Not sure I fully agree with Wiring Matters > Years > 2025 > 107 - September 2025

I. have come across many heat damaged spurs and the occasional socket over the years supplying immersion heaters so agree with that part. I personally always fit. a 20 amp DP switch. I don't see the need to have to lock it off as you will be working right next to it. If it needs locking off for other reasons do it at the consumer unit as it should be a dedicated circuit although I have seen an odd boiler or heating control tagged onto the same circuit.  Rotary switches are clunky and could be prone to being inadvertently damaged or knocked off the wall in an airing cupboard.

Gary

if you can lock it off at the CU and are feeling cheap you can probably drop the switch rated for isolation part altogether.

Its main use is not for us to work on it once in a blue moon, but for the householder to turn the water heating off when not needed - while on hols or similar . 

Fused spurs only make sense when the supply (dedicated or shared with others), is of a higher rating - e.g a 32A ring. 
To me another obvious fix if you must have one, may be  to put the fused spur unit outside the cupboard. much as one used to put conventional light switches outside bathrooms. being visible  also reduces the risk of it being left in the wrong state for a long time.
Mike

Not like you to post such gibberish Mike presuming this post was meant as a reply to what I wrote.

Gary

mapj1 said:

Its main use is not for us to work on it once in a blue moon, but for the householder to turn the water heating off when not needed - while on hols or similar . 

Or to turn it on once in a blue moon when the boiler has conked out.

mapj1 said:

Fused spurs only make sense when the supply (dedicated or shared with others), is of a higher rating - e.g a 32A ring.

Agreed!

Whilst a BS 1363 device would not be my first choice, I would not prohibit the use of one.

I do not know how efficient immersion heaters are, but given that almost all the energy is released into the tank, it should be pretty good.

Assuming a supply at 10 ºC, a thermostat setting of 60 ºC, and 100% efficiency, 3 kW for an hour will heat 220 l of water, which is enough for 2 baths, or at least 4 showers. So where does the prolonged load come from?

If the immersion is drawing current for a significant proportion of the day, the leccy bill is going to be a bit eye-watering.

BS 1363 devices may be rated at 13 A, but they are type-tested at 14.0 V ± 0.4 V with a maximum ambient temperature of 25 ºC and maximum allowable temperature rise of 52 K over 4 - 8 hours.

If the cylinder is adequately lagged, the cupboard ambient temperature should not be significantly above room temperature.

So what will a customer think?  I can plug in a 3 kW heater all day, so why not the immersion? If it says 13 A on the tin, it should be suitable for 13 A.

Well indeed, and double sockets that dont handle 2 full 3kW loads are a recurring source of surprise to hapless users- usually tumble dryer and washing machine sharing.

There is I think, an engineering argument that the passing bare minimum BS136x   13A plug/socket and 13A fused spur are all a bit under rated for modern life, what with overnight full loads such as EV charging and so on. (and modern houses are hotter to start with. )

But, rather like the car with enough petrol to put the driver into orbit stored in a single skin tin can tank a few inches off the road surface, the chance of changing the established practice now is probably almost nil.

Equally if you invented it now it would probably not get approval. I suspect the "13A' would not either, and might end up being nearer 10A .

The  European 16a Shucko plug fares no better, and also cooks with a long term load but is equally too long established, and I guess there is no appetite for inventing a peak current 20A domestic type thing that is utterly incompatible.

(Ignoring the fact that actually the ever interesting Brazilians did just that back in 2007, and their 'new standard' (NBR 14136) plugs  allow 20A on 4.8mm dia pins or 10A on  4mm dia pins, in a socket that may be supplied by either 110v, or 220v or 110-0-110 split phase, depending on where in the country you are. They also seem to have quite a few cooked looking sockets and a light sprinkling of every flavour of connector imaginable, so let's not copy that, shall we.)

Mike.

Are you suggesting that BS136x possibly needs an update or a revision 2?

mapj1 said:

13A fused spur are all a bit under rated for modern life, what with overnight full loads such as EV charging and so on. (and modern houses are hotter to start with.

Maybe a new product standard is needed?

Should we consider making the average UK domestic dwelling be now run in 4mm2 CSA rather than the standard 2.5mm2 CSA T&E?  Some people say a domestic Ring/Ring Final or Radial should be 2.5mm2 CSA because it has always been that way.  However the cost of a drum of 2.5 vs 4mm T&E will be a limiting factor

mapj1 said:

tumble dryer and washing machine sharing

Kettle and toaster every morning. :-)

mapj1 said:

tumble dryer and washing machine sharing

Kettle and toaster every morning. :-)