FCU not permitted for immersion heaters, really?

[EFLImpudence]

I've had a quick look and it seems the diameter of 2.5mm² flex is 2mm greater than 1.5mm².

Would that make using the clamp impossible?


It can't do because everyone does it.

 

[Harry Bloomfield]

T&E has never been acceptable to feed an immersion heater. I'm only talking round cable here.

T&E is perfectly normal, for the fixed wiring of an immersion heater circuit. The round cable is the flex, connecting from the accessory, to the actual immersion heater, at the cylinder. The usual flex to use is 2.5mm, high temperature flex, not for current rating, but for robustness.

 

[Harry Bloomfield]

On a recent job the cable clamps supplied with the immersion heaters were not suitable for the existing 2.5mm² round cable....so the contractor binned off the cable clamps and left the elements with no proper cable restraint.

I have never come across a 2.5mm heat resistant flex, as normally used for such, not fitting the cable restraint.

 

[Gasguru]

In this case the 2mm extra diameter of the larger cable makes it impossible to fit safely...the cable clamp will not accommodate that size and due to the ever decreasing size and cheapening down of immersion heaters the cores would be under significant strain when routed from the clamp to the terminals.

Perhaps many many years ago the cable clamps would accommodate 2.5mm² round cable (also remember that most/many thermostat manufacturers have now incorporated the overheat thermostat into a single larger unit reducing space).

How can T&E ever be acceptable?....no cable clamp has ever been fitted to an immersion heater that safely clamps that particular wire construction. Although it's common to see T&E fitted it does not make it correct. The same argument applies to the wall outlet from the switch/FCU.

Is it not the case that we generally have cables rated somewhat higher than the protective device?
If this was the case with older rewirable fuses that might explain the reason for 2.5mm² to be a common size in the past.
And now with a 16A MCB being the norm why 1.5mm has become the size that immersion heater manufacturers require with there smaller clamps.

 

[AndyPRK]

TBH I have some 2.5mm T&E (it's actually bigger) going to an immersion of 2011 vintage. I don't recall a fight with the cable clamp

 

[SUNRAY]

Likewise 2.5mm² always was the cable used in the days of fuses and the habit has continued, I've never found the clamp to be an issue but occasionally it has to go through a hole in the cover (rare) and makes it extremely fiddly sliding the cover into position but very much easier with 1.5mm². I'm finding it's changing in recent times, particularly so when dual elements are used for E7 systems requiring 4 core flex and more wholesalers now sell 1.5mm² by the metre than used to.

As to T&E into the heater, Yes I have seen it, I think 3 times but it's a sh1tee botch. One was a temporary botch for an event and one was intermittant N-E RCD tripping after ~10 minutes where the insulation had softened and mishaped then made contact against bare metal when things got hot.

 

[JohnW2]

How can T&E ever be acceptable?....no cable clamp has ever been fitted to an immersion heater that safely clamps that particular wire construction.

I don't really understand that. Although running T+E to an immersion is unusual, I've certainly seen it done, and have not noticed any problem with the clamping.

 

[Gasguru]

Here's common heater fitted to some unvented cylinders...there's barely room for 1.5mm cable.

 

[JohnW2]

Here's common heater fitted to some unvented cylinders...there's barely room for 1.5mm cable.

Fair enough, but I'm much more familiar with ones like this, whose clamp would, I think, have no problem accommodating 1.5 mm² T+E (I've certainly seen it done).

 

[EFLImpudence]

Here's common heater fitted to some unvented cylinders...there's barely room for 1.5mm cable.

I too have come across things which are badly designed.

 

[AndyPRK]

Arh. The unvented ones

 

[Risteard]

Everyone ok with using 1.5mm² to the immersion heater (that's what the cable clamps designed for) on a 16A breaker?

Of course. Why wouldn't you be?

 

[Risteard]

Are they? I can't see there would be much difference.

Anyway, in my experience, nearly everyone uses 2.5mm² heat resistant flex for immersion heaters.


Yes.

An FCU would not be necessary.
Although they should be adequate but the fuses seem to become loose with the constant full load which leads to more overheating.
The same with plugs - yet I have known one that lasted 'for ever'.

Yes, they are - in the same way that plug tops are designed for a maximum of 1.5mm^2 flex. You'll notice this with the cord grip and the space in the outlet which will prevent larger cables from fitting correctly.

And no sensible person uses 2.5mm^2 flex for an immersion heater. Certainly heat resistant though.

 

[Risteard]

I don't really understand that. Although running T+E to an immersion is unusual, I've certainly seen it done, and have not noticed any problem with the clamping.

It's certainly not acceptable for the final connection to the immersion heater.

 

[securespark]

I seem to remember that immersions in the 70s were a little more powerful. Somewhere roundabout 14A. But I may have been mistaken. I must look in my 14th Ed. Regs.

My second house, built in 1968, had an immersion tapped off the ring final with a switched FCU.

Under the 15th, it was permissible to use 1 milli for "power" circuits. In theory, I could have used it to wire some of the shorter immersion heater circuits I have wired, even on a 16A breaker. Indeed at college, we did an exercise proving in certain cases, 1 milli was fine.

Not sure what the rules are now. You could probably use 1.5?

But my go to was 2.5 up to the local 20A DP switch, then 1.5 Butyl to the element.
But I didn't use HR flex.

The comments in this link are interesting, which effectively state that which we probably all agree with, the older accessories were much better built:

Immersion Heater & 13 Amp socket. - Wiring and the Regulations BS 7671 - IET EngX - IET EngX https://share.google/7i91oTRVGq8JGEHZU