What is the difference between a ring main, a radial circuit and a spur?

They are different ways of wiring sockets, and the distinction matters because it determines what you may safely add.

A ring final circuit (ring main) is a loop. Cable runs from the consumer unit, around all the sockets on the circuit, and back to the same breaker. Current can therefore reach any socket by two paths, which allows a smaller cable to carry a higher total load — typically 2.5mm² cable protected by a 32A breaker.

Rings are largely a British peculiarity, adopted after the Second World War to economise on copper, and they are why UK plugs contain individual fuses.

A radial circuit runs from the consumer unit to the sockets and stops — no return leg. Simpler, used everywhere else in the world, and increasingly used in new UK installations. Cable size must match the breaker: typically 2.5mm² on a 20A breaker, or 4mm² on 32A.

A spur is a branch taken off an existing circuit. On a ring, a spur is wired from an existing socket or a junction box and serves points not part of the loop — so they are fed by a single cable rather than two paths.

The rules that matter, and this is where DIY work goes wrong:

One spur per socket. You may take a single unfused spur from any one socket on a ring, feeding one single or double socket. You may not chain spurs off spurs.

You cannot spur from a spur without a fused connection unit (FCU), which limits the branch to 13A and makes it protected rather than unfused.

A spur cannot be taken from a socket that is itself on a spur.

How to tell what you have. Count cables at a socket: two cables generally means it is part of a ring; one means it is a spur or the end of a radial; three usually means a spur is taken from it, so no more may be added.

Testing is the only reliable method, and in England and Wales new circuits are notifiable under Part P.