A ring main (ring final circuit) is a wiring method unique to the UK where a single 2.5mm2 cable starts at the consumer unit, passes through each socket outlet on the circuit, and loops back to the same MCB terminals. Because the cable forms a complete loop, every socket on the ring has two paths back to the MCB - current can flow from either direction. This halves the effective current in each cable section and allows 2.5mm2 cable to feed a 32A MCB, giving a maximum load of 7.2kW per circuit.

How many sockets can be on a ring main?

BS 7671 limits a ring final circuit to a maximum floor area of 100m2 (though this is a rule of thumb relating to diversity, not a hard socket count limit). The number of actual sockets is not prescribed - it is the connected load that matters. In practice, a typical domestic ring main serves 8-12 double sockets without overloading the 32A MCB when normal diversity is applied. Spurs (branch connections from the ring) are permitted - each unfused spur may supply only one single or one double socket.

What is a spur on a ring main?

A spur is a branch cable connected to the ring at a socket or junction, extending the circuit to an additional socket that is not in the ring itself. An unfused spur may supply only one single or one double socket outlet. A fused connection unit (FCU) with a 13A or lower fuse can supply a further radial circuit of additional sockets or equipment from the ring. Spurs must not be taken from existing spurs (a spur from a spur), and there must be at least as many sockets in the ring itself as there are spurs from it.

Electrical noun

Ring Main

/ rɪŋ meɪn /

Also known as: ring final circuit, ring circuit, 13A ring circuit

Definition

A ring main (ring final circuit) is a wiring arrangement - largely unique to the UK - where a single 2.5mm² cable starts at a 32A MCB in the consumer unit, visits each socket outlet in turn, and returns to the same MCB terminals. The cable forms a complete loop, giving each socket two current paths back to the MCB. This halves the effective current in each cable section, allowing 2.5mm² cable to safely feed a 32A breaker with a maximum circuit load of 7.2kW.

In practice

A standard domestic property has two ring main circuits - one for the ground floor sockets and one for the upper floor - each protected by a 32A MCB with RCD protection. The circuits serve all 13A socket outlets on their respective floors. Dedicated radial circuits (not rings) are used for higher-load equipment: cooker (32-45A), shower (32-40A), dishwasher, and washing machine (typically 16A or 20A).

Additional sockets may be connected as spurs from the ring. An unfused spur may feed only one single or one double socket. A fused connection unit (FCU) with a built-in fuse can feed a further radial circuit. The most common fault found on existing ring mains in older properties is a missing return - the ring is incomplete and effectively a radial, overloading the cable. This is detected by the ring continuity test carried out as part of an electrical inspection.

Building Regulations

Relevant PartPart P - Electrical safety in dwellings

StandardBS 7671:2018+A2:2022 Appendix 15 - Ring final circuits

Cable size2.5 mm² twin-and-earth (1.5 mm² for lighting circuits)

MCB rating32 A Type B

Maximum floor area100 m² per ring circuit

BS 7671 Appendix 15 covers the design and installation of ring final circuits. Adding a socket or extending a ring main is notifiable work under Part P if it involves a new circuit or circuit extension in certain locations (kitchens, bathrooms, outdoors). Adding a spur to an existing ring is typically self-certifiable minor works that requires a Minor Electrical Installation Works Certificate (MEIWC) from the electrician.

Full Building Regulations guidance

Ring Main

Just so you know...