Ring main spurs

[lawnmowerman]

I am a new member and have found the information on the forum regarding ring main spurs very useful. As I understand it, a spur may be added to a ring main using the same cable as the ring main (2.5mm T&E) either by teeing off the back of an existing socket or by intercepting the ring main cable using a 30A joint box and teeing from the joint box. Spurs on spurs are not permitted and the total number of spurs must not exceed the total number of sockets on the ring.
So far so good.
My question is: I believe that the current carrying capacity of 2.5mm cable is 24A so a ring can potentially carry 48A, but is fused at 30A at the consumer unit - no problem there.
If however a spur is added using a single 2.5 T&E rated at 24A then it may be possible to load the spur cable up to 26A if it is a double 13A socket. This means that the current carrying capacity of the single spur cable can be exceeded by up to 2A. If one of the newer triple sockets are fitted it may be possible to draw up to the maximum 30A of the ring and the spur cable capacity may be exceeded by up to 6A before the consumer unit fuse blows at 30A. Is this likely to cause a problem or am I incorrect in the current carrying capacity of 2.5mm T&E? I am concerned that the extra current in the spur cable will cause it to heat up and possibly ignite.
Thanks.

 

[JohnD]

When the spur rules were thought up (over 50 years ago) it was assumed that he biggest load anyone would plug in would be a 3kW electric fire, and that "obviously" no one woud plug in two such fires next to each other. Other things like electric kettles and toasters are only on for short periods, and not often both on at the same time.

If however you are planning to connect two large loads to the same twin, for example a washing machine and a tumble drier, then it would be better to provide two outlets.

If you are using it for radio, TV, computer, table lamp etc then the load is very slight.

The curent carrying capacity of the cable depends on how it is fixed. If it is inside a wall it is lower, or passes through a hot area like an airing cupboard. If it is inside an insulated wall it is lower still.

 

[Steve]

here's one for you

Why is cable available in the sizes it is? I mean, WHY does it go 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35mm²??? Why were these random cable sizes chosen?

 

[plugwash]

they aren't as random as they loook

1.5 / 1.0 = 1.5
2.5 / 1.5 = 1.67 (grr why doesn't this forum support unicode, its really annoying not being about to use the two thirds symbol
4/2.5 = 1.6
6/4 = 1.5
10/6 = 1.67
16/10 = 1.6
25/16 = 1.5625
35/25 = 1.4
50/35 = 1.43

i'm afraid i don't knoe what the availible sizes are beyond that so i can't continue this myself from there.

so they go up by somewhere arround 1.6 times each time. my guess is they started with a sequence like that and then rounded it to nicer numbers and for the larger sizes the copper was becoming sufficiantly expensive to justify a closer spaced range.

 

[lawnmowerman]

Thanks JohnD.
I guess that I am just being pedantic about the cable loadings. Does anybody know the official current rating for 2.5 T&E?

 

[ban-all-sheds]

If however a spur is added using a single 2.5 T&E rated at 24A then it may be possible to load the spur cable up to 26A if it is a double 13A socket. This means that the current carrying capacity of the single spur cable can be exceeded by up to 2A.

No. If you did that you might have bigger problems than the cable to worry about, as twin sockets are only rated at 20A.

If one of the newer triple sockets are fitted it may be possible to draw up to the maximum 30A

Even more no, as triple sockets are fused, so their total load is limited by a 13A fuse.

Thanks JohnD.
I guess that I am just being pedantic about the cable loadings. Does anybody know the official current rating for 2.5 T&E?

It depends on how it's installed. http://www.tlc-direct.co.uk/Book/4.3.1.htm

 

[lawnmowerman]

Thanks ban-all-sheds.

I did not realise that a double socket is rated at only 20A. However if you moved into a house with a double socket installed, if you were not an electrician you may not necessarily know that the socket was connected quite legitimately via a spur. You may therefore plug in two appliances rated at 13A each thus drawing the full 26A. This may overload the socket and the cable!

Great web link for the regs - I have bookmarked it.

 

[ban-all-sheds]

Thanks ban-all-sheds.

I did not realise that a double socket is rated at only 20A.

It is a little known fact.

However if you moved into a house with a double socket installed, if you were not an electrician you may not necessarily know that the socket was connected quite legitimately via a spur. You may therefore plug in two appliances rated at 13A each thus drawing the full 26A. This may overload the socket and the cable!

Possibly, and probably not.

Great web link for the regs - I have bookmarked it.

Those are not the Wiring Regulations.

These are:

http://www.iee.org/Publish/Books/WireAssoc/index.cfm?book=WR 250



See here: //www.diynot.com/forums/viewtopic.php?p=75416#75416

 

[lawnmowerman]

Thanks

 

[Steve]

looking back at JohnD's post, wouldn't it be wise to correct these little snags regarding socket ratings? IIRC, even MK sockets will only take 22 amps indefinitely. Why aren't all sockets specced to 26A? As lawnmowerman stated, quite correctly, you could plug in 2 heavy loading appliances (2 ovens in a big kitchen, tumble dryer and washer, lectric heaters etc), not knowing it was a spur, and burn the house down. Given that the spur rules were thought up in the 50's, and electricity consumption has gone up massively since then, and consumption patterns in the home have also changed, isn't it about time for a little re-think?

And why are triple sockets protected by a single 13A fuse, yet double sockets can draw 26A? Someone's been designing things in their sleep i think.

 

[JohnD]

isn't it about time for a little re-think?

It's on the way, a unified european standard. I knew people who were working on it 10 years ago and it's not out yet. And whatever it is, you can be sure everyone will grumble about it.

 

[Steve]

EU standard for sockets?????? how the hell would that work???

 

[JohnD]

You will have noticed we already have unified cable colours and unified voltage...

Before you and I were born the the 13A square pin plug standard came in for the UK, and eventually everyone got them.

Soon Hungary and Slovenia will be updating to the new standard, and so will we. So will France, Germany, probably Turkey too. And Poland. sweden. Norway. Italy. Need I go on?

Look on the bright side. one of the design principles is that it shall be entirely new, so that no single country has an advantage in getting its old standard imposed on all the others.

When you go on holiday, you'll be able to take your Dualit toaster and your Russell-Hobbs kettle, and plug them in at your mediterranean villa. And you can bring back your Gaggia coffee machine and plug it in at your Surbiton bungalow. And if you're in the kettle-manufacturing trade you'll be able to build them with the same plug and ship them to any country, without needing duplicated stock in your warehouse.

If you're a buyer, you won't have manufacturers charging UK customers extra (like they do with right-hand-drive cars) because you can order one by post from Warsaw.

 

[plugwash]

EU standard for sockets?????? how the hell would that work???

well they have tried to push a new system from the IEC in the past but failed afaict. SCHUKO is becoming more spread accross europe though and i guess the beuracrats might eventually try to push it everywhere in europe but the problem with schuko is that its not feasible to design a transition socket that works with both schuko and our BS1363 plugs.

 

[plugwash]

Soon Hungary and Slovenia will be updating to the new standard, and so will we. So will France, Germany, probably Turkey too. And Poland. sweden. Norway. Italy. Need I go on?

got a source for that? and which standard exactly are you talking about?