Electrical Mains Wiring

[Murray Elliot]

Ok, I'm a little confused - any electricians in the house? :o)

I'm adding extra sockets in my kitchen. The downstairs is wired as a ring, so I've added a double socket spur off each socket on the ring. So far so good.

All the references I've read so far state categorically NO MORE THAN ONE double socket on a spur from a ring.

1. If I plug a 4/6-way extension into a ringmain socket - this is effectively a 4/6-way spur is it not? So how does that work, if I can only add a two-socket spur to the mains? Is it related to the extension being fused?

1. Does this mean that I can or cannot put a plug-in 2/4/6-way extension on the spur sockets at any time?

I'm very cautious that I don't overload any circuits or overheat any wiring so I'm taking no chances (although the only thing permanently connected is likely to be the fridge/freezer (one its own switched spur), and the maximum load is likely to be the fridge/freezer a kettle a microwave and a washing machine, at any one time.

Is there someone out there that is able to explain the issues to me, regarding spurs and rings etc?

Thanks in advance.

[Martin]

So far so BAD.

Although you could add a spur, you would have been much better off to run two 2.5mm to each new socket, and connect so it becomes part of the ring.

You should not make any assumptions about what others might plug in - you could end up with 2 x 13amps on a single spur cable.

Precisely - the extension is limited to 13amps, whereas your spur is not limited.

Makes no difference, since each multi-way extension will be fused at 13A total.

I hope you don't mean you'll be running all these off one spur...? :-(

As above, don't rely on future occupiers / user, limiting what they plug into any given socket / circuit.

My advice - especially as this is the kitchen - is play extra safe and replace the spurs you've fitted with sockets incorporated in the ring.

[Murray Elliot]

Oh dear!

Ok.

Isn't the current drawn by a spur limited by the fuse at the distribution box like all ring sockets?

Please don't think I'm questioning your judgement here, I'm sure your advice is absolutely sound, I'm just trying to understand the risks more clearly, for my own education...

Ok. One of my spurs has an FCU on it (the one feeding the fridge freezer), so I hope that's pretty much ok...

A second one is feeding a cooker hood off a single socket high on the wall, which will be behind the hood, so no chance of that being made use of for any other purpose.

There are only two more - one of which COULD be put on the ring, so I may do just that, and the other which would be a complete b*st*rd to put on the ring, so would a good option there be to feed that off an FCU also?

hehe, most definitely not.

Thanks again for the speedy response.

Carrying on from my question above, for my own education - there are x sockets on the ring, all fed from a 35A (?) fuse at the distribution box. Some of these sockets will have a plug connected to it, most limiting the current drawn by the appliance to 3A.

What is the main (excuse the pun) difference of fitting a socket to a spur, to adding a socket into the ring, is it because of the current carrying capacity of a single cable vs two cables?

Sorry for all the questions!

[Bob Watkinson]

The thing is Murray your fuse/mcb in the CU is probably a 32A/30A. Now this is fine for a ring but a single length of 2.5 twin/earth has a current carrying capacity of only 18.5A. So your fuse in effect isn't really protecting your radial circuit and using Martins scenario, the 26A load he mentioned is too much for the cable but the fuse won't trip/blow

What Martin has said is quite right.

d.

Yes that would be a decent option under the circumstances.

Yes this is the reason. It means that your current limiting fuse/breaker will trip before the current level exceeds the rating of the cable. It is a much better job if you extend a ring rather than spur off it.

[Martin]

Yes - but as Bob Watkinson says, the limit is too high for the single cable you've fitted.

That's no prob - this is a discussion group after all...:-)

Can't add much to Bob W's reply... but the thing about fuses is many people think they're there purely to trip the supply if the appliance goes faulty, but the main purpose is to protect the installed cabling. A fuse between the cabling and appliance (i.e. in the plug) is no protection for cable faults. One of the (deliberate) consequences is that if (say) loads of 13A fires are plugged into a ring, the mains fuse will trip, despite there being no faults, simple because the cable capacity is being exceeded. Put simply, it helps stop your house burning down...

As Bob W - Yes!!

No prob - be safe!!!

One other thought - there's been a lot of posts here recently re. new regs limiting what electrical DIY is permissible without professional / certified testing. I'm not expert on these, but others here may be able to comment / advise. Apart from safety, a breach could give rise to big probs when / if you come to sell...

[Murray Elliot]

Many thanks guys. It's all starting to make sense now, the cable limitation was the factor I was missing, and the adviseability of independantly adding a fuse to the spur to limit it's current-carrying capacity to protect the cabling makes the utmost sense.

I don't want to go on and on asking stupid questions (on the other hand, it's a good way to learn!) but... a ring main is made up of (essentially) a single strand of 2.5mm cabling is it not?

Is its capacity increased by the mere fact that it's a ring rather than a radial configuration? I'm really asking so that I know I can sensibly go ahead and extend the ring using 2.5mm cable as I used in the spurs. I presume a 30A junction box is the right way to extend a ring main, if not using existing sockets to do so?

OK, understood... thanks.

Ok, thanks.

Understood.

Indeed. I intend to get all the work inspected, but don't want to waste time doing work which will be rejected at inspection time...

Thanks again for all the advice.

[Murray Elliot]

Alternatively, is it considered safe to use 30A connector strip to connect intoa ring mains circuit?

[Murray Elliot]

Think I just answered my own question - on a ring, current has two paths whereas on a radial connection only one. the fog's lifting!

[EricP]

Yes, two pieces of wire server as one larger one. Weird thinking that one!

I'm really asking so that I know I can

Junction boxes are out of favour at the moment, unless you can access them at any time in case of trouble.

Better to extend the ring from two sockets by replacing an existing part of the ring with a new one running through the new sockets.

ie Existing is socket A joined directly to socket B

A-------------------------------------------------------------------------B

New is A----------X-------------------X--------------------X--------B

Only changes a 10 minute job into a 2 hour one.

[Bob Watkinson]

I think all but this one has been answered. Connector strip can only be used if it is inside an enclosure. Never use it, tape it up and stuff it under the floorboards.

[Paul Saunders]

Yes, part p limits you to replacing sockets and small works but not complete circuits. ALSO, I believe that special locations are areas where you can't do anything and kitchens and bathrooms are special locations. Have a look at

I'm a bit cynical about the whole thing but on the other hand can understand the need to regulate. An example is a couple of years ago, a woman was electrocuted in her kitchen because a cable had been installed diagonally in the wall, someone fixed a metal plate rack to it in the proximity obviously not realising there was a cable running behind it. The screw penetrated the cable making it live, coupled with the fact that her leg was touching the metal part of the open dishwasher door and that ws enough to kill her.

[ARWadsworth]

I belive that 2.5 T&E is rated at upto 27 amps if clipped direct and lowered to 20 or 21 amps if in an insulated wall. 20 amp 2.5 radials are certainly allowed.

So your fuse in effect isn't really

You definately would not want a washing maching and tumble drier running from the same double socket which is a spur from a ring as it is purely bad design

I wonder if the OP has confused running a spur from a spur (ie not allowed) with the fact that the total number of non-fused spurs should not exceed the number of socket outlets on the ring main.

Three spurs that supply, for example, an extractor hood, a fridge and a gas hob (electric ignition) from 3 ring main sockets if perfectly fine.

Adam

[Bob Watkinson]

Your figures are wrong, what is your reference for those figures? Table 4D2A IEE Regs is my reference, where it states as you rightly say 2.5 is rated at

27A clipped direct (method 1) but when enclosed in an insulated wall drops to 18.5A. (method 4).

[ARWadsworth]

I took the figures from memory of the OSG. But double checking seems that they are table 4D4A of the regs.

Adam

[Andrew Gabriel]

You want table 4D5A for T&E (Amendment No 1). You won't find it in out-of-date copies of the regs.

2.5mm² T&E is now 20A for method 4.

4D4A is armoured cable.

[Bob Watkinson]

Thanks for that Andrew. 4D4A was new to me. I've googled it and now have a copy.

[ARWadsworth]

That is my bad typing. Sorry.

It was on page126 of my OSG. And it is as you say, table 4D5A not 4D4A.

Adam

[Bob Watkinson]

Sneaky aren't they, sticking in new tables like that. I suppose I should shell out another 40 odd quid for a new copy of the regs :-(

[andrew]

You can download the update from IEE's website (or at least you could).