system a little out of spec. A 40A MCB will probably supply a sustained overload at 45A indefinitely. According to the trip curves in BS7671, even a 50A load would probably take 30 mins to cause a trip. As long as the cable is of adequate rating there is unlikely to be any harm caused.
(might be worth checking what your nominal voltage is, If it is below
240V then that is in your favour. If however you routinely get 250V then I would be less inclined to do as above.
Is the Wylex NB range thermally compensated? if not, and the 40A rating is at 40Deg.C ambient then at typical UK temperatures the thermal trip would be at a higher current. As a shower is a resistive load the current it draws is proportional to the voltage at its terminals.If your cable run is long enough to drop the supply by 4% the current will also drop by 4%. But then of course the mains is 230V +10/-6% Aaaaaah.............
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