Electric shower installation question

[spoony77]

Hi people
I am not a diy'er or an electrician but i am learning as i go.
I have just had a new bathroom fitted and an electric shower installed by my friend a qualified electrician. He used 10mm cable and fitted me a pullcord switch in the bathroom.
I bought a 10.5kw triton shower as reccomended on some websites but when my sparky came to wire it in to my consumer unit we ran into some problems.
I have a wylex NB 100amp consumer unit and ive now found you cant by 50amp breakers for this unit.
My electrician says i need a 50 amp breaker but my plumber says a 40 amp will do. I asked for a second opinion from another electrician friend who said a 40 amp breaker should 'hold it'???
So what do you guys reccomend - should i buy a 40 amp breaker and see how it goes?
My electrician said he could adapt my consumer unit to fit a different type of 50 amp breaker (a type NSB rather than NB which doesnt fit otherwise) but this sounds like a risk job to me??

Any advice would be much appreciated.

Cheers

 

[andy]

10.5KW is 45A. put a 40A MCB in and itll trip after a few mins. get a 50A. also, you should have it RCD protected. if your existing CU is not RCD protected then you should get a seperate shower CU and split the tails

 

[plugwash]

i suspect a 40A will hold

another option is to put a henly block in the tails and run to a seperate shower CU

 

[ban-all-sheds]

I am not a diy'er or an electrician

What are you then?

To be pedantic, 10.5kW is almost certainly the rating at 240V, so 43.75A, not 45, but it makes little difference - a 40A breaker is not quite up to it. It may not trip, particularly if you don't have teenage daughters, but it will get warm, and it will get stressed.

45 or 50A is the way to go.

 

[ban-all-sheds]

10.5KW is 45A. put a 40A MCB in and itll trip after a few mins.

Shouldn't do - Inf for a 40A Type B is 45.2A.

 

[spoony77]

Hi there

I'm a chef for the person who asked!

One of my sparkys said im running 220v - could this make a difference?

What do you guys think of the idea of adapting my consumer unit to make a slightly larger NSB style 50 amp breaker fit - something ive been told is possible??

Cheers again

 

[Adam_151]

220v would mean it draws about 41.7A but that is irrelevant as calculations are done with 230v (and still overloading the breaker)

In practice, it will not trip, but the calculations show it is overloaded, and therefore bad practice to do

On another note: the fact that you are only getting 220v will mean that the power output of your shower is little more than 9kw (9175w to be more precise)

 

[Adam_151]

I am not a diy'er or an electrician

What are you then?

Someone who knows his limitations and therefore doesn't do any work?

 

[Scoby_Beasley]

We've got a triton 10.5kw shower, which quite clearly stated on the box that it was a 9.6kw shower at 230v. running ok on a 40amp b type mcb (okay then its tripped once during installation !)

 

[Damocles]

The amount of heating in a mcb depends on the current and its rating. I would guess that a 40A running at 40A would get exactly as hot as a 50A running at 50A. And that a 40 running at 45A will only be 5/40 hotter.

Better to overload the mcb than the cable.

 

[mapj1]

Better to overload the mcb than the cable.

certainly, that is the true point of it all. Moreover the trip will hold for ever unless you are in a very hot country, at 25deg ambient, the trip will carry 1.25 times rated current indefinitely. The only penalty is that if you ran the shower day and night you might notice that the MCBs life was shortened from about 50 years to maybe 10 or 20. (the contacts oxidise and one day it will overheat and trip, and refuse to be reset.) However, don't panic, as the shower is NOT run day and night, and the shower will almost certainly die of natural causes long before it. And you will have a new CU by then....
regards M.

 

[felix]

Just because I feel a bit pedantic today ---

Surely a 40 amp breaker running at 45 amps will generate about 25% more heat (2025/1600) so - and I'm about to step onto thinner ground here - its temperature will rise about 25% higher above ambient.

 

[ban-all-sheds]

the trip will carry 1.25 times rated current indefinitely

Where did you get that figure from?