Adding an additional socket in ring circuit question

[appletree]

Hi,

I am a DIY person (ie. not a qualified electrician) so I am sure this is a basic question but please bear with me.

I am looking to add a socket to the garage. I was looking at various online resources on how to wire this up and came across many videos and articles that said that if you are adding a spur off a socket in a ring circuit, you can only have one, not two spurs. The reason being that if two double spur sockets were added (in series), then the total load could be 52A (13A x 4), which would overload 2.5mm cable between the socket in the ring and the first spur.

OK I get that but isn't the whole ring circuit protected by MCB rated at 32A anyway, so way before you get to 52A, the MCB would trip? Also I understand that 2.5mm cable is rated at around 25A so even if you have one double socket spur, you could overload that piece of cable (between the socket in the ring circuit and spur) by plugging in two 13A appliances.

Or is it that,
1. Plugging in two 13A appliances would create 26A but it is just about at the max rating of 2.5mm cable so it's fine.
2. Since MCB is 32A, there is a range between 25A - 32A, which would overload the cable between the socket and the first spur, while MCB will not trip.

Can someone explain to me the thinking behind this?

Thanks.

 

[telectrix]

basically a 13A socket is rated at 20A, single or double. so if you were to plug in . say 2 x 3kW heaters into a double socket on a spur, the socket would be toast before 2.5mm cable overheated.

 

[appletree]

basically a 13A socket is rated at 20A, single or double. so if you were to plug in . say 2 x 3kW heaters into a double socket on a spur, the socket would be toast before 2.5mm cable overheated.

Thanks, I didn't realise even a double socket is rated at 20A. That explains that one double socket cannot overload the 2.5mm cable, since you cannot plug in two 13A appliances into one double socket.

I can deduce the answer to my first question. If the first spur uses 20A and second spur uses 10A (total 30A), it will not trip MCB but it will overload the 2.5mm cable, rated at around 25A. Therefore one is not allow to add two spurs in series off a socket in a ring circuit.

 

[littlespark]

It’s an oversimplification by saying you can’t have a spur off a spur, but it is right.

if the initial spur is fused down by a 13A fuse, then it’s basically a glorified extension lead and you can have as many sockets as you want, as all of them will be limited by the 13A

 

[appletree]

It’s an oversimplification by saying you can’t have a spur off a spur, but it is right.

if the initial spur is fused down by a 13A fuse, then it’s basically a glorified extension lead and you can have as many sockets as you want, as all of them will be limited by the 13A

Hi thanks for your reply.

Yes I was just talking about the non-fused spur case. Adding a first spur with a fuse makes absolute sense.

 

[Mikegh]

Interesting discussion

I work in the ROI not familiar with bs7671

Big fan of the Ring circuit in general not so much for domestic
We brought in a rule outlawing unfused branches off the domestic ring final circuit

 

[Taylortwocities]

Thanks, I didn't realise even a double socket is rated at 20A. That explains that one double socket cannot overload the 2.5mm cable, since you cannot plug in two 13A appliances into one double socket.

I

not quite. The test specification for a double socket is that it must be able to pass a minimum of 20A for a certain period without overheating. (14A on one socket 6A on the other IIRC) So you can plug two 13A appliance into a double socket. Some sockets will handle this, some don’t.