Ring final circuit vs radial circuit: what is the difference?

Glossary · Wiring, circuits and faults

How ring and radial circuits work

A final circuit is the last stage of wiring, from a protective device in the consumer unit to the sockets, lights or appliances it feeds. For sockets there are two layouts.

On a ring final circuit, the cable leaves the protective device, passes through each socket in turn and returns to the same device, forming a loop. On a radial circuit, the cable leaves the protective device, runs to each socket in turn and stops at the last one. Twin and earth cable suits both layouts, according to its maker.^{[3]Prysmian, Twin and Earth Cable}

The IET points out that ring final circuits are more commonly and erroneously known as ring mains.^{[1]IET, Back to the Forum - The ring final circuit} The right name matters, because a main supply and a final circuit are different things.

Why the UK has ring circuits

The IET traces ring final circuits to 1942 and the reconstruction effort after the Second World War. A committee convened that June was tasked with recommendations to support the building of one million houses, and with materials in short supply ring final circuits were conceived to minimise the copper required. The IET says a ring typically needed 30% less copper and could save up to 25% in cost.^{[1]IET, Back to the Forum - The ring final circuit} The 13 A socket and fused plug were introduced at the same time.^{[1]IET, Back to the Forum - The ring final circuit}

The rules for a ring final circuit

NICEIC sets out the conditions BS 7671 applies to a ring that supplies standard 13 A sockets, with or without unfused spurs. It must be protected by a 30 A or 32 A device. It must be wired in copper with line and neutral conductors of at least 2.5 mm². The cable’s current-carrying capacity, corrected for the installation conditions, must be not less than 20 A. And the load in any part of the circuit must be unlikely to exceed what the cable can carry for long periods.^{[2]NICEIC, Overcurrent protection - overload}

Branches off the ring are called spurs. Our guide to the fused spur covers them, and the guide to cable sizes explains the figures above.

How the two compare

The comparison follows the IET. It says continuity testing of ring final circuits can be time-consuming and requires the supply to be isolated, and that installing two radial circuits takes less time and uses less cable than one ring, which offsets the additional protective device.^{[1]IET, Back to the Forum - The ring final circuit} It also notes that modern dwellings other than the kitchen are unlikely to have equipment with high power demand, so a radial could be considered appropriate.^{[1]IET, Back to the Forum - The ring final circuit}

Example

Aisha buys a 1930s terrace in Beeston. Her EICR describes the downstairs sockets as one ring final circuit and the upstairs sockets as another. Her electrician explains that the ring layout is normal for UK homes. When Aisha later builds an extension, the electrician adds a new radial circuit with its own protective device. That keeps the existing ring undisturbed and avoids the risk of overloading it with new work.

What people get wrong

Many people think a ring circuit has to be wired as a ring and that a radial is a cheaper, lesser option. The IET says BS 7671 permits both and each should be judged on its merits. Both are safe when designed, installed and maintained properly.

The second mistake is changing a ring without understanding it. The IET warns that if ring final circuits are added to or altered incorrectly, a dangerous situation can arise with overloaded cables.^{[1]IET, Back to the Forum - The ring final circuit} Adding sockets is work for a registered electrician, who can test the circuit before and after.

The bottom line

A ring final circuit loops back to its protective device and a radial circuit stops at the last socket. Both are permitted for sockets, and both are safe when properly designed, installed, tested and maintained. You cannot tell which you have from the sockets, so ask a registered electrician, and never alter either one yourself.