Immersion heater - timer wiring

[EFLImpudence]

In any case, 13A accessories are not suitable for a 16A circuit.

Like 13A accessories are not suitable for use on a 32A radial or ring final circuit?

Well, no. I thought '13A' sockets were rated at 20A

Even single sockets?

We must have been here before but -

the springs for the plug may be 13A but the conductor terminals, surely, have to be 20A (for a ring).


Back to the timer.

I thought my interpretation was accepted/common practice for immersion heaters.

However -
If rated at only 13A, does that mean the timer should be protected by a 13A fuse?
I know, I know - the mcb protects the cable but the timer can be overloaded and, as such, should not be on a 16A circuit.
As it is rated at 13A (not 16A) this would indicate that 13A is all it can handle.

Alternatively, if it is acceptable to use a 13A fuse for an immersion heater then there is no reason that a 13A plug on a ring circuit cannot be used
as with the 3kW kettle (albeit on for three minutes, though).
For some reason immersion heaters should be on a dedicated circuit and are usually wired with hugely oversized conductors.

 

[JohnW2]

We must have been here before but - the springs for the plug may be 13A but the conductor terminals, surely, have to be 20A (for a ring).

Of course. In fact, more than 20A, since one doesn't have to have too unbalanced a load on a ring final for the current in one leg to exceed 20A. However, as I said, I thought we were talking about published 'ratings', and I do seriously doubt that you'll find a 13A single socket, or an FCU which the manufacturers say is 'rated' at more than 13A.

Back to the timer. I thought my interpretation was accepted/common practice for immersion heaters.

I actually thought the same about what I was saying - but it transpires that, by chance, it seems that I've only ever been exposed to immersion timers which were rated at 13A.

However - If rated at only 13A, does that mean the timer should be protected by a 13A fuse?

Well, that's someone that people might expect me to say - but,as you go on to point out....

I know, I know - the mcb protects the cable ...

...which is the response I usually get when I make such suggestions, but then you go on to say:

but the timer can be overloaded and, as such, should not be on a 16A circuit. As it is rated at 13A (not 16A) this would indicate that 13A is all it can handle.

Do I take it that I have a convert ? Would you therefore have a problem with, say, 10mm² cable (clipped direct) on a 50A MCB supplying a 7.5 kW shower (provided you were not constrained by MIs)?

Alternatively, if it is acceptable to use a 13A fuse for an immersion heater then there is no reason that a 13A plug on a ring circuit cannot be used as with the 3kW kettle (albeit on for three minutes, though).

I think that '3 minutes' is probably crucial, and brings us back to the apparent deficiencies of BS1363 plugs/sockets. We often discuss the fact that double sockets seem unable (and may well not be 'rated' to) safely cope with a sustained load of 2 x 13A, but we discuss less the fact that a single plug/socket seems at risk of getting hot, if not worse, with a sustained 13A load.[/quote]

For some reason immersion heaters should be on a dedicated circuit and are usually wired with hugely oversized conductors.

I have always assumed that the reason for recommending dedicated circuits for high fixed (and often semi-continuous) loads was that socket circuits (which are probably the ultimate in a 'diversity' situation) were never really intended for such things, which effectively limit the capacity of the circuit to provide power to sockets - plus, as above, probably a feeling that plugs/sockets are not a good means of connecting a high sustained loads.

Kind Regards, John.

 

[EFLImpudence]

However - If rated at only 13A, does that mean the timer should be protected by a 13A fuse?

Well, that's someone that people might expect me to say - but,as you go on to point out....

I know, I know - the mcb protects the cable ...

...which is the response I usually get when I make such suggestions, but then you go on to say:

but the timer can be overloaded and, as such, should not be on a 16A circuit. As it is rated at 13A (not 16A) this would indicate that 13A is all it can handle.

Do I take it that I have a convert ?

No. That's not the same.

The immersion timer is not a current using appliance at the end of a circuit.
It is merely an accessory between the OPD and the load and should be capable of carrying the current.
The same as a switch. You would not use a 20A (let alone a 6A) for your shower below.

They (timers) do fail from overheating quite frequently.

Would you therefore have a problem with, say, 10mm² cable (clipped direct) on a 50A MCB supplying a 7.5 kW shower (provided you were not constrained by MIs)?

Technically there is no problem - but a >50A switch would, of course, be required.
However, if new, it would be considered bad design.

Alternatively, if it is acceptable to use a 13A fuse for an immersion heater then there is no reason that a 13A plug on a ring circuit cannot be used as with the 3kW kettle (albeit on for three minutes, though).

I think that '3 minutes' is probably crucial, and brings us back to the apparent deficiencies of BS1363 plugs/sockets. We often discuss the fact that double sockets seem unable (and may well not be 'rated' to) safely cope with a sustained load of 2 x 13A, but we discuss less the fact that a single plug/socket seems at risk of getting hot, if not worse, with a sustained 13A load.

I would think single sockets are ideal. Do they get hot?

For some reason immersion heaters should be on a dedicated circuit and are usually wired with hugely oversized conductors.

I have always assumed that the reason for recommending dedicated circuits for high fixed (and often semi-continuous) loads was that socket circuits (which are probably the ultimate in a 'diversity' situation) were never really intended for such things, which effectively limit the capacity of the circuit to provide power to sockets - plus, as above, probably a feeling that plugs/sockets are not a good means of connecting a high sustained loads.

I was being sardonic and you are, of course, correct.

 

[ban-all-sheds]

They ought to be, but sadly they are not.

Off topic, but last night I was fitting a longer flex to a wall-mounted 4-socket strip.

Opened it up.

All the internal conductors are PCB tracks, and the receptacles for the pins are small loops of copper strip over holes in the board. It oozes quality.


Not.


IICBA I'll open it up again and take photos.

 

[EFLImpudence]

I note that the timers with a rating of 13A also state that that is up to 3120W.
This is at 240V.

We assume that it is correct but if merely a mathematical calculation then using the now nominal voltage of 230V the result is 2990W.

It all depends.

 

[kai]

For my Thermtec immersion (only used as a backup to the gas central heating!) - the manufacturer's instructions state that a 13amp fuse is required in the supply feed to the heater, through a double-pole isolation controller switch with min 3mm gap. The MK-K330-WHI fcu immediately springs to mind.

 

[EFLImpudence]

For my Thermtec immersion (only used as a backup to the gas central heating!) - the manufacturer's instructions state that a 13amp fuse is required in the supply feed to the heater, through a double-pole isolation controller switch with min 3mm gap.

Just to be certain.

These instructions were for the actual immersion heater itself, not the timer ?

 

[JohnW2]

Do I take it that I have a convert ?

No. That's not the same. The immersion timer is not a current using appliance at the end of a circuit. It is merely an accessory between the OPD and the load and should be capable of carrying the current.

It should be capable of taking the load current, yes.

The same as a switch. You would not use a 20A (let alone a 6A) for your shower below.

Of course not, given that it would be switching an ~31A load current!

Would you therefore have a problem with, say, 10mm² cable (clipped direct) on a 50A MCB supplying a 7.5 kW shower (provided you were not constrained by MIs)?

Technically there is no problem - but a >50A switch would, of course, be required.

Would it, if it were supplying a fixed-wired load of around 31A?? Again, isn't it the size of a (fixed) load which counts, not the potential of the supply?

However, if new, it would be considered bad design.

Would it? Why? Would it be bad practice to run a dedicated circuit (say for an alarm, doorbell or whatever) for a very small load (say <1A) in 1mm² cable protected by a 6A MCB? ... and isn't 'future-proofing' often a sign of good design (although I admit one could just run the 10mm² cable but use a smaller OPD for the present)?

... but we discuss less the fact that a single plug/socket seems at risk of getting hot, if not worse, with a sustained 13A load.

I would think single sockets are ideal. Do they get hot?

Bad wording on my part - I actually meant a single plug/socket interface, whether the socket be single or double ... but, yes, I think they can get pretty warm with sustained 13A loads (which they are rarely asked to carry), don't you?

Kind Regards, John

 

[JohnW2]

I note that the timers with a rating of 13A also state that that is up to 3120W. This is at 240V. We assume that it is correct but if merely a mathematical calculation then using the now nominal voltage of 230V the result is 2990W. It all depends.

Either way, it's not over 13A.

I presume that, given around 13A at running temperature, currents appreciably over 13A might occur at switch-on if the immersion is cold (in tank full of cold water). However, I would imagine that's unlikely to be a significant issue for a '13A' switch, or relay/timer contacts, since most problems are likely to occur on breaking (rather than making) the circuit, when the element will be hot and hence down to 13A or less.

Kind Regards, John.

 

[EFLImpudence]

I note that the timers with a rating of 13A also state that that is up to 3120W. This is at 240V. We assume that it is correct but if merely a mathematical calculation then using the now nominal voltage of 230V the result is 2990W. It all depends.

Either way, it's not over 13A.

If the voltage rises it would be. I think that is why 13A timer is not adequate.

Especially as a 16A can be fitted at no extra cost.

 

[JohnW2]

Either way, it's not over 13A.

If the voltage rises it would be. I think that is why 13A timer is not adequate. Especially as a 16A can be fitted at no extra cost.

That's all true - but, mind you, all sorts of designs might show inadequacies if supply voltages rose to 253V.

In case it hasn't come over clearly enough, I'm not arguing with you at all. I don't believe in 'brinkmanship' so, had I known of the existence of 16A rated immersion timers, I'm sure I would have used them. It's just that all the ones I've ever seen and used (mainly Smiths/Timeguard) have had 13A written all over them, so I assumed (clearly wrongly) that the same was true of all of all makes.

Kind Regards, John.

 

[riveralt]

to be honest in my absolutely limited experience, I can't understand why something needs to be so complicated. Take a switch, replace it with an automated switch. Surely it should be a straight swap, except that the component now automates what was previously being turned on and off by hand.

To the OP, if you are still listening - can I talk you down of the ledge - you have two choices.
Firstly, fit the timer you have bought and follow the wiring instrucitons as per my information AND the manufacturers instructions.
Alternatively, buy this from screwfix.
http://www.screwfix.com/p/mechanical-24-hour-immersion-timer/89388
Its a 16Amp rated timer and only costs a fiver.

 

[EFLImpudence]

Would you therefore have a problem with, say, 10mm² cable (clipped direct) on a 50A MCB supplying a 7.5 kW shower (provided you were not constrained by MIs)?

Technically there is no problem - but a >50A switch would, of course, be required.

Would it, if it were supplying a fixed-wired load of around 31A?? Again, isn't it the size of a (fixed) load which counts, not the potential of the supply?

It is.

However, (as I said "If new") I would consider it poor practice to 'future proof' the cable but have a smaller capacity accessory fitted in the cable run with a 50A mcb - or not to fit a 32A mcb.

However, if new, it would be considered bad design.

Would it? Why? Would it be bad practice to run a dedicated circuit (say for an alarm, doorbell or whatever) for a very small load (say <1A) in 1mm² cable protected by a 6A MCB?

You're splitting hairs, there are no smaller mcbs or t&e.

Plus any accessories would be compatible with the circuit - the ubiquitous 3A fused switch, perchance.

... and isn't 'future-proofing' often a sign of good design (although I admit one could just run the 10mm² cable but use a smaller OPD for the present)?

See above. If it would be good design to fit the larger cable then why not equally good to fit an equivalent switch.

... but we discuss less the fact that a single plug/socket seems at risk of getting hot, if not worse, with a sustained 13A load.

I would think single sockets are ideal. Do they get hot?

Bad wording on my part - I actually meant a single plug/socket interface, whether the socket be single or double ... but, yes, I think they can get pretty warm with sustained 13A loads (which they are rarely asked to carry), don't you?

Perhaps I've never come across a sustained 13A load.

What would it be - an immersion heater wrongly connected?

 

[EFLImpudence]

Either way, it's not over 13A.

If the voltage rises it would be. I think that is why 13A timer is not adequate. Especially as a 16A can be fitted at no extra cost.

That's all true - but, mind you, all sorts of designs might show inadequacies if supply voltages rose to 253V.

Agreed. I have definitely argued in the past that that would be alright as it must have been taken into account in the figures...

but, these immersion timers are particularly prone to overheating (the diagnosis usually being that the installer did not tighten the neutral sufficiently) I don't think the digital ones are very well made.

In case it hasn't come over clearly enough, I'm not arguing with you at all. I don't believe in 'brinkmanship' so, had I known of the existence of 16A rated immersion timers, I'm sure I would have used them. It's just that all the ones I've ever seen and used (mainly Smiths/Timeguard) have had 13A written all over them, so I assumed (clearly wrongly) that the same was true of all of all makes.

Ok.

 

[JohnW2]

That's all true - but, mind you, all sorts of designs might show inadequacies if supply voltages rose to 253V.

Agreed. I have definitely argued in the past that that would be alright as it must have been taken into account in the figures...

Indeed. As we've often discused, I'm sure there are some pretty enormous margins in some of the official figures. The regs really aren't all that clear (at least, not to me) about all this - but the implication of a lot of what they say is that it is acceptable to design an installation on the basis of the nominal supply voltage.

but, these immersion timers are particularly prone to overheating (the diagnosis usually being that the installer did not tighten the neutral sufficiently) I don't think the digital ones are very well made.

Yes, I gather that overheating of these things is quite common. However, if, as you say, it's usually due to insufficient tightening of the terminals, one can hardly blame the timer for that - the same could happen with any component in an installation, no matter how adequately rated and how well made! I'm also intrigued by your saying that it's usually the neutral that isn't tightened sufficiently - does this imply some sort of 'colour bias' on the part of installers?

Kind Regards, John.