Can I use TWO lots of 6mm T&E for a 10KW shower?

[PaulIG11]

I have *plenty* of 6mm T&E cable (100m spool).

For a shower, I know that I can't run anything bigger than around 8KW off of a single run.

But what about if I use TWO runs (ie, if I run TWO 6mm cables from the consumer unit to the corded switch in the bathroom & TWO cables from there to the shower itself? (ie to run the cables in parallel - you could think of it like a ring main but with only one feed point & one draw point)

Prime question IS IT SAFE to do this?

2nd question: Is it LEGAL to do this (even if unconventional)?

3rd question: If this is safe and allowed, what would the maximum size of the shower I'd be able to have then be?


The reason why I ask this is that I already have 100 metres of 6mm cable that I don't have to pay for !!!! If I end up having to buy 10mm cable then it's an expense I'd rather avoid if possible.

Any observations?

 

[RF Lighting]

Do it right with 10.0mm² cable.

Your shower is probably the most lethal appliance in your house. To try and bodge it for the sake of a few quid is madness.

Weigh the 6.0mm² in at the scrappers and use the money to buy the proper cable.

This work is notifiable to your LABC

//www.diynot.com/wiki/electrics:diy-electrical-work-and-the-law

 

[PrenticeBoyofDerry]

Prime question IS IT SAFE to do this?

Debatable whether it is safe to do so, as you could have an event where maybe you have a loose or discounted live conductor, which would not render the shower powerless but would cause a fire hazard.
Also you will have problems terminating 4 sets of 6.00mm at the pullcord isolator.

2nd question: Is it LEGAL to do this (even if unconventional)?

unconventional, illogical, impracticable but not illegal.

3rd question: If this is safe and allowed, what would the maximum size of the shower I'd be able to have then be?

It would be difficult to say what a safe current would be, as there is no defined rating for 6mm pairs, but I would be surprised if it was much more than that that would be offered by using 10mm t&e.

The reason why I ask this is that I already have 100 metres of 6mm cable that I don't have to pay for !!!! If I end up having to buy 10mm cable then it's an expense I'd rather avoid if possible.

What I would do is not scrap the 6.00mm reel but sell it, either by ebay or local ads or local sparks, as you will get more money than you would as scrap.
Then purchase the desired length of 10mm t&e that is required.

 

[EFLImpudence]

It's a STUPID idea,

Save even more money and don't buy the shower.

 

[PrenticeBoyofDerry]

I have *plenty* of 6mm T&E cable

For a shower, I know that I can't run anything bigger than around 8KW off of a single run.

Not true, you could supply a 9.0kW shower providing no de-rating factors of cable or voltage drop. 6.00mm will carry max current of 47A and using a 40A MCB. This would allow a 9kW shower to be installed.

 

[JohnW2]

3rd question: If this is safe and allowed, what would the maximum size of the shower I'd be able to have then be?

It would be difficult to say what a safe current would be, as there is no defined rating for 6mm pairs, but I would be surprised if it was much more than that that would be offered by using 10mm t&e.

433.4.1 of the regs says that, for the purpose of determining the requirement for overcurrent protection, the Iz of conductors in parallel should be taken as the sum of the individual conductors' Iz figures (i.e. double the CCC of single run of 6mm² for 2x6mm²) if current is shared equally between the conductors. Although it goes on {invoking first para of 523.7.1(i)} to imply that equal lengths of same CSA T&E (without branches) are deemed to have equally shared current, I really am not happy that one could ever be totally sure that the current was equally shared (and, as you say, remained equally shared).

The bottom line is that I agree that, whatever the regs might say, it's not a good idea at all.

Kind Regards, John

 

[plugwash]

If the cables are the same CSA and length and termination resistances are negligable then to all intents and purposes the current will be equally shared.

However the problem comes if there is a bad termination. In addition to cooking the termination said bad termination may cause one of the cables to be overloaded. Worse still one wire could just completely fall out or snap off leaving the other to take all the load and not necessarily creating any sign of a problem at the accessory.

If everything was terminated on quality (either dual screw or a single screw with provisions to stop the wires sliipping up the side of the screw) terminals with the connection able to be inspected in it's final loation and not subject to stress after termination I'd consider the risk of this to be negligible.

Unfortunately shower isolators (whether pullcord or wallmount) are not designed in a way that lends itself to reliable termination of large conductors. The cables must be terminated first and then the accessory fitted to the backbox. With large cables this process is pretty much inevitably going to put the terminations under strain. Further the terminals themselves are not generally of what I would consider to be high quality designs (they are usually single screw affairs with no measures to stop conductors going up the side of the screw). Terminal failures on shower isolators seem to be worryingly common.

Also note you need to take grouping factors into account when determining the current carrying capacity of each run so two paralelled 6mm T&E in close proximity will not have double the capacity of a single T&E. Still i'd imagine it would be fine for most showers.

All things considered I don't think you could find any grounds in the regs to fail a shower install with doubled up 6mm but I don't like it because of the risk of terminal failure.

 

[ericmark]

As already stated you can run conductors in parallel I have run two 120mm cables in parallel instead of a single 240mm as we could not physical get a 240 mm cable in the space.

As with the standard ring main the problem is to ensure it does use both cables not just one. Any poor connection could result in just a single cable carrying the whole load. With correctly crimped lugs and bolted connections used with my 120mm cable there was no real way it could become disconnected but with a compression joint used with most isolators, showers and MCB's the question is can you be 100% sure of a sound connection?

We find isolators and showers with signs of bad connections with a single cable all too often however the real question is will the LABC pass the work if you use two cables? On a long run the loop impedance will show if one or two cables are taking the load. But on a short run from isolator to shower it would be near impossible to test with loop impedance tester. Clamp on ammeters may work but you need a large enough tail to be able to get the clamp around the wire.

If I was the electrician sent by the LABC to test that work I would not be happy passing it. And to do all that work only to have the electrician refuse to pass it in the end would be rather a waste so what you have to ask yourself is it really worth it? Even with an A1 job it can be a bit of a fight to get the LABC to pass the work. As a non scheme member I have a few times crossed swords with the LABC to try and get them to accept my credentials to be able to sign the installation certificate nothing to do with the work done. And I have all the exams passed required. So they don't need to fail it on the use of two conductors they can easy find another reason.

Personally to do such a small job as DIY is just not worth it. The minimum charge from the LABC covers all work up to £2000 so the charge for a £100 job is more than the job costs it's self. A few years back in Wales it was £100 plus vat in England varies county to county. So for the work you are doing which is notifiable it is just not worth doing as DIY.

 

[PaulIG11]

Hello everybody,

Thanks to everyone who has replied to me. I had considered the "terminals" issue to both the shower itself and the switch to go in the ceiling ..... and the fact that one of the cables' conductors might in itself come out leaving the other to take the entire load....

The idea behind this shower is to build in redundancy - last year a diaphragm went in the boiler and we had no HW for a week.... Stripdown sink washes and stinking wasn't pleasant

I will explain that we already have a gas-powered (ie gas water heated) shower in the upstairs bathroom. We currently have a "utility room" downstairs however which is 6' by 6' or so - immediately adjacent to a the the wall where a 2nd consumer unit is mounted supplying our kitchen extension. {Note, the consumer unit is not inside the utility room and I am not going to pour water over it!!! - If we do install a shower, the feed will come from this 2nd consumer unit through a double skinned (previously external) brick wall into the space in the ceiling of the utility room}. The 2nd consumer unit runs off of 10mm armoured cable which runs along the wall on the outside of our house and has a 65A fuse at the prime consumer distribution board.

IF installed, the cable runs will be less than 1 metre from the distribution board to the pull switch and then probably 3 metres 4 at most from the switch across the ceiling in the space between the joists then down the wall into the shower unit itself.

This is only my 2nd post so I don't want to upset anyone but I will say I hate this part P malarky, notifications, approvals and the like.... Before those who have swallowed the H&S manual come back to me singing its praises, please read my original question and the order they were placed in - I don't consider myself to be a fool or worthy of the Darwin awards at all {why I originally sought advice and the views of others}.... Complete idiots will still use an unprotected spur off a spur off a spur off a spur from the ring main to power a hairdryer and competent DIYers are being prevented from installing things such as additional power points in kitchens.

 

[JohnW2]

If the cables are the same CSA and length and termination resistances are negligable then to all intents and purposes the current will be equally shared. ... However the problem comes if there is a bad termination.

Exactly. That's why I wrote "I really am not happy that one could ever be totally sure that the current was equally shared (and, as you say, remained equally shared)"

All things considered I don't think you could find any grounds in the regs to fail a shower install with doubled up 6mm but I don't like it because of the risk of terminal failure.

I totally agree - and I hope that was apparent from what I wroteyesterday.

Kind Regards, John.

 

[JohnW2]

IF installed, the cable runs will be less than 1 metre from the distribution board to the pull switch and then probably 3 metres 4 at most from the switch across the ceiling in the space between the joists then down the wall into the shower unit itself.

Many would argue that, given the fact that doubling up cable is not ideal (for the reasons discussed here), about £20 for 5 metres of 10mm² T+E would not be an unreasonable outlay.

This is only my 2nd post so I don't want to upset anyone but I will say I hate this part P malarky, notifications, approvals and the like.... Before those who have swallowed the H&S manual come back to me singing its praises, please read my original question and the order they were placed in - I don't consider myself to be a fool or worthy of the Darwin awards at all {why I originally sought advice and the views of others}.... Complete idiots will still use an unprotected spur off a spur off a spur off a spur from the ring main to power a hairdryer and competent DIYers are being prevented from installing things such as additional power points in kitchens.

Welcome to the forum. Whilst there will, of course, be some who will take great issue with what you say, I think you'll also find a fair bit of sympathy for your views amongst those here. The problem, of course, is in trying to think up a bureaucratic/regulatory system that could distinguish between 'complete idiots' and the 'competent' (everyday sense). Individuals make their own decisions as to how they are going to 'deal with' the regulatory system we currently have - although, for fairly obvious reasons, that is not usually discussed too much here!

Kind Regards, John

 

[ban-all-sheds]

I hate this part P malarky

This is Part P:






Why do you hate being required to make reasonable provision in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury?

 

[JohnW2]

Why do you hate being required to make reasonable provision in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury?

I strongly suspect that he doesn't hate that requirement (what sane person could?), and that his comments actually related to the associated bureacracy regarding notification etc.

Kind Regards, John

 

[PaulIG11]

Hi John (and everybody else),

All things considered, I will spend the money on getting the 10mm cable because although I glean from the answers kindly given that doubling up the 6mm cable wouldn't necessarily be illegal / unsafe to do so (if done properly), the needing to make sure that the 2 cables were both successfully anchored in the relative terminals properly / safely would mean having to in effect buy top of the range fittings (ie spend extra on making sure I had something which could certainly safely take 2 6mm cables rather than those just designed for single 10mm ones)..... what I might save on not having to spend around £20 on 10mm cable might be consumed on having to buy the very best / most expensive switch, shower, etc, etc.... Swings and roundabouts.

The next question is this: I have fitted showers in bathrooms, ring mains in kitchens and consumer units into garages in the past (prior to 2004) - I do understand that fuses protect cables / cables get hot [though I would be the first one to admit that I don't know the mathematical formulae behind it all..... but before the H&S brigade jumps down my throat saying "that's why people like me shouldn't be allowed to change a plug", I would say I don't know why cyanide kills people - I just know that it does so I would avoid ingesting it - it wouldn't stop me from transporting it or storing it in a cupboard while following the advice on the label!!!!] - Can someone explain for me the actual process of telling the council / what that route is likely to cost me (ie doing this myself and having my work checked) - or whether I should just pay an electrician to do something I think as a competenet person I ought to be able to do myself but for the hassle of doing so? If I do the work myself and tell the council, can I use the appliance before they come out to check it or would I have to leave the fuse out. Do I have to tell them before I start, during the time of install or afterwards? If afterwards, how long have I got to tell them before I would be considered to be "offending"?

The ludicrous thing about part P is that I can't extend the ring / add another patress in the kitchen to where Mrs IG11 might want one..... But I could always plug in an extension lead and trail it around the back of the sink not forgeting to make sure that the 2nd extension into the first has the join directly behind the taps !!!!!!! Had this been brought in to say "If you want to trade as a sparky, you need this qualification" I'd be all for it - But I really don't want to sort out the wiring in Mrs Brown's loft @ number 22..... I just want to maintain my own property! Being cynical, I think that this has been done PRIMARILY to stop sparkies making a few quid on the side at weekends and not paying income tax for doing cash in hand jobs - people's safety is only secondary, I'd guess.

Another observation about safety would be the colour of the wiring: RED means D A N G E R.... My wife really did think that brown is the new colour for earth - until she was asked what's the yellow & green one for. As I said in a post above somewhere - the idiot running the freezer in the shed on bell cable is still out there and they will continue to do stuff like this - when is someone going to set a modest course (if that's what is needed) to ask simple competence questions for self certifying work on one's own domestic property - other than connecting the earth to the plumbing at various points with an adequate conductor size, do I really need to know voltage drops & coulombs per second / megajoules / impedance / henrys / farads, yada, yada, yada - rather than making sure that the terminals are clean, dry, well insulated and that the cable is screwed down nice and tight to all connectors???? I would not object to paying £25 to get a certificate for my own domestic work only; I don't want to get fleeced whichever way I go and I am sure that there are loads of other people like me out there too.

Rant over !!!

 

[JohnW2]

Rant over !!!

There's little of what you say that I personally disagree with significantly. I do think that forking out on the 10mm cable is probably sensible.

As for the mechanisms of 'certification' of notifiable work, there are essentially two routes. You either notify (in advance) the LA of the work you intend to do (and pay their substantial fee) or else to get an electrician to do the work who is allowed to 'self-certify' (which involves paying a trivial fee to LA). What you theoretically cannot do is undertake work and then get a 'self-certifying' electrician to certify it - they are only allowed to certify their own work. The notification fees are substantial (vary around country but one hears of figues varying from £150 to £400) such that, if one wishes to play by the rules/laws, it is generally (financially) sensible to pay a self-certifying electrician for small jobs.

Kind Regards, John