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Fixed double wall socket in new(ish) house regs question

Postby nathanmnm » Thu Apr 21, 2011 10:16 am

Hi, just moving into a ~8 year old house (so I assume all the electrical wiring is up to spec) and have a question about double fixed wall sockets.

The room where I plan to have my home cinima setup and computer has two fixed double sockets, they are on opposite walls but I am unsure about current limits.

If I power on all the home cinima equipment together then the current could spike well above 13A, probably above 20, I have read that fixed double sockets are probably 20A limit but this raises some questions,

1 - What current limit are fixed double sockets (its not twice 13A obviously but some people claim 13A total and others 20A)?

2 - Are both double sockets in the room going to be on the same ring, i.e. is it 13A or 20A for both double sockets together?

Will the main circuit box trip if the current is exceeded? i.e. how do I know if I have gone too far - before the smoke alarm goes off...

Basically can I take both double sockets up to 20A or is this asking for trouble? If I cant then do I need to put some of the gear in a different room - will this even make any difference... I assume there is a 100A line comming into the house, humm

Thanks for any info!
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Postby stoneyboy » Thu Apr 21, 2011 8:51 pm

It's likely but by no means definate that both sockets are on the same ring main. This will probably be protected by a 32A mcb and there is no reason why you should not use 2x 13A from the double socket. If you did this with both sockets and they are on a ring the mcb will trip.
Try with all your equipment and see what happens.
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Postby cmhicks » Thu Apr 21, 2011 9:04 pm

Assuming that the wiring has been done to usual modern practice then you can draw up to 13A from any given socket, up to a maximum of 30A off any given ring main. It would be very unusual for the sockets in a given room to be on different rings, so that would imply that you can draw up to 30A total in your cinema/computer room subject to (i) there being a maximum draw of 13A from any one socket, and (ii) there being no other loads on the ring.

You can draw 13A from each half of a double socket.

However, I have to ask, what sort of cinema is it, that can draw 20A (except for inrush current at switch on, which could be several times that, but which won't trip a normal breaker because it is so short)?

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Postby ericmark » Fri Apr 22, 2011 8:05 am

I have also read that a twin socket is rated at 20A and really it is all about time. Plug in two kettles which will boil and switch off again in 6 minutes then there is no problem. Plug in two storage radiators which will run for 6 hours and there is a problem.

As to ring main the normal supply is 32A and the designed current is 20A at central point and 12A spread around the system. As a result when working out volt drop it is calculated on a draw of 26A. If what you have really draws over 13A continuous then there could be a problem but it is unlikely. The heat from such equipment would be far too much for a normal room you would be watching in a sauna.

If you want to be sure the energy meters are relativity cheap now and you can plug one in and measure. I would think it is no more than 6A for the lot except for in-rush as it is all switched on.

8 years old will be to BS7671:2001 and we are now BS7871:2008 so likely there are quite a lot of items not complying with current regulations. This would have been shown on the "Electrical Installation Condition Report" was called "Periodic Inspection Report" which should be carried out every 10 years or change of occupier. Although it may not comply that does not mean it is not safe. The reason for highlighting the code 4 items (Complies with previous edition) is that if you want to alter the electrics then you may need to upgrade first.

I think you are worrying about nothing but without viewing all you have I can only guess. Except where something really silly like two storage radiators plugged into same socket outlet it is rare for sockets to be overloaded without blowing a fuse.

What may be a problem with IT equipment is the earth leakage. Most IT equipment have some sort of spike arrestor in the supply which to work leaks a little power to earth. Likely less than 1ma however where there are many items used the total can exceed the 15ma threshold at which a 30ma RCD can trip. (Expensive types can be 90% not 50% but we always test at 50%) So plugging in a load of IT equipment may trip the RCD which under 2008 regulations covers all sockets under 20A unless dedicated and all cables buried at less than 50mm unless special cables plus all items in the bathroom (Including lights).

The reduce the build up of earth leakage you may need RCBO's instead of just 2 RCD's but this is only a problem with very new houses since 2008.
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