What cable is my incomer?

[Ben Blaukopf]

I've got a PME supply. I have two incoming cables which appear from conduit in the ground. These two cables are black, and 13.6mm in diameter (or to put it another way, exactly the same diameter as some

2x2.5mm swa I have elsewhere in the system).

If I had to guess, I'd say they were single core swa, except that the smallest swa single I can see is 50mm... Also, they don't terminate at the cutout with anything resembling an swa gland - more a big grommet that's probably designed to stop people ramming extra cable in there...

The cutout is 16A immersion heater

13A plug in fan heater 10A electric fire in the living room 6A of underfloor heating I haven't yet installed.

That's 45A. I've based my calculation on "What's likely to happen if the central heating isn't working and it's a cold day". Chuck in the oven (16A after diversity) kettle, and dishwasher, which are quite likely to be used at the same time, and I'm finding it hard to justify a diversity calculation that keeps me under 60A.

If it turns out to be unfeasible to upgrade, then I'll have to get creative (e.g. rip out the electric fire in the living room, which is pretty useless anyway).

Ben

[<me9]

With PME most likely concentric, with the second cable looped on to the next consumer.

[Ben Blaukopf]

What for? Just to give another path to earth? Presumably it has to go to the next consumer using the same phase?

Ben

[The Wanderer]

To give a supply to the next consumer. It was quite common practice to supply up to eight customers from one four-way joint, looping each service cableto feed two consumers, especially on housing estates planned with Gas CH.

No.

Yes

[Ed Sirett]

Please list out the the entire set of fuses/breakers you have and their purposes. You do not need to worry about what is on each circuit only what they do.

There is a simple and conservative method which is given in the OSG to take 100% of the biggest circuit and 40% of all others.

Example: Shower 40A - 100% 1 ring, 1 lighting, 32A + 6A * 40% = 15.2A Total 55.2A.

[Ben Blaukopf]

Currently:

2 rings at 32A 2 lights at 6A Imm heater at 16A Cooker 30A outside power (garage/shed) 20A 3 spare ways

The diversity calculation worked when I had a 6 way fusebox, but not afterwards. Big deal, it's not like I changed the usage of the system by putting a CU in. However, I'm rewiring the kitchen, adding more sockets, adding a bit of ufh, and generally making it more likely that more power will be used.

The eventual layout is:

3 rings at 32A (kitchen is going on its own ring) 2 lights at 6A cooker 30A imm heater 16A ufh 6A outside 20A

Ben

[Tony Bryer]

So if your house suffers a major fire, the neighbours lose their electricity?

[Lurch]

On Tue, 13 Feb 2007 18:07:52 GMT, Tony Bryer mused:

Yes, although if the service head went up in flames chances are the local sub station would do something along the lines of cutting power in some sort of automatic fashion so it would take at least a phase out anyway.

It's a farly common practice, LEB's have been doing it for years.

[Owain]

Exactly.

Adding more sockets doesn't increase the power consumption - using more appliances does.

So the only thing you're adding is 6A of underfloor heating. 3 rings are not going to use any more power than 2, and I bet that 20A outside circuit is hardly ever used to capacity. Also, you haven't mentioned any electric heating, so the CH boiler probably heats the HW and you won't be using the immersion?

Owain

[Colin Wilson]

If it's already a looped service, the odds of getting your fuse uprated is lower than with a normal 1ph incomer, but typically we would be able to uprate the fuse to 80A.

Going to 100A is more likely to be a chargeable job as it's likely to entail a new service off the main.

In some more unusual circumstances, the incoming service cable might have been doubled up - are you in a rural location, over about 30m from the main ? (perhaps 35mm hybrid was in short supply at the time ?!?)

[The Wanderer]

Yup. But how often does that happen? It's hardly a ten-a-day occurrence.

[The Wanderer]

Hmmm. That was always a no-no. I've known that happen perhaps 2 or 3 times at most to meet a commitment, and it was always corrected as soon as 35mm was available. Safety and protection, of course, are the problems.

[Ben Blaukopf]

Of course, but adding more sockets makes it more likely that lots of appliances will be used simultaneously. That said, it's a bit of a silly argument, because the high load appliances all have their own socket already.

Not if it's cold enough to have the heating on full blast :) I could stick a massive diversity reduction on the outside ring, because it's used for things like saws and shredders - so only drawing large current for short periods.

Also, you haven't mentioned any

Yeah, my scenaro is "What happens if the CH is off, and it's cold".

Anyway, if I can get upgraded to an 80A fuse (and I probably can) then I'll be more than happy....

Ben

[Ben Blaukopf]

Thanks, that's useful to know.

Makes sense.

Nope, it's not rural by any stretch of the imagination. 1980 build, for reference.

Ben

[Colin Wilson]

Mind if I ask roughly where in the country you are ?

[Ben Blaukopf]

Cambridge.

[Owain]

Exactly. And it's really only the high-load appliances that count.

How many heaters do you have available to plug in...? Once the house is warm they will cut in and out on thermostats (or be manually controlled) anyway, so there will still be some diversity.

Owain

[David Hansen]

On Wed, 14 Feb 2007 23:52:47 +0000 someone who may be Owain wrote this:-

There may well not be diversity while they are warming the house up though. When switched on in the morning they may all be running for an hour or more to warm up the house. The length of time they are running depends on the construction of the house, weather and desired temperature, but if too many rooms are to be heated the result will be no heating.

It is because they are on for long periods that immersion heaters should be treated as continuous loads.

[Bob Mannix]

Both my current house and previous house had overhead feeds feeding my house and next door via joints and then two concentric cables. Yes a major fire will remove next door's supply. The major problem was building an extension - next door's supply had to be brokren and rerouted through an extended cable trunked through the roof void in the new extension, to avoid going all around it. One cable feeding two semis via a joint on on of them very common supply practice round here (Oxfordshire)

[Lurch]

On Thu, 15 Feb 2007 08:48:00 -0000, "Bob Mannix" mused:

It's very common up and down the country to feed several properties with one overhead, or down a row of terraces.