Wiring immersion heater

Vital! The terminals on the element run hot by conduction and ordinary PVC goes very limp and insulation could easily be displaced.

Thanks. How do I determine whether the existing flex is heat resistant? :)

To the actual unit? Essential. Many fit a JB etc close to it so only need the heat resistant flex for the last little bit.

All I can tell you that I've had one wired up since the 1970s with a short piece of three core large flexible pvc covered wire to the switch and as I've had the heater changed twice I know that no damage has occurred to the wires at all. Obviously its not got hot enough to melt pvc. Now whether there are fault scenarios that might suggest that the specs now would call for higher temp sleaving I'd not know but certainly I've not seen any problems, indeed even the small wires for the thermostat are pvc covered. Brian

Interestingly, my emersion heater terminals do not run very hot at all, and none ever have. I suspect it depends on the type and thermostat settings. as I say, I've seen no sign of soggy sleaving on mine since the 1970s. Its just pvc, not ptfe. Brian

They follow the temperature of the hot water. Plus any heat generated by the terminal resistance.

There are two common types. Butyl which has a cream sleeve and much softer to the touch than PVC. There is also a heat resistant version of something like PVC which is usually engraved as heat resistant. That is much less flexible than butyl.

PVC's temp limit is only a bit above DHW temp. Ordinary 1.5 would be fine if the terminals were cold, but they're above DHW temp, so you need heat resistant pvc plus thicker flex to minimise the temp rise in the flex due to the current it carries.

As Brian points out PVC can be run above its temp limit, but doing so is not really safe. It goes very soft, conductor migration occurs easily and only a little contact resistance sends it over the edge.

NT

you can also get black butyl - used on theatre lanterns

It will also tend to become brittle over time, and in the worst case the insulation can fall off.

If unsure - why not just replace with heat resistant 2.5 : it won't break the bank and job done

I will do. It's just a bit thick and inflexible to work with. It's got to come out of a switched outlet, then do a sharp turn through the side of a kitchen unit, etc.

Adam will be along in a minute to talk about 'proper' 10mm stuff, but I don't have to deal with that and it would be a bit tricky to get in

You could probably use 1.5mm all the way from the CU!

Any markings on it? Usually HOR etc?

The butyl is suitable for use in contact with grease and oil and so often seen used on cooking appliances.

It also, as you said is more flexible than the standard heat resistant stuff.

It's one of those myths that was started by a bloke in the pub that "once knew an electrician" that tells you it has to be 2.5mm^2:-)

Probably the same bloke that tells you about the pike called "Pump house terror" that once killed and ate a Jack Russell terrier that was paddling in the local mill pond. And that is true because "a fisherman that he once knew" told him.

Heat resistant flex would be preferred as that works up to 90 deg.

It might have the cable type 3093Y printed or embossed on it for heat resistant, or 3183Y for non-heat resist

Thanks. I'll check the markings on what's there.