Reference
Radial, ring final & spur circuits
The three ways UK socket outlets are wired — and the specific rule that catches out most DIYers and even some tradespeople: you are permitted to spur off a ring, but you are never permitted to spur off another spur.
Reference only
Diagrams simplified for teaching. All installations must be designed and installed by a competent person to BS 7671:2018+A4:2026 and IET On-Site Guide (Appendix 15).
Legend
- Ring / radial cable
- Spur (permitted)
- Not permitted
Radial socket circuit
One cable leaves the consumer unit, visits each socket in turn, and terminates at the last socket. Simple, easy to fault-find, and the only sensible option in modern flats and small extensions.
- • 20A radial — 2.5 mm² cable, 20A MCB/RCBO, floor area up to 50 m².
- • 32A radial — 4 mm² cable, 32A MCB/RCBO, floor area up to 75 m².
- • Unlimited sockets on the circuit within that floor area.
- • Spurs are permitted from any point on the radial, following the same "one spur per point" rule as a ring.
Radial circuit — 20A / 32A
Single cable feeds each socket in a chain; ends at the last socket.
Ring final circuit
A cable leaves the consumer unit, visits each socket in turn and returns to the same MCB/RCBO terminal — forming a "ring". Current can flow to any socket from both directions, so a smaller conductor can carry a larger circuit. Uniquely British; still the default for larger UK homes.
- • 32A ring final — 2.5 mm² cable, 32A MCB/RCBO.
- • Floor area up to 100 m².
- • Unlimited sockets within that floor area.
- • Spurs are permitted (see next section).
Ring final circuit
Cable leaves the CU, loops through every socket, returns to the same MCB.
Spurs — what the rule actually says
A spur is a branch cable taken from a point on the ring (or radial) to feed one extra accessory. The IET's On-Site Guide (Appendix 15) is specific:
- • A spur may be taken from any socket outlet, junction box or connection unit on the ring.
- • The number of unfused spurs must not exceed the number of socket outlets and stationary equipment connected directly to the ring.
- • An unfused spur may feed one single or one twin socket, or one fixed appliance.
- • A fused spur (through a 13A fused connection unit) may feed multiple socket outlets or equipment — because the FCU limits the load to 13A.
- • You may NEVER take a further spur off an existing spur. This is the classic mistake.
The reason is load protection. A ring can carry 32A because current arrives from both sides. A spur cable is a single length of 2.5 mm² — only rated to 27A on its own. Add a second spur onto it and you can end up feeding four sockets, potentially 20–25A of load, through a cable and terminations never designed for that duty.
Permitted: one spur from the ring
A single socket on a spur taken from an existing ring socket.
Not permitted: spur off a spur
Adding a second socket onto an existing spur — never allowed.
Fused connection unit — the get-out clause
If you need multiple new sockets in one place but the ring is already fully spurred, take a single spur to a 13A fused connection unit. Downstream of the FCU is now a mini circuit protected at 13A — you can wire multiple sockets, kitchen appliances or an SFCU-fed sub-radial without overloading the spur cable. This is how extra sockets are added to a full ring compliantly.
Extending safely: spur → FCU → multiple sockets
One spur off the ring feeds a 13A FCU; the FCU protects the downstream cable.
Quick summary
- • Radial: 20A on 2.5 mm² (≤ 50 m²) or 32A on 4 mm² (≤ 75 m²), unlimited sockets.
- • Ring final: 32A on 2.5 mm², up to 100 m², unlimited sockets.
- • Unfused spur: one single or twin socket only, taken from a ring/radial point. Never from another spur.
- • Fused spur (13A FCU): can feed multiple sockets/appliances downstream — the FCU limits the load.
- • All socket circuits in a dwelling require 30 mA RCD protection (BS 7671, Reg 411.3.3).
Need extra sockets adding — compliantly?
Paul designs and installs new radials, extends existing rings and adds FCUs so you get the sockets you need without overloading the circuit or breaking the spur rule.