Types of Electrical Circuits: Ring, Radial & Lighting

An overview of the main circuit types used in domestic electrical installations and when each is used.

9 min read Guide M. Davies, Electrical Engineering InstructorLast reviewed: March 2026

Understanding Domestic Circuit Types

Every domestic electrical installation is made up of individual circuits, each designed to supply a specific type of load. Understanding the different circuit types, their cable sizes, protection ratings, and applications is fundamental knowledge for any electrician.

This guide covers the main circuit types found in UK domestic installations, including their standard configurations, cable sizes, and the protective devices used. All specifications are based on BS 7671 (the 18th Edition Wiring Regulations) and common UK installation practice.

Ring Final Circuits

The ring final circuit is the most common circuit type for socket outlets in UK domestic installations. For a detailed comparison with radial circuits, see our ring main vs radial circuits guide. It is unique to the UK and a small number of other countries. The circuit starts at the consumer unit, routes around the installation connecting each socket outlet, and returns to the same MCB terminal, forming a complete ring (loop).

Standard Ring Final Circuit Specification

✓Cable: 2.5mm squared twin and earth (T&E)
✓Protection: 32A MCB (Type B)
✓Socket type: BS 1363 (standard UK 13A sockets)
✓Maximum floor area served: 100 square metres
✓Both ends of the ring terminate at the same MCB in the consumer unit

How Ring Final Circuits Work

Because both ends of the cable connect to the same MCB, current can flow to any socket outlet from both directions around the ring. This means the current is shared between the two halves of the cable, which is why a 2.5mm cable on a 32A MCB can safely supply multiple 13A sockets. The effective current-carrying capacity of the ring is approximately double that of a single radial cable of the same size.

Spurs

A spur is a branch cable that connects to the ring at one point and runs to an additional socket outlet or fused connection unit. There are two types:

✓Unfused spur: connected directly to the ring, can supply one single or one twin socket outlet. Limited to one per spur.
✓Fused spur: connected via a fused connection unit (FCU) with a 13A or lower fuse. Can supply multiple outlets beyond the FCU.

Radial Circuits

A radial circuit starts at the consumer unit and runs to the furthest point of the circuit without returning. It is a simpler configuration than a ring and is used for both general power and specific dedicated loads.

20A Radial Circuit

✓Cable: 2.5mm squared T&E
✓Protection: 20A MCB (Type B)
✓Maximum floor area served: 50 square metres
✓Suitable for: kitchens, utility rooms, small rooms, or additional socket circuits

32A Radial Circuit

✓Cable: 4mm squared T&E
✓Protection: 32A MCB (Type B)
✓Maximum floor area served: 75 square metres
✓Suitable for: workshops, garages, or areas needing higher capacity on a single run

Radial circuits are increasingly popular in modern installations, particularly for kitchens where dedicated circuits for appliances (dishwasher, washing machine, fridge freezer) provide better protection and fault isolation compared to a shared ring.

Lighting Circuits

Lighting circuits supply the fixed lighting in a property. They operate at a lower current than power circuits because the total load from light fittings is relatively small, especially with modern LED lamps. Understanding electrical symbols is essential for reading lighting circuit diagrams.

Standard Lighting Circuit Specification

✓Cable: 1.0mm or 1.5mm squared T&E
✓Protection: 6A MCB (Type B)
✓Typical load: 8 to 12 lighting points per circuit
✓Usually two circuits per property: one for ground floor, one for first floor

Lighting Circuit Wiring Methods

There are two common methods for wiring lighting circuits in domestic installations:

✓Loop-in method: the supply cable loops in at each ceiling rose or junction box, with switch wires running down to wall switches. This is the most common method in modern installations.
✓Junction box method: a junction box at each lighting point distributes the supply. Less common but used in some installations, particularly with downlights.

Two-Way Switching

Two-way switching allows a light to be controlled from two different switch positions, such as the top and bottom of a staircase. This requires three-core and earth cable between the two switches and specific wiring arrangements. Intermediate switching (three or more switch positions) requires additional intermediate switches and four-terminal connections.

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Cooker Circuits

Cooker circuits supply electric cookers and ovens, which are high-current appliances requiring dedicated circuits. The cable size and MCB rating depend on the power rating of the cooker.

Standard Cooker Circuit Specifications

✓Small cooker (up to 13kW): 6mm squared T&E, 32A MCB
✓Medium to large cooker (over 13kW): 10mm squared T&E, 45A MCB
✓Connected via a cooker control unit (usually incorporating a 13A socket outlet)
✓The cooker control unit must be within 2 metres of the cooker

Diversity

When designing a cooker circuit, diversity can be applied. It is unlikely that all elements of a cooker (oven, hob rings, grill) will be at maximum load simultaneously. BS 7671 allows the first 10A of the cooker rated current to be taken in full, plus 30% of the remainder, plus 5A if a socket outlet is included in the cooker control unit. This diversity calculation often means a 6mm cable on a 32A MCB is adequate for most domestic cookers.

Shower Circuits

Electric shower circuits are among the highest-rated circuits in a domestic installation. Modern electric showers range from 7kW to over 11kW, requiring appropriately sized cables and protective devices.

Shower Circuit Specifications

✓7kW to 8.5kW shower: 6mm T&E may be acceptable on a 32A MCB (check voltage drop and installation method)
✓8.5kW to 9.5kW shower: 10mm squared T&E, 40A MCB
✓10kW to 11kW shower: 10mm squared T&E, 45A or 50A MCB
✓Connected via a double-pole isolating switch (typically a ceiling-mounted pull-cord switch in the bathroom)

Voltage Drop

Shower circuits are particularly susceptible to voltage drop because of their high current and often long cable runs (from the consumer unit, typically on the ground floor, up to the bathroom). Always check that the voltage drop does not exceed the maximum allowed by BS 7671 (typically 3% for lighting or 5% for other uses). A larger cable may be needed for longer runs.

Other Common Domestic Circuits

Immersion Heater Circuit

✓Cable: 2.5mm squared T&E
✓Protection: 16A MCB
✓Connected via a 20A double-pole switch (usually adjacent to the hot water cylinder)
✓Most domestic immersion heaters are rated at 3kW

Smoke and Heat Alarm Circuit

✓Cable: 1.5mm squared T&E for supply, plus interconnecting cable between alarms
✓Protection: 6A MCB (some installations use a 3A fuse)
✓Must be a dedicated circuit not shared with other loads (in new installations)
✓All alarms interlinked so that activation of one sounds all alarms

Outdoor Circuit

✓Cable: SWA (steel wire armoured) for underground runs, or T&E in conduit
✓Protection: appropriate MCB plus 30mA RCD protection (mandatory for outdoor circuits)
✓Must be designed for the specific load (garden lighting, outbuilding supply, etc.)
✓Underground cables should be buried at minimum 500mm depth with warning tape above

Electric Vehicle Charging Circuit

✓Cable: typically 6mm or 10mm squared T&E or SWA (see our three-phase power guide for larger installations)
✓Protection: 32A MCB with Type A or Type B RCD (depending on charger specification)
✓Dedicated circuit from the consumer unit to the charging point location
✓Must comply with BS 7671 and IET Code of Practice for EV charging

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Circuit Design Considerations

When designing circuits for a domestic installation, several factors must be considered beyond simply matching cable size to MCB rating. The consumer unit must accommodate all circuits with appropriate protective devices.

Key Design Factors

✓Current carrying capacity: the cable must be rated to carry the design current of the circuit, accounting for installation method and grouping
✓Voltage drop: must not exceed the limits in BS 7671 (typically 3% for lighting, 5% for others)
✓Earth fault loop impedance: must be low enough for the protective device to disconnect within the required time
✓Disconnection time: 0.4 seconds for final circuits in TN systems, 0.2 seconds for TT systems with RCD protection
✓Thermal constraints: the cable must withstand the energy let-through of the protective device under fault conditions
✓Grouping factors: where multiple cables are bunched together, their current carrying capacity is reduced
✓Ambient temperature: high ambient temperatures reduce cable current carrying capacity

Frequently Asked Questions

What is the difference between a ring final circuit and a radial circuit?

A ring final circuit starts at the consumer unit, loops around the installation connecting all socket outlets, and returns to the same MCB, forming a complete ring. A radial circuit starts at the consumer unit and runs to the furthest point without returning. Both serve socket outlets, but ring circuits can serve a larger area on the same cable size.

How many sockets can I have on a ring final circuit?

There is no limit on the number of socket outlets on a ring final circuit. The limit is on the floor area served: a ring final circuit should not serve an area exceeding 100 square metres. In practice, this means a typical ring circuit serves one floor of a domestic property.

Can I add a spur to a ring final circuit?

Yes. A fused spur (connected via a fused connection unit) can be added to a ring final circuit at any point. An unfused spur (connected directly) can supply one single or one twin socket outlet. The total number of unfused spurs should not exceed the number of points on the ring.

What size cable do I need for a shower circuit?

The cable size depends on the shower rating. A typical 8.5kW to 9.5kW shower requires 6mm or 10mm cable on a 40A MCB. A 10.5kW to 11kW shower requires 10mm cable on a 40A or 45A MCB. Always refer to BS 7671 current carrying capacity tables and account for installation method and grouping.

Why are lighting circuits rated at only 6A?

Domestic lighting circuits are rated at 6A because the total load on a lighting circuit is relatively small. A typical circuit serving 8 to 12 light fittings with LED lamps has a total load well under 6A. The 6A rating is more than adequate and allows the use of smaller, more economical 1.0mm or 1.5mm cable.

Can I use a 20A radial circuit instead of a ring final?

Yes. A 20A radial circuit using 2.5mm cable can serve a floor area of up to 50 square metres. For smaller areas such as a kitchen, utility room, or single bedroom, a radial circuit is a perfectly acceptable alternative to a ring final circuit.

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