Wiring load question

[EricP]

Just looking at my house ring main and a bit bothered with my kitchen sockets.

The socket ring is one loop from a 30 amp fuse using 2.5 T&E.

In the corner of the kitchen the ring goes down from the ceiling into an above worktop double socket and then down under the worktop into another double socket.

I always assumed the ring would go into the top socket with one wire unbroken and on to the bottom one and the other wire into and out of the socket. ie both sockets on the full ring.

I have just found the ring stops at the above worktop socket and the bottom one is a spur.

Fine, except the bottom one has a washing machine and a dishwasher plugged in. If both machines are on and the heaters go on, it seems to me that the load on the spur wire is near maximum.

To do what I thought would be the accepted thing would have involved only an extra meter of cable and avoided three sets of wires in the top socket terminals.

Any comments please?

[Bob Watkinson]

2.5mm flat tw/e has a current carrying capacity of 20A. The IEE regs do allow a spur to be wired in 2.5 despite the potential 26A loading. It's not a situation I would design for though and if you can incorporate it into your ring I would do it.

[EricP]

Thanks for the figures, exactly what I want. As the appliances are the type where the maximum loading could be prolonged, I will extend the ring. The installer put conduit in and everything, why not the extra little bit of cheap cable? He must have spent longer twisting the three pairs of wires and fitting them in the back of the socket.

I may be able to pull through the conduit, but if not, it's only 2m of bolster work through decent plaster. Could have done without it though.

Again many thanks :))

[meow2222]

No, it is rated to carry 20A continuous. That means it has higher current carrying capacity, but more significantly will happily carry much more intermittently.

because its fine.

no theyre not.

If you switch both machines on and monitor the cable temp, you'll find all is well. PVC is rated to operate at upto 70C. 60C is hot enough to hurt.

NT

[Bob Watkinson]

That may be so but when designing circuits we have to follow the regulations irrespective of our assumptions of whether or not full load current is to be carried intermittently or not. The regs allow diversity in some situations but not in others and when they do then specific calculations are made to determine an assumed load. As an electrical engineer it would have been irresponsible to recommend anything other than what I did.

[meow2222]

even though the socket on 2.5 is both safe and within regs?

NT

[Bob Watkinson]

Yes. If you read again my post to th OP you'll find I said the regs covered it but I don't like to design it that way personally. It's not good practice to under-rate a cable given a potential load. Far better to do the job properly. There are many thing that are technically within the regs but you wouldn't do it. Would you wire a whole house in 16mm and protect it with a single 60A fuse. Course you wouldn't it would be silly but the regs allow it.

[Christian McArdle]

There's no problem with the spur per-se. It will handle the current. You do have a problem with a designed in point loading on the ring, which would not be considered a good idea these days. This point load would be the same even if the socket is on the ring proper and would not be solved by a circuit reconfiguration.

The correct solution these days is to have a dedicated circuit to the laundry appliances, leaving the ring for portable appliances. This is how I wired mine up. I've got a 32A ring and a separate 32A radial for the dishwasher, washing machine and tumble dryer, run in 6mm cable. I can run all the appliances simultaneously in complete confidence. The main kitchen ring ends up quite underloaded, with only intermittent loads, like kettles and microwaves, which is the best way to run them.

Christian.

[EricP]

No fear of you having problems then. :))

Thanks for the input.

[Bob Watkinson]

Well it would actually. With a ring you have two current paths instead of one so the current effectively is halved in the conductors.

Nothing wrong with your solution, though it depends on having sufficient ways in the CU and is only really achievable in a rewire. When you say this is the correct solution, how is it more correct than the OP wiring his appliances into the ring (which is what I would do using FCU's as they are fixed appliances)

This is how I

IMHO I would say a little overkill. If you really wanted to keep these items off the main ring they would have all have sat quite happily on a 32A dedicated ring wired in 2.5mm. A much easier task than wrestling with 6mm and even with all three on I doubt you would get tripping.

The main kitchen

[Christian McArdle]

The current wiring regulations require you to consider the ring balance. In particular, it is not permitted to "design in" substantial and foreseeable point loads. Personally, I think running all the laundry off one double socket is a substantial and foreseeable point load and is poor practice when designing the circuits. That is not to say that an existing system should be regarded as dangerous or requiring rectification, it is just good practice when designing a modern installation.

Christian.

[Bob Watkinson]

Which regulation are you referring to? I believe you're mistaken, I've never seen such a requirement in the regs or any revisions. In current distribution the current is distributed evenly along parallel paths irrespective of where the load is.

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