wiring dual immersion heater switch

8 Jan 2010
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United Kingdom

I need some help with wiring a dual immersion heater switch, it is the MK one available from TLC, there is no info. on their or the MK site. I have got the old switch with the wires still connected but this has little in the way of marking.

It looks like the live (red) from the 20a dp on/off switch is going into the common on the 2w switch, the neutral (black) is going into an insulated connector block. There is a 3 core cable going to the heaters wired as follows: black into earth block, red on 1 (sink) and green on 2 (bath).

There is no earth currently going to the immersion heaters.

The cylinder is a direct combination with 2 separate immersions, both 3kw, one at the bottom (bath) and one at the top (sink).

I am replacing the cable back to the CU (changed about 7-8 years ago), there is a 16a MCB and I will run 4mm tw+e to the switch.

Am I right that I should then run a 4 core cable from this switch? Wired as follows:
1 to earth right through
2 from N on main switch to neutral right through
3 from 1 to sink immersion
4 from 2 to bath immersion
Link from L on main switch to common on 2 way switch

If not what should I be doing???

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i guess you have a twin immersion tank?

do you know what immersion to wire to which? ie sink, bath?
yes, a twin immersion tank... top one to sink, bottom one to bath
you wire a seperate 3 core for each element...

I've never fitted one and can't find the wiring diagram on the TLC or MK website so can't advise on the switch wiring..

if it's an either / or kind of job ( ie only 1 element on at a time ) then you're ok with the 16A feed..
if however you can have both on at the same time then you'll need a 32A feed as each element draws 12.5A when on..

also, are you aware of Part P of the building reg's?
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It's an either or situation, the sink setting turns on the upper element and just heats the top of the tankgiving you a small amount of water - the hot water is drawn from the top of the tank. The bath setting turns the top element off and turn on the bottom element which heats the whole tank.

Can't find any 4 core heat resistant larger than 0.75 so think I will run 2 supplies as you suggest.

I'm loosely aware of part P, worked in the trade after leaving school for 12 years but gave up about 10 years ago as it felt like there were new regs every week. I'm happy that it will be safe, and definately safer than the old cable i'm replacing!
what do we recon chaps? replacing a single damaged cable?? not in the bathroom ( I take it the tank is in a cupboard somewhere with a door? )
The switch has two sides to it:
the left hand side is an on off switch (double pole),
the right hand side is a 2 way switch.

So L + N in the 'feed' side of the on/off switch.

Neutrals from both immersions in the 'load' Neutral connection.
Fit a wire link between L on the load side and 'C' on the sink/bath side.

Bottom immersion L to L1
Top immersion L to L2.
(Check these are the correct way round by operating the switch and checking to see which element is heating).

I wouldn't have thought this were notifiable as its probably in a cupboard?
Thanks for your help londonboy, this was where i'd got to...

I just wanted some clarification as i'd never wired one up before, couldn't find a circuit diagram for the switch, the old switch doesn't have marking except COM and the existing heaters are not earthed as the 3rd core had been used as live to the 2nd heater.

The tank is in a cupboard and i'm replacing the cable back to the CU as when I took the old switch off the insulation cracked on the existing supply.
I'm not sure if 4 core heat resistant flex is available, I usually use two 3 cores (one to each heater).
Please make sure you earth both heaters and both cables!

Let me know if you get stuck. ;)

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