# Immersion Heater Timer

https://www.electricalsafetyfirst.org.uk/guidance/safety-around-the-home/plugs-and-fuses/ says
"Plugs for appliances rated between about 700 watts and 3000 watts (the maximum rating of a wall socket) should be fitted with a 13-amp fuse (coloured brown)."

There are various calculations online. Some take UK voltage at 230 and some take it at 240 . If 230 then they suggest multiplying by 1.25 ... (watt/volt) x1.25
If 240 then they leave it as is...

But i understand that the cable rating also needs to be taken into account when deciding on the value of a fuse...
Though these are for plugs (which i think in my case is the same as a switch)....

is that right ?

this is all fascinating btw...

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240 volts divided by 17.7 ohm = 13.56 amps

So , either way, a 13 amp fuse is in the original switch. Which I'm guessing is fine. (?)
The new timer-switch has also a 13A rating (for a resistive load) and a 13A fuse.

Thanks John... I understand that if a fuse is lower than necessary, it runs the risk of 'blowing' if there is a surge...
So in a way, the lowest possible fuse is 'safer' but runs the risk of cutting out....
Am i right?

In which case, the current switch I have has 13A fuse anyway. And it's a simple on/off switch. I have no other means to turn it on or off. It was like that when I moved in. (Which i'm guessing is not ideal)....

I've learned so much through this discussion... (Though it has distracted me from my own work!!)

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But i understand that the cable rating also needs to be taken into account when deciding on the value of a fuse...
Though these are for plugs (which i think in my case is the same as a switch)....

is that right ?
The cable rating is all that needs to be taken into account.
Fuses and MCBs are to protect cables; not appliances.

13A is the maximum available for UK plug fuses.

Thanks John... I understand that if a fuse is lower than necessary, it runs the risk of 'blowing' if there is a surge...
So in a way, the lowest possible fuse is 'safer' but runs the risk of cutting out....
Am i right?
Don't worry about that, The actual current, where a BS 1362 fuse will blow, is about 1.66 times the rated value, so for a 13A fuse this will be 21.6A.

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Thanks everyone ! I am biting the bullet

What brand/ make is it ? Is it GreenBrook (kingshield)
You didn't respond to this ,but nevertheless you have a fused ,switched connection unit ,and you can replace it with the timer which is effectively a fused ,switched connection unit with integral timer.And to answer your original question ,the earth conductors go together into the terminal on the back of it.

With a resistive load, the amps go UP when the volts go up.

Working from the nominal figures:

We know that the nominal voltage is 230

We think that the nominal amps are 13

x volts ÷ y amps = z ohm

230 volts divided by 13 Amps = 17.7 ohm (rounded)

Calculating on 240v:

x volts ÷ z ohm – y amps

240 volts divided by 17.7 ohm = 13.56 amps

Here we go again. The UK voltage is 240v and has been since nationalisation. (Some idiots decided to call it 230v a while back but nothing has changed). His immersion heater is most probably rated at 3kW at 240v so Harry Bloomfield is correct.

Yes, Harry and Winston are correct.

With a resistive load, the amps go UP when the volts go up.
They do.
That means they go DOWN when the volts go down.

Working from the nominal figures:
We know that the nominal voltage is 230
We do.

We think that the nominal amps are 13
We do not.

x volts ÷ y amps = z ohm
230 volts divided by 13 Amps = 17.7 ohm (rounded)
Calculating on 240v:
x volts ÷ z ohm – y amps
240 volts divided by 17.7 ohm = 13.56 amps
What about?

x volts ÷ y amps = z ohm
240 volts divided by 12.5 Amps = 19.2 ohms (exactly) and the approximate resistance usually found in the elements.
Calculating on 230V:
x volts ÷ z ohm – y amps
230 volts divided by 19.2 ohms = 11.99 Amps.

2760W ÷ 230V = 12Ω

Thanks All and especially EFL for the photo evidence to confirm that what I knew was correct, was in fact correct. I have not seen many immersion heaters, but all of those I have come across have been rated 3Kw at 240v. I have come across JohnD's argument that the rating is 3Kw at 230v before, but I have never seen such an immersion heater in the UK.

immersion heater circuits are usually fused at 16A (they should not be on socket circuits), and their timers don't usually have fuses in them.

As is my immersion circuit a 16amp MCB, but that is more to do with the fact that they don't manufacture a 13amp MCB. The 16amp is to protect the 2.5 T&E and the final section of 2.5mm flex. It isn't to protect the 240v 3kw 12.6amp immersion heater element.

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