current capacity of spur socket

[squowse]

hi, am i right in thinking that a socket on a spur from a ring using 2.5mm cable could only safely deliver 20A? the what size cable section on the "how to..." states 20A as the safe load for this size but maybe it is more complicated in this situation as it is only a short length? (usually).

this could easily be exceeded on a double socket supplying toaster and kettle for example.
so it seems to me that any spurs should be fused below the rating of the cable to be safe?

is this why some state that it is always better to put new sockets in the ring than on spurs.

 

[sparkyspike]

Yes, you've pretty much got it. When a spur is made, the potential loading is taken into account - which is why they are normally avoided in kitchens etc. And they are usually avoided completely in new installations.

Unfortunately, the misuse of spur radials is the cause of many dangerous DIY bodges.

 

[squowse]

i've got lots of spur sockets i've discovered. need to check the wattage of the devices i've got plugged in to them. one supplies my toaster and kettle, i imagine that's quite a common arrangement.
seems to me that a 4mm cable or should be used for all spurs to double sockets unless it is separately fused.

 

[Adam_151]

A spur should only supply one single or double socket outlet, both of which are rated at 13A max... which means you should not plug a combined load that exceeds 13A into a socket, regardless of whether its on the ring or is a spur

Having a double socket supplying a kettle and a toaster is unlikely to result in problems in the real world however, because both loads are for short duration.... a washing machine and tumble drier however might overtime cause damage to a socket outlet.

A 2.5mm² cable for a spur is perfectly adequate as long as people don't abuse it and daisy chain spurs!, terminating 4mm² and 2x2.5mm² in a socket might not be easy, and could probably count is installing it it in a way that goes agaisnt the product standard (BS1363), if you want to be picky!

 

[squowse]

A spur should only supply one single or double socket outlet, both of which are rated at 13A max... which means you should not plug a combined load that exceeds 13A into a socket, regardless of whether its on the ring or is a spur

i wasn't aware of that. but thanks for the reassurance on short duration loads. like i say, i'm sure i'm not the only one in this situation.

 

[ericmark]

If you go Here and down load BS 7671:2008, Corrigendum (July 2008) at the end you will find a copy of the details in the regulations as to use of radials, rings and spurs. They made a mistake so had to re-print it. Handy really.

 

[Master of None]

I also did not know until recently reading posts on this forum that a double socket wired in any configuration is rated at 13A total. I understand this is because you may only draw 13A from any single point on a ring final. Is there any double sockets on the market where each socket is wired individually for use in high load areas? Obviously this would mean a very short link of T+E or singles in Back box to effectively become 2 points on ring final. Come to think of it why aren't the terminals just beefed up to cope with 26A load. Or am I missing something?

 

[flameport]

If two large loads need to be connected in one place, the usual method is to install 2 single sockets instead of 1 double. The usual situation where this occurs is a washing machine + dishwasher, or washer + tumble drier.

Double sockets are not rated to 26A, as the standard they are made to does not require this. Making one would be possible, but pointless, as it would cost significantly more and have very limited applications.

For normal sockets, even in a kitchen, it is actually fairly difficult to overload it. Some kettles are 3kW, but plenty are not. Most toasters are far less than 3kW, even 4 slice ones.
Dishwashers and washing machines are often less than 3kW, and that load is only applicable when the heating elements are on - which as with kettles and toasters, is only for a short time.

 

[JohnD]

The BS for 13A sockets was drawn up in the late stages of WW2.

The design is very ingenious, but at the time no-one had dishwashers or tumble driers, hardly anyone had a washing machine, so the team envisaged someone sitting with a 2-bar electric fire (2kW, the highest common domestic load but a 3-bar 3kW was also available) in his living room, who might want to plug in a wireless set at the same time. The idea that anyone would want to have, in a single room, more than an electric fire, a wireless set, a table lamp and a sewing machine would have seemed crazy at the time. This is why older houses used to have so few sockets.

IIRC double sockets did not become common until the 1970's, and in most cases they are very lightly loaded (hardly anyone uses plug-in electric heaters these days).

When a 13A double does get overloaded, it is almost always due to the simultaneous use of a washing machine and tumble drier. Although professionals and skilled DIYers are aware of the problem, there are unfortunately still cases where a double socket is used in this way. Hopefully the more common use of dedicated appliance outlets in modern kitchens will reduce the problem.

 

[taycast]

Out of interest, I have seen a washing machine and tumble drier plugged into a double socket, contolled above the worktop from a tenby grid switch.

Would it be acceprtable to change the double socket to 2 singles in a dual box, with one switch receiving a supply from the isolator, then feeding a spur to its neighbour?

 

[RF Lighting]

Were the isolators just 20A DP switches straight off a RFC, or was there a fuse incorporated into the circuit for the spur?

 

[taycast]

No fuses - just dp switches

What I'm not sure about is whether the double socket might be overloaded - rectified by the singles - or the supply cable overloaded. Or just the fact that you cant have 2 singles on a non fused spur?

 

[RF Lighting]

You really ought to replace the twin socket with a single, and install an additional switch for the extra socket.

You can NOT supply a twin socket, or two singles either through a 20A DP switch if there is no fuse in the circuit.