Here we go again, 6mm cable x 10kw shower x whack a 40A mcb in.

[old man sparky]

Just been to another one of those !!
Plumber replaces shower for upgraded version. 40A mcb replaces 32A. No problem mate !! that will work fine.
So 12 months later the owners girlfriend moves in with him.
So that poor old 6mm that was just starting to break a sweat for 12 months, suddenly ain't getting that quick 10 minutes and off cycle.
Nope its 10 minutes followed by 30 minutes to include washing those golden locks.
Over heated cable over heated shower switch. What's that funny burning smell ?
And that my friends is why 40A mcb's on 6mm cable cus now I have a much more powerful shower.
Is just pushing your luck a little to far, and one day the luck runs out.
Will they never learn ??

 

[old man sparky]

There was a discussion on this forum some months ago about whether 6mm was inadequate for modern showers. The question was asked if anyone had come across a shower supply cable that had suffered damage from overheating and no one had. The consensus was, was that the switchgear was the issue regarding overheating.
While the situation with ever increasing wattages needs to be addressed it must also be recoqnized that a 6mm cable clipped direct is rated to carry 46 amps continuously (24 hrs) That means it will carry 46amps for a half hour without registering any or little heat to hand touch.
My shower (in a rented house) is a Triton T90 (8.5 kW). It's wired in 4mm. We like our showers. Inspected the cable every year. It's in perfect nick.
Is it possible the particular 6 mm was of anon standard variety.? I have come across T&E that was hard to strip not because of overheating but due to poor quality?

Provided heat can dissipate and the cable run is moderate. The average on load times for showers will mean borderline installations can show little signs of adverse affects for years. Its when heat cant escape and starts to build up due to a change in usage time that those borderline situations become unstuck. The problems occur when you start taking ideal conditions and applying it to every install. There are few manufacturers that would quote 6mm cable for a 10.5 kw shower 101 installed for most of an 18 meter run. 10mm is the better option long term. As said you can get away with it in some scenarios but its not good practice.

 

[LastManOnline]

Provided heat can dissipate and the cable run is moderate. The average on load times for showers will mean borderline installations can show little signs of adverse affects for years. Its when heat cant escape and starts to build up due to a change in usage time that those borderline situations become unstuck. The problems occur when you start taking ideal conditions and applying it to every install. There are few manufacturers that would quote 6mm cable for a 10.5 kw shower 101 installed for most of an 18 meter run. 10mm is the better option long term. As said you can get away with it in some scenarios but its not good practice.

I comepletely agree that the situation is not ideal. The shower circuit is without question the most heavily loaded in the house. To have a situation where a 40 amp rcbo is taking 43 amps (10 kW, shower? ) is as you already mentioned in a previous post not acceptable and is very poor design. Most circuits use just a fraction of their capacity.
New shower installs should really be wired in 10mm and we should have a 50 amp rcbo. That future proofs if.
On the other hand I think we need to assess the situation in such a way as to avoid becoming alarmist. It's gotten to a stage over here that we are expected to condemn existing installs wired in 6 mm, though they are according to the wiring method and shower wattage perfectly safe.
The most sensible approach would be to compel shower installers to look first at the electrical installation supply and let the homeowner know what wattage shower is permissable. Currently we have a cart before the horse approach

 

[LastManOnline]

I guess it depends on how long the 6mm cable is and how it is run e.g clipped direct or fished under loft insulation.

JW did a vid showing how 2.5mm clipped can take about 60-65amps before it started sweating and a solid 70amps before it started smoking. This was only about a metre of cable clipped to a bit of mdf but it shows cable can take a lot more amps that the tabulated values depending on how its installed

Our wiring systems are very, very robust. Which is how it should be. Somewhere between around 10 of electrical installations here have DNO mains of 6mm or less. I have once seen tails of 2.5 mm from the meter to the CU. Was called to the house for a tripping rcd. On arrival someone was using the electric shower. It was evening time, fridge, telly etc all on. In disbelief I could, nt resist feeling the 2.5 mains. It was warm to the touch but certainly not boiling.
I am a big fan of lightly loaded circuits. I have begun recently to install socket radials with 16 amp mcb, s to futureproof the circuits. My issue is with installers walking in to homes and condemning 6mm shower installs as a fire risk without first assessing the install from an electrical perspective.

 

[Dustydazzler]

I suspect you could run 100-125a down a short run of 6mm cable clipped direct for ages, As i say several youtuber sparks have done random experiments passing comfortably more amps down a cable than its tabulated value permits. To no ill effect. You really need to ramp up the amps to 2x or even 3x it max permitted value to get some heat and smoke going...

 

[Soi disant]

New shower installs should really be wired in 10mm and we should have a 50 amp rcbo. That future proofs if.

Only just.

 

[old man sparky]

I suspect you could run 100-125a down a short run of 6mm cable clipped direct for ages, As i say several youtuber sparks have done random experiments passing comfortably more amps down a cable than its tabulated value permits. To no ill effect. You really need to ramp up the amps to 2x or even 3x it max permitted value to get some heat and smoke going...

Having started in this game back in 1969 I hope I have managed to learn something over the years. I will be putting my feet up when the other lads let me. I hope !! But for now I can only say that things have changed so much that the entire trade has turned into a dog eat dog, back stabbing, where theres blame theres a claim set up. You have to watch your backs these days and air on the side of caution. I would have agreed without reservation with most of the comments you guys have made, and I still do. But bad design is bad design. Even if in good faith you can prove it should be ok in most circumstances. I am thinking of YOU. Not the customer or some up there own --- scheme provider who will always hide behind the book stab you in the back, and say you should know better. So if you see a 10.5 kw shower on a 6mm cable. For your own sakes go by the book. Cover your asses. Your not doing yourselves any favours saying well !! Should be fine. Because BS7671 will prove you wrong and thats what will be relied upon when and if things go south. Sad but true, so think of yourselves, and start covering yourselves.

 

[old man sparky]

Quick update, jobs done. 10mm installed. fed from 45A rcbo. (I was lucky as the high integ side of the board had been used for a security alarm leaving a spare way on the busbar) So not on either of the dual rcd's my choice, and close to the main switch. Stud wall meant the ceiling mounted pull isolator is binned and a 50 amp MK logic is on the landing side of the wall. I'm happy, customer is happy, who also found the installation manual for the shower. I wont say what size cable they recommend as that would be too obvious. I know this is simple house bashers stuff and not what a fully qualified electrician should be bothering with. But at least the next youngster who comes along will see a proper job.
Can you see a wind up comment in my reply cus its deffo in there. ???

 

[nicebutdim]

Quick update, jobs done. 10mm installed. fed from 45A rcbo. (I was lucky as the high integ side of the board had been used for a security alarm leaving a spare way on the busbar) So not on either of the dual rcd's my choice, and close to the main switch. Stud wall meant the ceiling mounted pull isolator is binned and a 50 amp MK logic is on the landing side of the wall. I'm happy, customer is happy, who also found the installation manual for the shower. I wont say what size cable they recommend as that would be too obvious. I know this is simple house bashers stuff and not what a fully qualified electrician should be bothering with. But at least the next youngster who comes along will see a proper job.
Can you see a wind up comment in my reply cus its deffo in there. ???

If you don't mind my asking, why place omit RCD protection?

 

[DPG]

If you don't mind my asking, why place omit RCD protection?

He didn't, he used a separate RCBO. Not sure why the reluctance to use the existing RCDs though I must admit

 

[stevethesparks]

If you don't mind my asking, why place omit RCD protection?

I assumed an rcbo was used
Snap

 

[nicebutdim]

He didn't, he used a separate RCBO. Not sure why the reluctance to use the existing RCDs though I must admit

Missed that in the opening sentence ?‍

 

[old man sparky]

He didn't, he used a separate RCBO. Not sure why the reluctance to use the existing RCDs though I must admit

I prefer rcbo populated boards to dual rcd boards for probably the same reasoning behind the preferences of dual rcd boards to a single rcd. At least if at a later date an earth fault develops within the shower it wont take out half the board this way. So just taking discrimination to the best level really. Just a personal choice nothing more than that.

 

[DPG]

I prefer rcbo populated boards to dual rcd boards for probably the same reasoning behind the preferences of dual rcd boards to a single rcd. At least if at a later date an earth fault develops within the shower it wont take out half the board this way. So just taking discrimination to the best level really. Just a personal choice nothing more than that.

Yep fair point. Can't beat an all RCBO board.