Spur from Ring Main

[andyh]

I'm looking to spur off the ring main to supply an FCU for a fixed piece of equipment. The spur will be from an existing double socket. Question is whether it is ok to use 2.5mm cable or whether the connection to the FCU should be in 4mm? The FCU is fused at 13A so it can't draw more than this but what happens if there's a fault in the ring main supplying it causing a current in excess of what the single 2.5mm can take?

 

[cagedanimal2010]

you can spur one point in 2.5 but tbh i always use 4.0 thats way your always covered
hope this helps you!

 

[cornburn]

As you say, it's limited by the 13A fuse so definitely no need for 4mm, there'd be no problem using 1.5mm for that spur but most would use 2.5 anyway

 

[cagedanimal2010]

only reason i use 4.0 is before i have seen people spurring of a spur in 2.5, in which you end up with 3 or 4 sockets fed via a single 2.5 covered by a 32a mcb

 

[andyh]

Thanks for the replies. I always thought it was 2.5mm as standard. All I'm thinking about is the case where a current fault in the ring overloads the single 2.5mm or can this not happen? The 13A FCU protects the load side but what protects the link between the ring and the FCU?

 

[cornburn]

only reason i use 4.0 is before i have seen people spurring of a spur in 2.5, in which you end up with 3 or 4 sockets fed via a single 2.5 covered by a 32a mcb

The regs don't allow spurring more than one socket though without fusing down, and I'm not sure spurring several sockets in 4mm off a 2.5mm ring would comply either? By the book I would have thought one socket per spur without fusing down, whether the cable is minumum required 1.5mm or oversized to 2.5, 4mm, 6mm or whatever. If someone comes along afterwards adding spurs onto spurs onto spurs that's something a competant electrician wouldn't do and we can't go overkill on everything just in case a DIYer decides to bodge more on there in the future.

 

[cagedanimal2010]

as far i was aware you can spur of a ring in 4mm to several sockets as the cable is sufficient to carry the current? please let me know if this is incorrect.

 

[ste mac]

The way I was taught was that:

Only 1 spur off any socket or juction point of the ring circuit.

No more than 1 double socket on that spur - unless the spur is protected by a 13A FCU.

Minimum cable size of 1.5mm for 13A FCU, 2.5mm for double socket.

The reason you are not allowed to spur off of a spur is to limit current draw through the terminals on the back of sockets to prevent overloading and overheating of the terminals of the original ring circuit socket.

 

[Amp David]

as far i was aware you can spur of a ring in 4mm to several sockets as the cable is sufficient to carry the current? please let me know if this is incorrect.

Regs don't allow this to be done from a ring, only a 32 amp radial. There was a thread about this very same topic a week or so ago.

 

[Guest123]

as far i was aware you can spur of a ring in 4mm to several sockets as the cable is sufficient to carry the current? please let me know if this is incorrect.

Yes, that is incorrect, you will be overloading the ring at the point of connection.

Have a look in the back of the regs book at final circuit arrangements.

 

[andyh]

The reason for my initial query is a thread on the IET forums here:
IET Forums - Adding a Fused Spur To A Spured Socket

One poster is convinced that you CANNOT use a 2.5mm cable to connect a spur from a ring main.

Can anyone confirm or deny this logic?

 

[Guest123]

As I said, have a look in the back of BS 7671, it's all there in black and white.

 

[Ponty Massive]

As long as the spur's are on the "load" side of the FCU then the max you can pull is 13a. The 13a fuse is protecting the cable.

 

[ste mac]

One poster is convinced that you CANNOT use a 2.5mm cable to connect a spur from a ring main.

Can anyone confirm or deny this logic?

The maximum load (theroreticaly) that can be drawn through a double socket is 26A (2x 13A), the CCC of 2.5mm T&E when clipped direct is 27A, so why would you need a larger conductor CSA?

As mentioned by Lenny, page 362 of the BRB. Its all there cable CSA unfused, fused spurs, etc.