10.8kW shower cable size

[DSpark1]

Hi,

I'm installing a 10.8 kw shower and looking at other forums everyone is saying 10mm is OK however i can't see how.

10800W/230V=47A

breaker size 50A

the only way it can be a 10mm is if its clipped direct with no insulation as it can take 64A however if it is any other reference then the rating is less than 50A.

so how is 10mm enough to carry the load?

what have i missed?

 

[snowhead]

Don't the Regs over rule what people on forums say?

Using your first method, it complies.
Otherwise it doesn't.
So half the people are correct, or is that all of the people are half correct?

 

[Octopus]

What is the purpose of the MCB?

 

[DSpark1]

The purpose of the breaker is to protect the cable from overloading or burning due to overload
I think you just helped me answer the question as it can't overload as a shower is a fixed equipment. Under fault it will trip anyway. However 10mm is boarderline as the current carrying capacity of it is 45-47 amp with ref method 100 and 102 which are the most common for domestic

 

[Octopus]

Bottom line is that you are the one signing off the new circuit/notification so you need to decide.

Maybe a bit of 16mm is the way to go......

Call your scheme's technical helpline and see what they say.

More information required me thinks

 

[Risteard]

However 10mm is boarderline as the current carrying capacity of it is 45-47 amp with ref method 100 and 102 which are the most common for domestic

I agree with what was stated on the IET Forum to you, which is that Clipped Direct (C) is by far and away the most common reference method in a domestic installation. Also, as said, the shower is a fixed load which cannot overload and so the protective device need only provide fault protection.

 

[Darkwood]

The mcb provides both overload protection for the cable and fault protection for the circuit, to suggest that just because the load is fixed and cant go any higher is wholely irresponsible, what if you have a insulation fault either in the cable L/N or shower(partial element short reducing its resistance) this can raise the current without showing a fault. If i was installing any cable through a loft i would fit a trunking 2/2 where it passes through the insulation and have the cable run above any possible insulation level, and with goverment policies to have all homes insulated at some point i would install the trunking whether insulation there or not, if the cable calcs dont comply then dont cut corners either upsize the cable or choose a different route or method.

 

[Risteard]

The mcb provides both overload protection for the cable and fault protection for the circuit, to suggest that just because the load is fixed and cant go any higher is wholely irresponsible

How so? The Wiring Regulations specifically permit the omission of overload protection where supplying a fixed load which cannot overload.

what if you have a insulation fault

Hence why you must ensure that it provides adequate fault protection as I stated.

 

[Darkwood]

A fixed load that cannot overload is like a motor with overload protection or a fixed device with its own fusing, this case the current can't exceed the overload setting or device fuse, you may install a cable of less than 3m where the protective device upstream is higher as long as the device downstream protects the cable from overload.
Now this is how i was taught and if things have changed with the plight of rcd's on everything il stand corrected as extra protection has been afforded.

 

[needasparks]

I remember at college I worked out an 8.5kw shower with all the factors and cable run come to a 16mm,
It was a question that was designed to see how working out was acheived by having above normal problems to overcome but the main question was how were we to get a 16mm into the terminal block.

 

[Farmelectrics]

10mm is good enough surely not come across a shower wired in 16mm myself would you physically be able to terminate a 16mm in to a shower

 

[Darkwood]

Example of reg 433.3.1 (ii).. would be fitting a 2way board with 2 x 16a mcb's you could effectively feed board of an Henley block in 6mm tails regardless of how large the fuse up-stream was because it cant draw long term a load exceeding the value of both mcb's.

 

[Deleted member 9648]

433.1.1 (ii) states that the rated current of the overload device must not exceed the current carrying capacity of the circuit cable...in which case a 50a mcb on a 47a rated cable will not comply.
But...433.3.1 states overload protection may be ommitted where the characteristics of a load means an overload is unlikely,provided the device provides fault protection.
IMO a fixed load like a shower is unlikely to overload.

 

[IQ Electrical]

433.1.1 (ii) states that the rated current of the overload device must not exceed the current carrying capacity of the circuit cable...in which case a 50a mcb on a 47a rated cable will not comply.
But...433.3.1 states overload protection may be ommitted where the characteristics of a load means an overload is unlikely,provided the device provides fault protection.
IMO a fixed load like a shower is unlikely to overload.

Agreed.

 

[Darkwood]

And as i said il stand corrected, of which i do... having thoughts regarding a shower it does have thermal fusing within too as a back-up but yes as long as cable does not underate mcb then il also agree, my prehistoric teachings only included loads with their own protection for overloads but regs change so much im bound to miss the odd change.... viva la 15th edition

 

[IQ Electrical]

And as i said il stand corrected, of which i do... having thoughts regarding a shower it does have thermal fusing within too as a back-up but yes as long as cable does not underate mcb then il also agree, my prehistoric teachings only included loads with their own protection for overloads but regs change so much im bound to miss the odd change.... viva la 15th edition

It's not exactly a widely known caveat and crops up quite often on forums, it just doesn't 'feel' right even though it complies....

 

[Deleted member 9648]

I suspect this is the reason why we can spur from a ring in 2'5mm cable fused at 32a......effectively the spur is limited to a load of 26a under normal circumstances.

 

[IQ Electrical]

I suspect this is the reason why we can spur from a ring in 2'5mm cable fused at 32a......effectively the spur is limited to a load of 26a under normal circumstances.

Yes, 0.5mm flex on a pendant etc etc.

 

[Guest111]

Is it just my maths or does anyone else get a figure a tad bigger than 47A for 10,800 Kw as per OP?

 

[Deleted member 9648]

46.95 according to my calculator!.....


Edit......OOHH just read your post properly........46956.5a.......might have to go to 16mm.

 

[trev]

10.8kw @ 230v=46.9A
10.8kw @ 240v=45A mate.

 

[Guest111]

10.8kw @ 230v=46.9A
10.8kw @ 240v=45A mate.

Come on Trev read the post properly he said 10800 Kw not 10.8 Kw

 

[trev]

Phil I swear I just saw your bit about a fundamental error and I said "10800kw for starter"
No idea where the post has gone
Update
That ones in the arms Dammit

 

[needasparks]

Example of reg 433.3.1 (ii).. would be fitting a 2way board with 2 x 16a mcb's you could effectively feed board of an Henley block in 6mm tails regardless of how large the fuse up-stream was because it cant draw long term a load exceeding the value of both mcb's.

This is like what was know as the busbar rule, with the reverse fusing. As far as i know its still acceptable as long as the fault protection covers upstream supply. But its one of the old school methods and would not be concidered by most now.

 

[beaver74]

table 4d2 is still for T+e bs6004 and gives a value of 52A in trunking or am I missing something

 

[Darkwood]

This is like what was know as the busbar rule, with the reverse fusing. As far as i know its still acceptable as long as the fault protection covers upstream supply. But its one of the old school methods and would not be concidered by most now.

Still see it alot, have you tried getting 25mm tails into a 2way board lol, but yes im currently fitting isolators of a 200amp busbar with tails from 16mm to 70mm coming off it into switchfuses.. waste of copper and money otherwise.

 

[cbw]

Well.....Bottoms to you all, I have a 10.8Kw shower at home and I installed it on the 40A 10mm circuit..............................

Its fine!

No melted cables, no house fire.

HOWEVER>>>> Would I do this at a clients house Hmmmmm?

Can you imagine trying to install 16mm T+E into an MK shower isolator?????

 

[DSpark1]

I agree with what was stated on the IET Forum to you, which is that Clipped Direct (C) is by far and away the most common reference method in a domestic installation. Also, as said, the shower is a fixed load which cannot overload and so the protective device need only provide fault protection.

but that would mean there no insulation in the property, and most if not all new builds have insulation in them so i can't see how it would b ref method c

 

[drew35]

I was surprised to see the other day a shower isolator light switch! I couldn't find a shower isolator so a search in the loft above revealed a 16mm cable to a sub board, with a double pole contactor as the isolator, switched by a light switch on the outside of the bathroom. This was a recently installed 10.8kw shower carried out by a bathroom company. So there's your answer, 16mm cable, contactor, rewire, rebuild etc!

 

[Knobhead]

DS, if you’re that worried about it put a 16mm in. Two forums have advised you, but you seem unwilling / incapable of taking advise.
Do it your own way!

 

[Des 56]

if you’re that worried about it put a 16mm in.





Or get one of these

​

Triton T80Z 7 5 KW Electric Shower White Chrome SP8007Z

 

[DSpark1]

10mm on a 50Amp breaker it is.

just wanted to be comfortable in understanding it so i can explain myself if I have to.

Thank you very much all

 

[beaver74]

table 4d2 is still for T+e bs6004 and gives a value of 52A in trunking or am I missing something

am I right or not ????????

 

[Risteard]

but that would mean there no insulation in the property, and most if not all new builds have insulation in them so i can't see how it would b ref method c

Rather than derating it's generally better to avoid the insulation! No reason why in almost all cases you shouldn't be able to prevent it being a factor.

 

[Darkwood]

If you have 400mm of 2/2 trunking fitted vertically where it passes through the insulation you really dont need to apply methods related to cable in trunking.... the trunking is open top and several orders larger than the cable so any thermal dissipation restrictions will be neglegible and clipped direct can be used as normal.

 

[DSpark1]

Thanks, yep point taken on the ref method.

 

[DSpark1]

table 4d2 is still for T+e bs6004 and gives a value of 52A in trunking or am I missing something

Table 4D5 on the 17th (green one) pg 340 is T+E

 

[beaver74]

Table 4D5 on the 17th (green one) pg 340 is T+E

yes I know that but it does not give ref method B(T+e in trunking)table 4d2 is also T+e but with other ref methods.
Am I not correct