Just checking ... immersion heater spur?

21 Dec 2004
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United Kingdom
Happy and Safe New Year all.

I have just installed two security lights (150W each) that I have not yet connected, with cable runs into the loft.

There are currently 11 light fittings and 2 extractor fans on the upstairs lighting curcuit, so if I connect the security lights to the upstairs light curcuit, that should still consume less than 1200W with everything on (assuming 60W bulbs elsewhere).

However, I'd rather shift the load for the security lights to something other than the lighting circuit, just to give more flexibility in the future. Searching the forums, I found this thread ... //www.diynot.com/forums/viewtopic.php?t=11432 which suggests it's okay to run a spur off the immersion heater. For me this is by far the easiest way to get power to the loft ... particularly since we never use the immersion heater.

So what I'd like to do is put a radial from the immersion heater (16A MCB) switch out to two FCUs in the airing cupboard ... a 5 amp up to a socket outlet in the loft and 3 amp for the security lights. There's no specific use in mind for the socket ... just a nice-to-have; eg plug-in light for the darker corners etc. Obviously if the heater was on, I wouldn't be running much from it anyway!

I intend running 2.5 cable to the two FCUs, then 1.5 on the load side of the FCUs (assuming stated fuse ratings).

Does this sound reasonable?
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It sound safe, and I suppose reasonable in the sense that reasons for doing it could be produced.

However... It is custom and practice to use a dedicated circuit for the immersion heater. The first reason is that the circuit is pretty much matched to deliver the amount of power needed to run an immersion heater, with not a lot left over. The second reason is that it is possible to isolate just this circuit, without affecting anything else. Cicuits are supposed to be designed so that there is a sensible breaj up of what they do.

What happens if someone decides to change the two 150W for 500W? Perhaps this is not very likely, but one of the reasons for not running big lamps off the lighting circuit is their relatively big load compared to the power available. It would probably be better to (fused) spur them off the socket ring.
I was very surprised to read the thread that suggested that doing this was acceptable. I'd always thought of immersion heaters as dedicated circuits. As you say, there's not a lot of juice left over. But the argument "what if someone adds x, y, and z" could surely be used on any circuit ... there's always a point at which it becomes overloaded ... as long as it's still the fuse/breaker that's the weakest point.

I'd really love to take a spur off the ring ... my problem is that it's very awkward to do without a fair bit of hacking around.

If the immersion heater spur was a legal (and you would therefore suppose reasonably safe) thing to do, then it would be a better option for me. If there are strong arguments against then I'll find another way ... but that's why I'm asking the question again.
Just knew you would say that the ring was inconveniently far away. Always is.

It is not permissable to design a circuit which you know will be overloaded. Which is the reason for having circuits dedicated to one thing. For example, it is not permissable to put two 30A showers on one 45A circuit. Because even if you know you will never shower in both bathrooms at once, someone will do it.

On the other hand, a ring is allowed to have as many sockets as you like. The idea is that it is very unlikely you will use them all at once. Simply because people just don't use that many pieces of household equipment at the same time.

So, within reason you can ignore the generalised rules and add things.

Now, considering part P. A certified and qualified alectrician still has discretion to ignore the standard rules. He always has. Only know an official inspector is there to look over his shoulder and impose his own interpretation of the rules. Quite apart from what the inspector thinks about the greater crime of plainly departing from them. How many inspectors are going to allow departures from the rules? More than their jobs are worth.
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Ahem, there you go then! No, it's a new house, and I can't find (believe me I've looked) a straightforward way of getting a cable up behind the plasterboard into the loft. And by choice I'd rather not bung a run of trunking up. I can find sockets that have space behind them, but when I feed a rod up behind there's always some obstruction. Not having a clear picture of how the walls themselves are constructed doesn't help.

But that was my point about overloading ... the appliances I want to use won't overload it. On any circuit, why should anyone else believe they can come along, add x fixed appliances to it, and it won't overload? (I'm discussing rather than arguing here btw). It's not like something that plugs in where you might reasonably expect to put any appliance in and have it working. If you replace or add any fixed appliance on any circuit surely you have to consider the capacity of the circuit, what's already on the circuit and the load that you want to add to it?

I completely agree with your original point about the circuit being matched to the appliance and no; there ain't much to play with. Thanks for your comments - I'll have a look back at the ring and see what I can do there as a first choice.
Yes, I agree. I wouldn't just add a shower to the ring. But a floodlight, even 500W, is reasonably small compared to 30A. Now if it was a floodlit arena, might think again. If it was two or three big floodlights then perhaps a brand new dedicated circuit would be a good idea.

Perhaps one reason for specific rules is that people may come along and add things without thinking at all whether they might overload the circuit.

Guess you also know what you are supposed to do. Start smashing holes in the walls. Mindful, of course, of permitted routing for cables.

Don't you hate it when a new house needs ten million extras?
Oops, missed the last paragraph of your last but one post Damocles. I presume you're referring to the design thing ...

It is not permissable to design a circuit which you know will be overloaded.
How would I be doing that if it's to work with two 150W lights and that circuit can accomodate the load? Again ... not being pedantic ... just genuinely trying to understand the reasoning. What if the socket outlet was taken out of the equation, would that be more comfortable? There would be two fixed appliances of 150W, which could be accomodated by that circuit. How would I be "designing a circuit which I know will be overloaded"?

Really don't wanna start chasing walls ;) .... but I'll drop it now, honest!
Sorry, wasn't really meaning your specific example. Just trying to show the reasoning. However..

I would not connect a socket to an immersion heater supply. That's a potential 13A on top of your immersion. How do you know when the immersion is on? If you have to use it for some reason, will people remember in 10 years time not to plug in the heater in the attick? If you stick a socket on a cooker supply you are supposed to assume 5A loading. But that is on a supply with a much bigger margin (30/45 compared with 16 here). Always safer to play the averages game when there is more to average.

You do not say what sort of attick. If it is the dust and fibreglass sort with no boards, then perhaps ok. But if it is in any way used, then I think not. And someone may come along in 10 years and say,' great, the power is already in'.

Seems to me quite apt to stick a shower pump on the same circuit as the immersion. Perhaps its just the way my mind works, but this combimation feels more in the spirit of the circuit dedication.

I have myself argued the case for putting in a socket on a 6A circuit. Admittedly, the example I had in mind was attached to the ceiling and unlikely to be used for a spare room heater. Also for a garden supply. I instinctively dislike supplying additional current on a circuit I know damn well is going to be shorted out regularly. But in general, people expect to be able to plug in anything with a plug into a 13A socket. Not desirable to design in trips unnecessaily.

This kind of discussion is beyond the simple cut and dried design rules. Interesting though.
mind you we had all our rings on 15A rewirables for years and the only ring we ever had any trouble with was the kitchen ring (and even then it was something like 3 times in 10 years)

you could also probablly upgrade the immersion cuircuit to 20A (assuming its 2.5mm cable) which would give you a lot more leeway (use a FCU for the immersion itself if you do this though)

noone calls for 5A fused extention leads made with 6A flex to be banned either do they?
No but they are probably supposed to have '5A max' written all over them
the reels are generally pretty clearly printed with ratings

plain leads on the other hand tend to at best have stickers that don't last very long.
This stuff is very creative and I would not complain about that. The original poster does not like the idea of knocking his house about so is looking for an alternative. But myself, I reckon the trend of the times is towards more individual circuits. Better discrimination, and much more fun. Have you seen some of those TV makeovers with just reams of cabling everywhere?
personally i think that the best method may be to wire everything back to a central point

if you really felt inclined you could then take all the wiring to some form of inlets (perhaps the 16A variant of iec connectors) which could then be patched to power directly or to dimmers or to remote switch boxes or whatever.

you are probablly familar with structured cabling for data and telecom you could do the same for mains
marmite said:
There are currently 11 light fittings and 2 extractor fans on the upstairs lighting curcuit, so if I connect the security lights to the upstairs light curcuit, that should still consume less than 1200W with everything on (assuming 60W bulbs elsewhere).
I'm very surprised that nobody has yet pointed out that you aren't supposed to make that assumption. Lighting circuit calculations should assume a minimum of 100W per lampholder..

For me this is by far the easiest way to get power to the loft ... particularly since we never use the immersion heater.
Then disconnect the immersion heater and re-label the circuit "Loft Supply". Follow the rules for a 2.5mm radial, with FCUs to supply the lights, and everyone will be happy.
damocles - I agree that most people assume anything can go into a socket. And normally I guess that should be a fair assumption. So yes ... I 've come round to thinking the socket, at least is a bad idea. Also, I know what you mean about the shower pump. And strangely, since that's something we may be aquiring shortly, that may be where it ends up!

plug - Nice idea about the structured cabling, though I'd prefer something quicker to install.

bas - Thanks, I couln't find a "standard figure" for the lighting circuit lampholders, though I've since found the 100W figure on tlc. The mere fact that 100W isn't uncommon is one of the reasons why I was reluctant to add the security lights to the upstairs circuit anyway (despite the fact that we've actually got low-energy bulbs throughout!).

Disconnect the immersion heater? Perhaps I should have said "particularly since we've never used the immersion heater - yet"! I don't fancy rewiring things just 'cos the central heating's on the blink!

I've read enough to convince me that: a) It's the right idea to keep the security lights off the upstairs lighting circuit; b) I should try and get a spur off the upstairs ring - somehow! If all esle fails, I'd put a 3A FCU onto the immersion heater spur with just the lights on it - but definitely no socket.

Thanks for all the comments - it's been really useful.

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