Shower tripping mcb

will155

Hi, desperate for some advice.  I've just had a new 8.5kw shower put in.  I've also had the cable changed to 10mm from 6mm, also had a new shower pull switch fitted.  A new consumer unit was fitted in Jan 2020 with a 40amp mcb on the shower.  My problem is the new shower keeps tripping after a few mins and the mcb turns off, not the RCD, tho.  This is the second new shower I've had put in, I've had 2 electricians out and I'm still having the same problem.  Can anyone shed any light on what I can do next as I'm tearing my dirty hair out with it all as well as spending too much money.  Thanks

will155

Actually just checked and it's 9.5kw, not 8.5

tiggerbodhi

IF only the MCB is tripping it is an over current issue not residual to earth, so could be a faulty cable or loose connection.
However your on the "cusp" for a 40A MCB with a 9.5KW Shower, will be drawing 39 ish Amps operating, if the circuit test OK, sparky should have done an insulation test, it would seem normal to change the 40A MCB to a 45A MCB

cpbackhouse

I am not an electrician but .... at some very rough maths if its 9.5kW assuming 230V supply it could easily be pulling more than 41A which I would hope would trip the MCB rated at 40A. 
Obviously not a professional opinion but sounds like the MCB is doing what its designed to and might need upgrading. 
Surprised the electricians who have been out haven't mentioned this though so I might be missing something obvious.

Mickey666

If the MCB is tripping, and not the RCD, then it points to an over-current issue.

UK mains voltage is specified as 230v +10% to -6% so it could be anything between 253v to 216v
Even at the nominal 230V, 9.5kW will draw 41.3 amps so I'm not surprised that a 40A MCB trips after a few minutes as it is operating right on its limit, which is generally not advisable.

I'm really surprised that two electricians couldn't fix the problem.  What did they say?

coffeehound

tiggerbodhi said:
 it would seem normal to change the 40A MCB to a 45A MCB

Looking at some of the cable rating tables, it appears the shower may be drawing more than the cable is rated for in certain installation circumstances.  Perhaps that is why the electricians didn't want to uprate the MCB?

tiggerbodhi

coffeehound said:

tiggerbodhi said:
 it would seem normal to change the 40A MCB to a 45A MCB

Looking at some of the cable rating tables, it appears the shower may be drawing more than the cable is rated for in certain installation circumstances.  Perhaps that is why the electricians didn't want to uprate the MCB?

did, possibly wrongly, assumed that when the cable was upped to 10mm the length and type of run was taken into consideration, personally always err on the side of caution and gone for 16mm, but if it's a short run and not buried in insulation 10mm should be fine.

Jeepers_Creepers

He's had 10mm2 cable fitted, which was the right move. That should be more than capable of handling 45A - it should even be fine for a 10.8kW shower.

Like others on here, I find the fact the 40A MCB is tripping isn't a mystery, but that two sparkies have seemingly not come to this same realisation is.

fenwick458

what brand of consumer unit have you got? 
has anybody had a clamp meter on to see what sort of load is being drawn when the shower is on before it trips? 
also amazed at the inability of 2 electricians to fix this, or at least do some checks so that things can be ruled out at least

coffeehound

The table shows that a ceiling installation under >100mm insulation, or in an insulated stud wall will reduce current rating to 36- or 32 amps, respectively.

grumbler

tiggerbodhi said:

coffeehound said:

tiggerbodhi said:
 it would seem normal to change the 40A MCB to a 45A MCB

Looking at some of the cable rating tables, it appears the shower may be drawing more than the cable is rated for in certain installation circumstances.  Perhaps that is why the electricians didn't want to uprate the MCB?

...if it's a short run and not buried in insulation 10mm should be fine.

And what if it's a long run and not buried?

The current will be only smaller.