2 spurs between 2 sockets on a ring.

[LegionOfBrad]

Hi there, having pulled up the floorboards in an upstairs bedroom i have found that there are two spurs coming from between 2 sockets on our ring main. One serving the hall downstairs and another an additional socket in the room.

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• LegionOfBrad
• 30 Jan 2015
• 1

I know that you can't take a spur off a spur from a socket. But i was wondering if there is anything in the regulations not allowing this double spurring method between sockets?

 

[ericmark]

It is permitted. There was an alteration with BS7671:2008 and you could down load the picture showing the options I expect it's still on the IET web site.

You can only take a single spur from any set of terminals so maximum of three wires in the terminals but be it the consumer unit, a socket, a junction box, or a fused connection unit it does not matter.

As to a junction box designed for four wires as with some maintenance free types it is open to debate if you could take two spurs. I would say no as it could cause overload.

 

[LegionOfBrad]

Thanks,

When i look at it again conceptually it is actually no different than if those two junction boxes were in fact sockets on a ring!

I'm going to swap them out for some maintenance free 32A ones before boarding the floor back up.

 

[JohnW2]

You can only take a single spur from any set of terminals so maximum of three wires in the terminals but be it the consumer unit, a socket, a junction box, or a fused connection unit it does not matter.

That's only really true if/because more than three conductors would exceed the capacity of the terminals. Manufacturers don't seem all that logical about their specified terminal capacities - for example the terminals of MK sockets are said to have a capacity of 3 x 4mm² conductors of 2 x 6mm² conductors, but only 3 x 2.5mm² ones. However, as you go on to say ...

As to a junction box designed for four wires as with some maintenance free types it is open to debate if you could take two spurs. I would say no as it could cause overload.

I don't think I agree - at least not as a generalisation. Whenever one is connecting sockets to a ring, particularly near one end of the ring, one has to consider the possibility of overloading one arm of a ring. However, there is essentially no difference between deriving two spurs from one JB (or socket, if it has adequate terminal capacity) and deriving them, separately, from two sockets or JBs a few inches apart.

Kind Regards, John

 

[JohnW2]

When i look at it again conceptually it is actually no different than if those two junction boxes were in fact sockets on a ring!

Exactly.

I'm going to swap them out for some maintenance free 32A ones before boarding the floor back up.

Probably a good move - although there is no requirement to bring existing installations up to current standards, you would certainly have to use MF JBs if you were installing them today.

Kind Regards, John

 

[ericmark]

I don't think I agree - at least not as a generalisation. Whenever one is connecting sockets to a ring, particularly near one end of the ring, one has to consider the possibility of overloading one arm of a ring. However, there is essentially no difference between deriving two spurs from one JB (or socket, if it has adequate terminal capacity) and deriving them, separately, from two sockets or JBs a few inches apart.

Kind Regards, John

My problem with a ring final it's a special type and what ever makes sense has little to do with what you are allowed. They were designed for electric heating during the war and now told no more than 2kW for non portable appliances.

 

[JohnW2]

My problem with a ring final it's a special type and what ever makes sense has little to do with what you are allowed.

"what you are allowed" is any ring final in which the designer does not feel it "likely" that any of the cable will become overloaded for long periods.

They were designed for electric heating during the war and now told no more than 2kW for non portable appliances.

that "being told" is actually part of the guidance regarding ways in which overloading of cables for long periods may be avoided. Designers are allowed to think for themselves.

Almost any ring final is a 'gamble'. Just two 13A outlets within the first 23% or so of the length of a ring are capable of 'overloading' a cable with a CCC of 20A.

Kind Regards, John

 

[ban-all-sheds]

They were designed for electric heating during the war and now told no more than 2kW for non portable appliances.

Although the requirement was pretty much the same

“minimum current carrying capacity Iz of the cable be not less than 0.67 x the rated current setting In of the protective device”

I expect that in practice they were pretty relaxed about where stuff was plugged in because in practice the chances of a house of that vintage having enough insulation to drag 7/.029 down to 20A CCC was vanishingly small.

 

[stillp]

And because electric heater were typically only 500W per element, with maximum 2 elements.