sockets on ring main

[Dave Plowman (News)]

Yes.

[John Rumm]

This is where the concept of diversity comes into play. A 32A circuit can in theory be loaded to maximum capacity with just three sockets at full load, but in fact may have 10s of sockets actually wired into it.

With circuits designed for general purpose use, one can make assumptions about the loadings based on statistical likelihoods. You can usually assume that loads will be distributed around the circuit, and will be a mixture of load types[1]. So for every large load there may be a dozen small insignificant ones etc. Ring circuits in general are very good at coping with today's pattern of usage which normally require lots of sockets, powering a large collection of mostly small load stuff, with the occasional higher load often imposed only for shortish durations.

[1] If you know for a fact that this is not the case when designing the circuit then you should take that into account.

[John Rumm]

or power consumption even ;-)

and a good many loads will be things like electronic kit drawing even less than that, or things like chargers etc that take almost nothing.

Big loads these days tend to be found in the kitchen / utility room (hence why a dedicated circuit for these is common).

[nightjar]

Yes. It would be potentially hazardous to anyone working on the sockets if it were possible to isolate the supply to some in one room, but not to others in the same room. I have one ring downstairs, one upstairs, one for the kitchen and one for outdoor sockets. the last two have individual RCBOs, rather than MCBs.

Only if you plug in high current devices. Mine run a lot of electronic equipment, that need lots of sockets but not much power.

Colin Bignell

[dennis]

Like the argument I had with an electrician wiring someone's mansi^W house..

Wanted to put in a ring for the kitchen when he had 20 ways left on the fuse panel.. argued about diversity when I said it needed more.. No it isn't very likely that someone will run the kettle, toaster, washing machine, tumble dryer, dishwasher, microwaves, etc. at the same time, but why take the risk when all it costs is a couple of meters of cable?

[John Rumm]

Depends a bit on the kitchen in question. The kettle and toaster are usually ignored in domestic installs since they are very short term loads. The others can be longer term loads, but are often thermostatically controlled, which increases their diversity. There is certainly no harm in laying on extra radials for potentially large appliances - or say a second ring in a utility room though.

[Stephen]

[David Hansen]

On Fri, 15 Aug 2008 09:34:14 GMT someone who may be Sam wrote this:-

To add to what the others said, it depends what is plugged in.

One method of thinking what is likely to happen in the worst case is to imagine that the normal heating has failed, the house is typically badly insulated and each room has a heater plugged in. Most rooms are unlikely to have more than one such heater plugged in, certainly a room is unlikely to have 16 heaters plugged in [1]. The heaters are likely to be 1 or 2kW radiant heaters or 2 or 3kW thermostatically controlled air heaters (fan or convector). The average load of this collection will be 1 to 2kW, probably nearer

2kW, though it will be higher for the first 20 minutes or so before the thermostats start to operate.

In the "typical" 3 or 4 bedroom house with one ring circuit upstairs and one downstairs under those conditions both ring circuits will be heavily loaded (assume two heaters in at least some downstairs rooms), but will be able to cope with both the startup conditions and the normal conditions and have some capacity for other loads. In a more modern better insulated house one ring main might be enough to cope in these conditions.

In a larger house there is likely to be more diversity, even under the worst conditions (for example it is unlikely that all rooms will be occupied when the normal heating fails) but eventually there comes a point where the circuit may become overloaded. That is why there are maximum floor areas for the standard socket circuit arrangements, even though the number of sockets on the circuit is unlimited.

Compared to the loads imposed by emergency electric heating, under normal conditions a ring circuit is unlikely to be that heavily loaded, even if every socket is occupied by an electronic gadget and the odd hair drier. The ring final circuit is a superb design, it allows large numbers of sockets to be installed and within reason anything can be plugged into any socket without problems. It also provides a useful cable running round the house into which fixed outlets can be easily connected, provided they are not to be used for space heating. There is seldom any reason to run a separate radial circuit for an electric towel rail, it can just be connected to the ring.

In normal use the place final circuits are likely to be heavily loaded these days is in a kitchen/utility room, which is why a separate circuit should be considered for these areas.

[1] I have seen computer suites under test where hundreds of convector fires were plugged into one room and throwing out heat. The above is only for domestic buildings, in industrial/commercial buildings other factors may well apply and circuits should be tailored to the building.

[Owain]

In practice, most people are unlikely to keep sufficient spare heaters to have enough for more than one or two rooms to be heated simultaneously.

Owain

[Jerry]

But that doesn't stop them going out on a chilly winters day and buying more from the local 'shed'...

[Chewbacca]

You pay your money and make your choice.

you cold put in enough rings so that you could run loads of heaters and ovens, water heaters etc, but the main supply fuse is 80 or 100A. So now that is not big enough......

[David Hansen]

On Sat, 16 Aug 2008 18:11:19 +0100 someone who may be Chewbacca wrote this:-

A "typical" 3 or 4 bedroom house can be heated electrically with that size of "electricity board" fuse. As I said, in a larger house it is unlikely that all rooms will be occupied when the normal heating fails and thus the occupants can keep warm.

Someone who has a very large house may want to pay for a suitably sized supply. A friend used to live in a house which had provision for a three phase supply. He had done the wiring so that it would be easy to convert into three phase internal wiring. He never did get the other two phases connected in the end.

[Jerry]

The thing about planning an electrical installation is to plan for the present and 'reasonable expectations', it's not unreasonable to expect a 3Kw electric heater in the main living room and 1Kw electric heaters in each (normally used) bedroom in the event of a boiler breakdown.

[Sam]

Sorry, I should have made myself clearer, I wasn't expecting sockets within the same room to be on different rings; I was wondering how many rooms were on the same ring.

I'm not sure about the broken heating scenario. If I put three 3kW fan heaters on a ring, wouldn't that blow a 32A MCB? So if you want to prepare for boiler failure, I think you could only have two rooms per ring. I suppose you would have to switch the fan heaters to their lower setting and be patient for the rooms to heat. Fitting so many rings sounds extreme for something that hopefully will never happen or will rarely happen.

I agree that most people (myself included) do not have lots of heaters but if the boiler broke, I would rush out and buy some.

How do people that use electric heating manage? Do they have dedicated radials to each heater?

Thanks, Sam.

[David Hansen]

On Mon, 18 Aug 2008 08:49:41 GMT someone who may be Sam wrote this:-

9kW at 230V is a little over 39 amps. According to the time/current characteristics a 32A MCB will not operate in less than 10,000 seconds at this current, probably a fair amount more as it is on the almost vertical part of the curve. That's 167 minutes or 2.7 hours. The wiring can withstand this if it was properly designed and installed. In a "normal" house long before this time the thermostats on the heaters will have been turning the heaters on and off as necessary.

Note that in a "normal" house the bedrooms are not likely to contain many 3kW heaters, 1 or 2kW is more likely. Also in a "normal" house there are unlikely to be more than three public rooms each with a

3kW heater in them on the ground floor circuit. Thus the ring mains are unlikely to be as heavily loaded, except for the say 30 minutes that the heaters are all on before the thermostats take over (or people turn off a radiant heater for a while as they are too warm).

If one is not in a "normal" house then the standard circuits are not appropriate and the design should be considered more carefully. To take one example, a castle is of such a heavy construction that assumptions about thermostats going on and off may not be realistic.

The current which any bit of electrical equipment can withstand is not one figure, it is a combination of current and time (the energy let through) which determines whether the item will be usable or not. It is surprising that the larger types of pvc insulated and sheathed cable, used as meter tails, can withstand well over 10,000 for the short time (until the protective device operates) without damage. Even twin and earth can withstand many hundreds or a few thousand amps for short periods (can't be bothered to look up the precise figures). Of course if the protective device is not there or fails to operate the results are spectacular, there are some great films from fuse manufacturers of what happens with and without the fuse at extreme currents.

They should do. It is essential for storage heaters and large space heaters should not be connected to a ring but instead wired on radial circuits.

Note that if there is a 1kW infra-red heater in a bathroom it might as well be connected to the ring in most circumstances.

[Owain]

And heat-and-light units used to run off the lighting circuit ... 750W I think.

Owain

[Sam]

I'm not sure I agree. Surely a "normal" house will not have any electrical heaters but if the boiler breaks the householder will rush out and buy a number of fan heaters. People that read this group know more than the average member of the public and would consider the loading of the ring but Joe Public might not and would just buy 3kW heaters and wonder why his fuses were blowing. What 1-2kW heaters were you thinking of?

[David Hansen]

On Tue, 19 Aug 2008 21:21:57 GMT someone who may be Sam wrote this:-

1-2kW heaters. Radiant fires tend to come in 1 or 2kW, convector heaters and fan heaters are increasingly 2kW. 3kW heaters are becoming rarer, though still widely available.

[ARWadworth]

In practice you should not need to design circuits for electric heaters in every room for a house with CH.

The boiler breaks down then it gets fixed. So you are cold for a few days. Tough.

Someone is making up fictional stories about houses loaded with electric heaters for no reason. Most people do not own one fan heater per room and will not rush out to buy one per room if the boiler breaks. Normal rational people will heat the lounge whilst they watch TV and the bedroom for an hour or so before they go to bed using an electric heater if the boiler fails. They will also not rush out and buy several electric heaters to heat every room in the house but instead spend their money on fixing the boiler.

Adam

[Frank Erskine]

It's easy to say that in this relatively warm (if wet!) weather :-))).

Actually though you're perfectly correct. If my fairly reliable (famous last words!) gas boiler breaks down I certainly wouldn't panic until it was fixed. (Then I would!!) The study, where I suppose I spend most waking time, is quite small and is probably adequately heated at least from two PCs and me (and the dog when he can be ar$ed to come up here!).

The kitchen can assume enough warmth from the cooking facilities and by the fact that it has large south-facing fenestration.

My bedroom (the master one) never seems unduly cold; with a suitable choice of tog-rating fror the duvvet it's never unbearable.

I DON'T have a fan heater...