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 ??

 

[Dustydazzler]

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

 

[Soi disant]

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.

Yup - 47A for Method C.

So not necessarily wrong to use it for a 10kW shower. Unlike using a 40A OPD.

 

[Soi disant]

Over heated cable over heated shower switch. What's that funny burning smell ?

Maybe the smell was from the switch, occasioned by a poor connection?

Did you find damage to the cable anywhere except at the switch terminals?

 

[old man sparky]

Thermal damage showing at the mcb and isolator not down to loose connections. Shower actually rated at 10.5 kw. 6mm running 18 meters ref 101 for a good part of that. I'm not making excuses for a set up showing this kind of evidence. And after talking to the home owner about how and for how long the shower is usually used. This is clearly a case of pushing the limits and what happens when you do. Shower is all good its the cable size, the length of run, and the installation method that's causing the problems. Its a simple fix. Just needs to be done correctly. Its a classic all going fine until usage habits change. Then the weakness within the design will show, just like this has.
10mm install and you wont find yourself second guessing with depends on this depends on that ?

 

[telectrix]

with 10mm you'd need the click or the crabby 50A isolators. no way to get 10mm into the cheap shyte even with a decent hammer and exposing basic insulation above ceiling.

 

[old man sparky]

with 10mm you'd need the click or the crabby 50A isolators. no way to get 10mm into the cheap shyte even with a decent hammer and exposing basic insulation above ceiling.

About time the ruddy things were banned. Another case of manufacturers churning out the same crap from bygone days long after its use has become obsolete.

 

[DPG]

Was the 6mm cable definitely overheating? Or was it at the terminations in the switch etc?

 

[buzzlightyear]

Was the 6mm cable definitely overheating

more like old man sparky .lol.

 

[Pretty Mouth]

Was the 6mm cable definitely overheating? Or was it at the terminations in the switch etc?

I would bet good money it's loose connections. Otherwise the overheating would be apparent over the entire length of cable.

 

[old man sparky]

Was the 6mm cable definitely overheating? Or was it at the terminations in the switch etc?

Definitely overheating but not as a result of loose connections. 2 meters of slack up in loft. When stripping back a good meter from the isolator. The insulation was still bonded to the conductor. Always a good indication that the cables been running at high temps. Seen this stuff before and its not difficult to put it right. Trouble is people buy replacement showers a good 1.5 to 2 kw above the original and some plumbers stick it on the end of the existing 6mm, whacking a 40 amp breaker in the board and thinking the jobs a good un. As said its all down to how long the shower is running. If its a 10 minute session you can get away with it for years. But when the load is sustained for long periods. The heat starts to build and damage occurs. Remember, the accessories ie mcbs and switchgear have max operating temperatures as well. And copper easily transfers heat to those items. Its something I have seen happen where xlpe is used. Cables ok running at higher temps but connected control gear start to suffer when those higher operating temps are called on. Its all about designing fit for purpose. Not rocket science.

 

[LastManOnline]

Over heated cable over heated shower switch. What's that funny burning smell ?

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?

 

[nicebutdim]

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?

I posted in that thread about the inlaws asking me to look at this very situation - shower upgraded to 8.5kW on 6mm cable and (after the sort of change in circumstances described by OP) melting 30A fuse carrier.

No sign of thermal damage at board, switch or shower and cable clipped direct in a run totaling roughly 6m. My advice was to get a spark in fairly sharpish, but they did as expected and waited until the carrier was beyond use.

Spark came out after advising that cable & CU would likely need replacing, but was happy with cable and apparently replaced fuse carrier with 40A MCB (possibly RCBO - not been over and can't remember if circuit was protected by RCD).

 

[LastManOnline]

melting 30A fuse carrier.

That harmonises with the vast majority of experiences. Sometimes, the fuse has overheated, not because of the fuse rating itself but because of not being screwed in tightly enough or just because of arcing over the years resulting in a poorer connection between fuse and backplate. I would advise you check there is an rcd on the shower.

 

[nicebutdim]

That harmonises with the vast majority of experiences. Sometimes, the fuse has overheated, not because of the fuse rating itself but because of not being screwed in tightly enough or just because of arcing over the years resulting in a poorer connection between fuse and backplate. I would advise you check there is an rcd on the shower.

All connections were secure and cable showed no signs of thermal damage. The fuse had apparently been replaced quite a number of times, so I can not comment on the exact cause (beyond unreasonably long use of shower).

I wasn't doing anything beyond taking a cursory look (for several reasons) and was fairly certain nothing would be done until that carrier was beyond use, despite having made clear that it needed attending to as a matter of urgency.

Trying to remember layout of board and am fairly certain it was protected by the single RCD, but the guy they got out has a good reputation and I'd be fairly confident that it has been left in a safe and serviceable condition.

 

[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.