9.5 kw shower on 6mm ?

[Guest55]

If you design your circuits correctly you must be aware that.
As a 9.5KW shower draws 41.3A you are unable to use a 40A MCB You need to use the next size up. This would be a 45A
it is correct that the tabulated current carrying capacity of a 6mm T&E cable is 46A this is the value prior to any correction factors being taken to account. I would suggest that the there are multiple cables exiting the DB at the same place so the grouping factor will reduce the cable rating below 41.3A without taking into account any other factors.
It is not acceptable to assume that no harm will be caused to the cable as the shower will only be on for a short while or the loading is only 1A over the 40A MCB rating.
When the electrical installation certificate is issued you are sating that the installation complys with BS 7671 and is a legal document in the event of future problems.
I would suggest if this size shower is to be installed a 10mm cable minimum cable should be used. Pay now or pay later.

Youre correct of course but your example is out of context to the original post.
The issue that was debated at the start is regarding utilizing an existing cable so as to reduce cost / inconvienience to the customer in having a bigger cable installed.
Its a judgement call that each person will assess differently , but in my experience a 9.5kw shower will run off 6mm with absolutley no problems for years , regardless of any calcs you want to throw at it.

 

[gaulty]

Youre correct of course but your example is out of context to the original post.
The issue that was debated at the start is regarding utilizing an existing cable so as to reduce cost / inconvienience to the customer in having a bigger cable installed.
Its a judgement call that each person will assess differently , but in my experience a 9.5kw shower will run off 6mm with absolutley no problems for years , regardless of any calcs you want to throw at it.

His example is not out of context. He's just shown the calculation showing that 6mm is not big enough! The MCB is not even big enough to take the load of the shower regardless what a clamp meter shows.

Don't take this the wrong way but "in my experience" will not stand up in court. Also how would u fill out the test sheet - "comments on installation- 'I know the calculations state a 40amp mcb is not suitable for a 9.5kw shower along with using 6mm t+e but in my experience it SHOULD be fine'

That's rediculas THERE IS NO GREY AREA

40A mcb + 6mm cable is unsuitable for 9.5kw shower according to the 17th edition IEE wiring regulations regardless of personal opinion, that is fact, law, gospel.

You have two options down size the shower to suit the circuit or upgrade the circuit to suit the shower.

 

[sparc]

Just done new CU installations on a property with three showers - two 9.5 one 8.5. They're all on 4mm on 32A breakers. Asked owner about trips but never appears to have happened. Feed water temperature to the shower is a key factor in power drawn - which is why clamping a shower in summer doesn't tell you anything. I think in the case of this property the cold feeds to all three showers travel through a lot of internal plumbing, so water never really that cold.

 

[nickblake]

One of the biggest problems is what happens after youve installed your shower you do all the calculations select the correct cable then along comes the home improvement energy saving bods and sticks 300mm of insulation right over the top of it , when you look at the de rating factors of cable then for showers we should be installing them in 16mm just in case they are covered in insulation in the future

 

[steveberry11]

Quite right when replacing a shower the electrician should make a judgment on the existing installation. If the cable is not big enough to feed the new load the customer should be informed, what is required to comply with BS7671. It is not a case of it will do or I have never had a problem before I have known these showers run on a 4mm cable with no problems. It is your responsibility to comply with the green book.
It is not acceptable to pass the responsibility to the customer by asking them to sign a piece of paper to cover you. The buck stops with you. As they are hiring you as a skilled and informed person.
The loading of a shower is not dependent upon the water temperature. The only effect this will have if any is the is the amount of time the element is heating this will have a negligible effect.

 

[mickys86]

Future note to new forum users :

Please let this thread die , at 8 pages long there is little of use to add that hasnt been argued to death.
;-)

Maybe you should've aimed this post at the regular posters??

 

[sparc]

One of the biggest problems is what happens after youve installed your shower you do all the calculations select the correct cable then along comes the home improvement energy saving bods and sticks 300mm of insulation right over the top of it , when you look at the de rating factors of cable then for showers we should be installing them in 16mm just in case they are covered in insulation in the future

That makes sense, Nick, but where do you draw the line? If you have to future-proof your installation you'd need to make worse case scenarios assumptions for all your circuit derating calcs - not just for showers.

 

[Guest55]

His example is not out of context. He's just shown the calculation showing that 6mm is not big enough! The MCB is not even big enough to take the load of the shower regardless what a clamp meter shows.

The mcb is to protect the cable not the load , which it does.

Don't take this the wrong way but "in my experience" will not stand up in court. Also how would u fill out the test sheet - "comments on installation- 'I know the calculations state a 40amp mcb is not suitable for a 9.5kw shower along with using 6mm t+e but in my experience it SHOULD be fine'

Court ? Who's going to court ? Has anyone ever ended up in court from connecting a 9.5kw shower to a 6mm cable ?
Stop being a drama queen.

That's rediculas THERE IS NO GREY AREA

Ridiculous even ?

40A mcb + 6mm cable is unsuitable for 9.5kw shower according to the 17th edition IEE wiring regulations regardless of personal opinion, that is fact, law, gospel.

Bs7671 isnt law , that is fact.

You have two options down size the shower to suit the circuit or upgrade the circuit to suit the shower.

I was trying to keep the debate polite and reasonable before all the pedantic know-alls all jumped in.
Get your facts straight next time.

 

[steveberry11]

Correct BS7671 is not law. However the Electricity at work regulations are and it refures to BS7671 as a standard so in a court anybody being prosecuted will have to prove compliance. This applies to any electrical work undertaken not just showers. The MCB is to protect the cable just as the rating factors are used to down rate the current carrying capacitys of the cable. I fail to see how good electrical theory can be classed as pedantic.

 

[sedgy34]

I was trying to keep the debate polite and reasonable before all the pedantic know-alls all jumped in.
Get your facts straight next time.

so now the boxing gloves are out!!!

 

[wade88]

Oooo brilliant i've just sat down for my lunch i definitely think ill be staying to watch this little firework display unfold. How exciting, yet ANOTHER feeble display of bickery.

I would have thought it was a fairly straight forward answer however? Stick with what the regs recommend - and you can do no wrong. Is that not the basis on which we as electricians design and guide our circuits around to adhere to safe and proper practice? Or have i missed something where personal opinion counts for more than the governing body by which we all follow. But i don't know.... that's just my opinion. Am i allowed to say that or do i have to note that as a departure :stuart:

 

[Guitarist]

I am still amazed at how many people think that British electric showers are a variable load. 8.5kw is 8.5kw, regardless of anything (except variations in supply voltage ofc). If you select the "economy mode", which tbh nobody ever does, then it will be a lower fixed load, but still fixed....

 

[Sherwoods]

The cable size must be suitable for the load, no matter what overcurrent protection is provided. No reference method has been mentioned, but if it is "A" then 6mm is rated at 32A, method 100 rated at 34A, and 101 would be only 27A.
A 9.5 kW shower would draw 41.3A.
I have seen a few cases where the shower isolator is starting to disintegrate due to the high load and a situation where the contact resistance continues to increase. This, along with the inadequate cable current capacity is a fire risk. The conclusion must be that the shower in the quoted case must be rewired or the shower replaced with the correct rating.
The overcurrent protection should match the load but not exceed the cable current rating.

 

[Guitarist]

Just to add my tuppence worth,
I have seen many, many 8kw and above showers wired in 2.5mm flex from the isolator to the actual shower unit in my time testing, none of which showed any signs of damage, poor terminals and terminations cause many more,
I think in an ideal world there should be a 'domestic shower diversity' added to the regs, it irritates be to give a code 6mm supplying showers in houses, in fact I often give it a code 3 rather then 2 if there is no heat damage.

I hope you are not condoning 8kw showers being fed with 2.5mm cable.

 

[sedgy34]

i cant be bothered to read through them 10 pages but has anyone thought what the manufacturer wants to keep the warranty in place normally correct fuse rating to the bs7671 and an rcd in place "just my 2penny worth"

 

[Guitarist]

i cant be bothered to read through them 10 pages but has anyone thought what the manufacturer wants to keep the warranty in place normally correct fuse rating to the bs7671 and an rcd in place "just my 2penny worth"

A lot of shower manufacturers state a minimum cable and MCB size. All this has been said so many times before, yet still people are arguing about whether to work from "experience" or adhere to the regs! We know that everything has a safety margin built-in, but that's exactly what it's for, safety.
I think that the phrase "well, the last house didn't burn down", is not going to be much use if it all goes belly-up.

 

[telectrix]

when we look at it, so a 6mm cable is installed method C. it is perfect for the situation at time of installation. OK, so in the future a muppet might cover it in itchy snot, thus derating it's CCC. but, in the future rats/mice may infest the loft, so do we install SWA or cable in steel conduit? client may, in the future, get a plumber to install a 15kW shower, do we allow for that?

 

[Engineer54]

I'm just wondering how many of these manufactures stated KW ratings are actually achieved in the real world??

 

[telectrix]

exactly.

 

[Dave 85]

Youre correct of course but your example is out of context to the original post.
The issue that was debated at the start is regarding utilizing an existing cable so as to reduce cost / inconvienience to the customer in having a bigger cable installed.
Its a judgement call that each person will assess differently , but in my experience a 9.5kw shower will run off 6mm with absolutley no problems for years , regardless of any calcs you want to throw at it.

And here we have the difference between a proper sparky and a regs jockey.