Immersion heater wiring - stop me doing something silly!

[AndyI]

Hi all.

Amateur, fair concept of mains electricity, but also some idea of my limitations....

Anyway, I've stuck an immersion heater into my hot water cylinder (thanks to the folks on the plumbing forum) and got a timer in place and ready to go, but have realised I'm an idiot and should have planned it better, as there's no satisfactory means of wiring it up.

There's a tempting mains outlet which controls the gas boiler and timer, which seems to be on one of the socket circuits from the consumer unit. I suspect I'll be needing the fire brigade if I wire the immersion heater into that circuit, but would be delighted to hear I'm worrying needlessly.

There is also a beefy cable coming from the plasterwork and entering the power shower. Now this seems to be a good candidate, but the problem is, I'm quite fond of having showers.

The consumer unit has some 32A breakers, plus one 40A breaker which is labelled 'immersion heater' - this controls the shower. I can only imagine that there used to be an immersion heater but no shower, though why the original immersion heater was removed (the hot water cylinder is the original) and replaced by a blank, I have no idea. Presumably someone couldn't think how to support both items on one circuit. This doesn't bode well! There are no spare breakers, or slots for breakers, in the consumer unit.

So..... (answers to one or more would be great)

1) can I use the nice, handy outlet on the 32A 'sockets' circuit? (expected answer - laughter).
2) would it be OK to tee into the cable on the 40A 'immersion' shower circuit (and if so, how)? (expected answer - sniggers)
3) do I need an electrician, and how much work will he/she need to do - dig big holes in the plasterwork? replace the consumer unit? (expected answer - yes, yes and yes).

Oh, and, out of interest, supplementary question 4) why can you plug a kettle into a mains socket safely, yet you need a dedicated circuit (or at least not a spur) to wire up an immersion heater of about the same power requirement?

Thanks!

 

[luminaire]

You need a dedicated circuit for an immersion heater, because in theory you could leave it on all day (and night) and the loading would be too much to add to another circuit.
You will need to get get various quotes to find out the cost.

 

[ricicle]

.


Oh, and, out of interest, supplementary question 4) why can you plug a kettle into a mains socket safely, yet you need a dedicated circuit (or at least not a spur) to wire up an immersion heater of about the same power requirement?

Thanks!

There is nothing wrong, from an electrical point of view, with plugging an immersion heater into a socket outlet circuit.It is the loading side of things where the concerns arise - if you plug it into the socket circuit you are reducing the capacity of your circuit by around 12 - 13A for long periods of time while the immersion is heating the water.
A kettle is usually only on for a couple of minutes or so leaving the socket circuit near full again.Washing machines only take a few minutes to warm the water, and tumble driers are nearer the 10 - 11A mark (and normally have dual temp settings)
So you can see it makes sense to have a known load on it's own circuit to free up the capacity of the socket circuits, otherwise you could end up with overload tripping.

 

[ColJack]

1 - no

2 - no

3 - "can't see it, can't quote it" as said by someone on here before..

it depends on the location and route... if the immersion is in it's own cupboard and you have floor boards and the CU isn't far away and you have spare ways then it's simple enough to run..

4 - see posts above...

there may be a good reason they took out the immersion heater..

if there has been a combi boiler fitted then this may just be used as a storage tank for hot water..

 

[AndyI]

Thanks for the input, guys.

As I say, I reckon the immersion heater was taken out because they wanted an electric shower. Of course we now have to boil a kettle if we need hot water during the summer, which is pretty crap really (still, if it's a straight choice between shower and immersion.....). There is no spare room on the consumer unit (and I'm assuming I'll need a new one - could someone confirm?).

I'm still a bit unclear - I have a 40A breaker running the electric shower and nothing else. What exactly stops me from using that circuit for the immersion heater too, provided the shower draws less than 20A? It's pretty weedy!

Thanks again.

 

[TicklyT]

The cable to the immersion heater would probably be rated to carry about 16 Amps maximum, so protecting it with a 40 Amp breaker would not be a very good idea.

Anyway, a pretty weedy electric shower would draw about 7KW.
7000 Watts / 230 Volts = 30.5 Amps as the crow flies.....

A 40A breaker for the shower labelled 'immersion' has set alarm bells ringing for me - Has the cable between the CU and the old immersion spur been replaced, or has someone stuck a 40A breaker on the end of the existing spur, and extended it to the shower

 

[AndyI]

Tickly - thanks for the answer. I'm not 100%, but the current cable to the shower runs uninterrupted from the wall plaster to the shower unit and is a good bit longer than a cable to the immersion would have been, so perhaps it's OK - I'd have expected some obvious junction if it had been extended. The cable looks pretty beefy, but I'll see if I can find out what its rating is likely to be. It's been working fine for at least 6 years, so I'm not expecting to come home and find a pile of ashes, but thanks for the warning.

Right. Decision made, sparky to be brought in for chin-stroking and teeth-sucking.

Thanks again everyone.