Correct MCB ???

[srl-8]

Hello guys. I am new on here and wanted to get involved.
Firstly, I am on route to becoming a domestic installer and now becoming very inquizitive when I see other peoples installations.. And for the record, I know i have a LONG way to go before I would consided myself competent !

At my parents house, they have had a new electric shower put in, but I am unsure if the electrican has used the correct rated MBC...Can someone clarify the below

Taken a new feed from the Hendy Block and feed a Small 2 way C/u (Just for the bathroom)
The new Cu has a 63A / 30ma RCD main switch, and then 6A MCB and a 32 A MCB which the shower is on.
The shower Cable is 10mm.
The shower is rated at 9.8kw / 9800 watts

I think this MCB is underated and should be a 45A MBC, however it have never tripped??

Can someone explain what is happening here

Thanks!!

SRL-8

 

[telectrix]

you are correct. a 40A or a 50A would be the correct MCB for 10mm cable, depending on the installation method ( insulation derating factors being taken into account). the shower probably is optimistically rated however. a clamp meter would tell you the actual current drawn. i would think that the MCB is being overloaded, but not for sufficiently long enough to trip. (MCBs will run at around 140% for 1 hour before tripping).

 

[malcolmsanford]

^^^^^^^^ about sums it up.

 

[srl-8]

Thanks alot. Guys!! As it has worked for the passed year with no trip, i can presume the installation is safe. However, If he had just installed a 45a MCB, we would not have questioned this.... tut....tut...

Thanks again.

 

[Paul.M]

It's good that your picking up on things like this seen as your just starting out.

The two way cu most probably came with a 6A and a 32A mcb and I bet the sparky didn't have a 40A mcb in the back of the van so just used the 32A. If you have an OSG have a look at the method that the cable is installed, use the tables in the OSG to work out what size mcb you need and change it. Just post up on here first so it can be checked before you change it.

 

[Dunc]

40.83A for a 9.8kW shower. The 32A will hold unless someone has a very, very long shower due to the slow reaction to slight overloads that MCBs have. The bi metallic strip within the MCB has to get warm enough to bend up and touch the mechanical parts of the MCB which will obviously take a bit of time. Seems a bit lazy not fitting a 40A MCB if he'd already gone to the trouble of running a 10mm.

 

[Deleted member 26818]

Whilst the cable is rated to carry a greater load than that of the 32A MCB, there is no requirement or reason to change the MCB.
What is the rating of the shower?
Is the shower one that heats the water, or one that just pumps the water?
It may be that while the cable can carry around 45-50A, the shower is only drawing 30A.
If so then the MCB is correctly rated, and the cable is sized to allow a shower with a greater draw to be installed in the future.
Obviously if a shower with a greater draw is ever installed, the MCB would have to be changed.

 

[GLENNSPARK]

You can divide the wattage by the voltage so: in this case you go 9800/230= amps and you can work out from that the size of breaker to use and that will then give you a starting point for cable size..........

 

[srl-8]

the shower is 9.8kw or 9800 watts (electric shower / not pumped power shower)

so 9800/230 = 42.6 Amps.

I guess it would only draw a max of 42.6 amps if you increased the temp setting to max, however a 32 MCB is not correct for this install. Should have been a 45 Amps MCB. Ageed?

 

[Deleted member 26818]

If the rated current draw of the shower is 42.6A, then technically anything below that would comply.
40A would be ideal, but may have to be special order.

 

[Paul.M]

If this were my problem to sort out I'd put a 40A mcb in. How many people put there shower on full heat? Btw is it rcd protected?

 

[Deleted member 26818]

Hi Spinlondon, shower is rated at 9.8kw so at flc it can pull 42.6A. If this were my problem to sort out I'd put a 40A mcb in. How many people put there shower on full heat? Btw is it rcd protected?

Yes I did not notice that the OP had stated the shower rating in the original post.

 

[Paul.M]

It's edited now, all 3 of us posted at the same time. Sorry spin.

 

[Dunc]

the shower is 9.8kw or 9800 watts (electric shower / not pumped power shower)

so 9800/230 = 42.6 Amps.

I guess it would only draw a max of 42.6 amps if you increased the temp setting to max, however a 32 MCB is not correct for this install. Should have been a 45 Amps MCB. Ageed?

Just as a quick reference point, although all calculations in BS7671:2008 are done at 230V, shower manufacturers rate their products at 240V. I know it doesn't make a huge difference but it's still a difference that you need to be aware of.

 

[robsparky1975]

the shower is 9.8kw or 9800 watts (electric shower / not pumped power shower)

so 9800/230 = 42.6 Amps.

I guess it would only draw a max of 42.6 amps if you increased the temp setting to max, however a 32 MCB is not correct for this install. Should have been a 45 Amps MCB. Ageed?

As previously said,the installation method of that 10mm cable would need to be checked and any derating factors be taken into account,for instance is it in contact with thermal insulation,bunched with other cables etc.If you have the regs book have a practise at a cable calculation.This will be most helpful for your training also.