How best to add sockets to a fused spur?

Question

I have an existing spur running to a double socket in my study. It was added before I moved in and I have a mess of extension leads to power all the office appliances.

My plan is to replace the current socket with a fused spur connector and then add three surface mounted sockets in convenient locations around my desk. This should never exceed 13 amps. Appliances are laptop, monitor, small amplifier, printer, and some low power LEDs.

My builder tells me firstly that the fused connector is not necessary, but secondarily that to do it properly I have to make a ring back to the main circuit.

Is he right on those points? Is there anything wrong with my original plan?

Answer 1

UK wiring standards.

Your builder is half right and half wrong.

(a) Is the circuit a ring circuit or a radial circuit? Standard circuits are:

• ring circuit, wired in 2.5mm cable with a 30A fuse or 32A circuit breaker

• radial circuit, wired in 2.5mm cable with a 20A fuse or circuit breaker

• radial circuit, wired in 4mm cable with a 30A fuse or 32A circuit breaker

If the circuit is a ring circuit then an unfused spur can supply a maximum of one single or one double socket. A fused spur, as you propose, can supply an unlimited number of sockets with a maximum load of 13A.

Your building is proposing extending the ring. To do this correctly a second cable would have to be run back to the point where the spur is taken from the ring, and the ring broken with each side connecting to one side of the new wiring. Connecting to another part of the ring randomly to form a 'figure 8' circuit is not permitted.

If the circuit is a radial circuit then it can be extended with an unfused spur-off-a-spur as the cable is sized to carry the whole load of the circuit at any point. The radial must be extended in the same size cable as the existing circuit.

Answer 2

What you propose is a spur circuit. Everywhere but one island (well, 1 and 1/4), people think "spur circuits" are the only legal thing, because ring circuits are banned.

The problem with ring circuits in the UK is that you have wires with 16A capacity, but the fuse/breaker is fused at 32A, on the assumption that no matter where the loads are, the current has 2 wires to get there. The problem is, if a wire breaks, there will be only 1 wire to get there, and this failure will happen silently. So your surviving 16A wire will now be carrying up to 32A.

And likewise, your spur circuit with 13A? 16A? wire, will have a 13/16A capacity, but a 32A breaker. That won't do. Hence the need for local fusing on the spur.