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.