What is the maximum floor area for a 20A radial circuit?

Under BS 7671, a 20A radial circuit using 2.5mm² cable can serve a floor area of up to 50m² for general-purpose socket outlets. This makes it suitable for smaller rooms such as bedrooms, home offices, and utility rooms. If the floor area exceeds 50m², you should either use a 32A radial on 4mm² cable (which serves up to 75m²) or install a ring final circuit. The floor area limits are guidance figures — Appendix 15 of BS 7671 is informative and words it as "historically, the floor area served has been limited to 50 m²", and the IET On-Site Guide gives the same figures in its standard circuit arrangements. They are not absolute regulation limits, but they represent accepted good practice and are the figures most competent person scheme assessors will expect you to follow.

Can I use a radial circuit instead of a ring circuit for sockets?

Yes. BS 7671 does not mandate ring final circuits for socket outlets. A radial circuit is a perfectly acceptable alternative, provided the cable size and protective device rating are appropriate for the load and floor area being served. In fact, many electricians and designers now prefer radial circuits because they are simpler to install, easier to test, and avoid the common faults associated with ring circuits (such as broken rings or cross-connections). A 32A radial on 4mm² cable protected by a 32A MCB or RCBO can serve a floor area of up to 75m² — comparable to a ring circuit. The key advantage of a ring circuit is that it provides two paths for current, which can allow smaller cable to carry a higher load. But modern consumer units with RCBOs make radial circuits a straightforward and reliable choice.

What cable size do I need for a radial circuit?

The cable size depends on the protective device rating, the installation method (reference method), and the applicable correction factors. For a 20A radial, 2.5mm² twin and earth (6242Y) is standard when installed using Reference Method C (clipped direct) or Reference Method A (enclosed in conduit in a thermally insulating wall). For a 32A radial, 4mm² twin and earth is the minimum. However, you must apply the Appendix 4 rating factors for grouping (Cg, Tables 4C1 to 4C6), ambient temperature (Ca, Tables 4B1 and 4B2), and thermal insulation (Ci). If the cable passes through insulation, you may need to increase the cable size. You must also check voltage drop against Table 4Ab: for an installation supplied directly from a public low voltage distribution system the limit is 5% for uses other than lighting (11.5V on a 230V supply) and 3% for lighting (6.9V), measured under the design current.

Do radial circuits need RCD protection?

Under BS 7671:2018+A4:2026, Regulation 411.3.3 requires 30mA RCD additional protection for socket-outlets with a rated current not exceeding 32A. Note that the 32A threshold is the rating of the socket-outlet, not of the circuit — a 13A socket on a 32A radial is squarely within scope. The only exception is a documented risk assessment, and that exception is not available for a dwelling. This applies to radial circuits serving socket outlets just as it does to ring circuits. For lighting circuits, Regulation 411.3.4 (introduced in A4:2026) now requires 30mA RCD additional protection on all AC final circuits supplying luminaires in domestic (household) premises — not only those in bathrooms or with cables concealed at less than 50mm depth. Radial lighting circuits are explicitly included. In practice, this means RCBO boards or split-load consumer units with RCD protection on every lighting circuit are now required for all new domestic installations and alterations.

How do I test a radial circuit?

Testing a radial circuit follows the standard sequence set out in BS 7671 Chapter 64 and GN3 (Guidance Note 3: Inspection and Testing). The key tests are: continuity of protective conductors (R1+R2 method), insulation resistance (minimum 1MΩ at 500V DC for a 230V circuit), polarity verification at every point, earth fault loop impedance (Zs) at the furthest point (which must not exceed the maximum Zs value for the protective device from Table 41.3 for circuit-breakers, or Table 41.2 for fuses at 0.4 s), and prospective fault current at the origin. For a radial circuit, the R1+R2 continuity test is simpler than for a ring circuit because there is only one path — you do not need to perform the ring circuit continuity test (the three-step method). The measured Zs at the furthest point must be checked against the tabulated maximum to ensure disconnection within the required time. Be careful with the old rule of thumb that fixed equipment gets 5 seconds — the Table 41.1 times (0.4 s on a TN system at 230 V) apply to final circuits up to 63 A with socket-outlets AND to final circuits up to 32 A supplying only fixed connected equipment. Regulation 411.3.2.2 holds the maximum disconnection times; the 5-second figure comes from Regulation 411.3.2.3, which covers distribution circuits and any circuit not covered by 411.3.2.2.

Is a spur allowed on a radial circuit?

Yes, spurs are permitted from radial circuits, subject to the same rules that apply to spurs from ring circuits. A fused spur (via a fused connection unit) can supply any load up to the rating of the fuse in the FCU. An unfused spur from a radial circuit is also permitted, but Figure 15B of Appendix 15 of BS 7671 is specific: an unfused spur run in 2.5mm² cable should feed one single or one twin socket-outlet only. Fixed equipment is fed from a fused connection unit (maximum 13A fuse), not from an unfused spur. The total load on the radial circuit, including all spurs, must not exceed the rating of the protective device. In practice, spurs from radial circuits are common for supplying fixed appliances such as extractor fans, waste disposal units, or additional socket outlets in locations where extending the main radial cable would be impractical.

What is the advantage of radial circuits over ring circuits?

Radial circuits offer several practical advantages. They are simpler to design, install, and test. There is no risk of a broken ring going undetected (a common fault in ring circuits where one leg of the ring becomes disconnected, causing the remaining conductor to carry the full load). Testing is faster because the three-step ring circuit continuity test is not required. Fault finding is more straightforward because the circuit follows a single path. Modern RCBO boards provide individual circuit protection, which removes the main historical advantage of ring circuits (sharing current between two conductors). Many European countries have used radial circuits exclusively for decades without ring circuits. The trend in UK domestic installations is increasingly towards radial circuits, particularly in new-build properties.

INSTALLATION GUIDE

Radial Circuit Explained: When to Use Radial vs Ring

Radial circuits are the simplest, most reliable circuit configuration in electrical installations. This guide covers how they work, when to choose a radial over a ring circuit, cable sizing from BS 7671, circuit protection requirements, and testing procedures for UK electricians.

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12 min readUpdated 2026-08-06Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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The short answer

What cable and MCB size does a radial circuit need?

A standard domestic socket radial circuit uses 2.5mm² twin and earth on a 20A breaker, or 4mm² on a 32A breaker for a larger floor area. Lighting radials use 1.0 to 1.5mm² on a 6A or 10A device. The cable current-carrying capacity (Iz) must equal or exceed the device rating after correction factors are applied (BS 7671 Reg 433.1.1).

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Key takeaways

01A radial circuit runs from the distribution board to each outlet in sequence, terminating at the last point — unlike a ring circuit which loops back to the board.
02Radial circuits are the preferred choice for dedicated appliance circuits (cookers, showers, immersion heaters) and lighting circuits throughout the UK.
03Appendix 15 of BS 7671 (informative) shows a 20A radial on 2.5mm² cable for socket outlets and notes that the floor area served has historically been limited to 50m²; the larger arrangement is a 32A radial on 4mm² cable, traditionally taken as up to 75m².
04Correct cable sizing depends on the protective device rating, installation method, grouping, ambient temperature, and thermal insulation — use the correction factors from Appendix 4 of BS 7671.
05Elec-Mate's cable sizing calculator applies all BS 7671 correction factors automatically and checks voltage drop against the BS 7671 Table 4Ab limits — 3% for lighting and 5% for other uses.

01 · Installation Guide

What Is a Radial Circuit?

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A radial circuit is an electrical circuit that starts at the distribution board (consumer unit) and runs to each outlet or load point in sequence, terminating at the last point on the circuit. Current flows in one direction only — from the supply to the load and back through the neutral and protective conductors. This is the simplest and most common circuit configuration used in electrical installations worldwide.

In the UK, radial circuits are used for lighting circuits, dedicated appliance circuits (cookers, showers, immersion heaters, electric vehicle chargers), and increasingly for general-purpose socket outlet circuits as an alternative to the traditional ring final circuit. The circuit is protected at the distribution board by a circuit breaker (MCB or RCBO) rated to match the cable current-carrying capacity.

The term "radial" distinguishes this configuration from a ring final circuit, where the cable forms a loop starting and finishing at the same terminals in the distribution board. Both configurations are permitted under BS 7671, and the choice between them depends on the load, floor area, cable routing, and design preference.

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02 · Installation Guide

How Radial Circuits Work

In a radial circuit, the line conductor, neutral conductor, and circuit protective conductor (CPC) all run from the distribution board to the first outlet, then continue to the second outlet, and so on to the last outlet on the circuit. The circuit ends at the last point — there is no return path back to the board.

Single path for current — all current flows through the same conductors from the distribution board to the load. The conductor nearest the board carries the total circuit current.
Progressive voltage drop — voltage drop increases with distance from the board. The furthest outlet sees the highest voltage drop, which must be checked against the BS 7671 voltage drop limits — 3% for lighting and 5% for other uses, for an installation supplied directly from a public low voltage distribution system (Table 4Ab).
Cable sized for full load — the cable must be rated to carry the full design current of the circuit because there is only one current path, unlike a ring circuit where current is shared between two legs.

This single-path design makes radial circuits straightforward to install and test. There are no cross-connections to worry about, no ring continuity tests to perform, and fault finding follows the cable from one end to the other. For electricians, this simplicity translates directly into faster installation and testing times.

03 · Installation Guide

When to Use a Radial Circuit

Radial circuits are the correct choice in several common scenarios. Understanding when to specify a radial rather than a ring circuit is a fundamental design skill.

Dedicated appliance circuits — cookers, electric showers, immersion heaters, electric vehicle chargers, and any other high-current fixed appliance should always be on a dedicated radial circuit with cable and protection sized for the specific load.
Lighting circuits — all lighting circuits in the UK are radial circuits. A typical domestic lighting circuit uses 1.0mm² or 1.5mm² cable protected by a 6A or 10A MCB.
Small floor areas — for rooms or areas up to 50m², a 20A radial on 2.5mm² cable is simpler and more cost-effective than a ring circuit. For areas up to 75m², a 32A radial on 4mm² cable is the equivalent alternative.
Extensions and additions — when adding circuits to an existing installation, a radial circuit is often the simplest option because it only requires a single cable run from the distribution board.
Commercial and industrial installations — radial circuits are the standard configuration for commercial socket outlets, typically using 20A or 32A circuits on appropriately sized cable.

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04 · Installation Guide

Radial vs Ring Final Circuit: A Direct Comparison

The debate between radial and ring final circuits has been ongoing in the UK electrical industry for years. Here is a factual comparison to help you make the right design choice.

Radial Circuit

Single cable path from board to last outlet
Cable must carry full circuit current
20A on 2.5mm² serves up to 50m²
32A on 4mm² serves up to 75m²
Simple to install, test, and fault-find
No risk of broken ring faults
Standard in commercial and European installations

Ring Final Circuit

Cable loops from board back to board
Current shared between two legs of the ring
32A on 2.5mm² serves up to 100m²
Smaller cable for higher load rating
Three-step ring continuity test required
Risk of broken ring or cross-connection faults
Unique to UK and Ireland

The historical advantage of ring circuits was that they allowed 2.5mm² cable to be used on a 32A circuit — saving on copper cost when it was expensive. With modern cable prices and the widespread adoption of RCBO boards, the cost difference is minimal. Many designers and electricians now prefer radial circuits for their simplicity, reliability, and ease of testing.

Neither configuration is inherently safer than the other when correctly installed and tested. The risk with ring circuits is that faults (such as a broken ring) can go undetected and cause overloading. Radial circuits do not have this failure mode because there is only one current path to monitor.

05 · Installation Guide

Cable Sizing for Radial Circuits

Correct cable sizing is critical for radial circuits. The cable must be able to carry the design current continuously without exceeding its rated temperature, and the voltage drop at the furthest point must not exceed the BS 7671 limit.

Standard Radial Circuit Cable Sizes

6A lighting circuit — 1.0mm² twin and earth (6242Y). Suitable for most domestic lighting circuits with LED loads.
16A immersion heater — 2.5mm² twin and earth. Dedicated radial to a 3kW immersion heater via a double-pole switch.
20A radial sockets — 2.5mm² twin and earth. General-purpose socket outlets serving up to 50m² floor area.
32A radial sockets — 4mm² twin and earth. General-purpose socket outlets serving up to 75m² floor area.
32A cooker circuit — 6mm² twin and earth. Dedicated radial to a cooker control unit with diversity applied.
40A or 45A shower circuit — 10mm² twin and earth for showers above 9.5kW. Cable size depends on the kW rating of the shower.

These are starting points for standard installation conditions. You must always apply the correction factors from Appendix 4 of BS 7671 for the actual installation conditions — grouping with other cables, ambient temperature, and thermal insulation. If any correction factor reduces the current-carrying capacity below the protective device rating, you must increase the cable size.

High Protective Conductor Current: Radial Socket-Outlet Circuits

A duplicate protective conductor on a radial socket-outlet circuit is not a general good-practice option — it belongs to one specific case. Regulation 543.7.2.201 applies where a final circuit has a number of socket-outlets or connection units intended to supply two or more items of equipment and the total protective conductor current in normal service is known or reasonably expected to exceed 10mA. That circuit must have a high integrity protective conductor connection. For a radial, the two acceptable arrangements are a single protective conductor connected as a ring, or a separate protective conductor provided at the final socket-outlet by connection to metal conduit or ducting. The IET On-Site Guide covers the same ground at 7.5.3 and adds that the duplicate protective conductor should be kept close to the circuit conductors to reduce electromagnetic compatibility (EMC) effects. Regulation 543.7.1.205 also requires information at the distribution board identifying circuits with a high protective conductor current.

06 · Installation Guide

Size the Cable for Your Radial Circuit

The tables above are the standard arrangements for standard conditions. Your circuit is rarely standard — it is grouped with five other cables in a joist notch, it runs through 150mm of loft insulation, or the last socket is 28 metres from the board. Put the real numbers in below and the calculator does the Appendix 4 work for you: it applies the grouping (Cg), ambient temperature (Ca) and thermal insulation (Ci) rating factors, checks the corrected current-carrying capacity against the device rating, and checks voltage drop at your cable length against the Table 4Ab limits — 3% for lighting, 5% for other uses.

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Cable Sizing Calculator

Professional cable sizing with BS 7671 compliance validation

Current Specification

Cable sizing parameters

Design Current (A)

Cable Length (m)

Installation method (BS 7671)

Selected method

Clipped direct to surface (Method C)

Cable type

Insulation Type

Protective device (BS 7671 Reg 433.1.1)

Device Type

Rating (In)

The cable is sized on In, not on Ib (Reg 433.1.1: Ib ≤ In ≤ Iz). A BS 3036 semi-enclosed fuse also applies Cf = 0.725 per Appendix 4 §5.1.1(c)(i).

Environmental conditions

Ambient Temperature (°C)

Standard: 30°C

Cables Grouped

Affects current rating

Voltage Drop Limit (%)

Lighting: 3%, Power: 5%

Load characteristics

Load Type

Diversity Factor

1.0 = 100% simultaneous load

System parameters

System Voltage (V)

Power Factor

Voltage drop, App 4 §6.2. Resistive 1.0, inductive 0.8-0.9

Empty: Design Current (A), Cable Length (m).

Cable selection factors

Cable sizing depends on multiple factors beyond current rating alone:

Current-carrying capacity
Voltage drop over distance
Installation method & ambient temperature
Grouping factors when multiple cables run together
Short circuit protection requirements

Always consult relevant electrical codes and standards for your specific application.

If the result comes back a size larger than you expected, the usual cause is insulation or grouping rather than the load itself — the correction factors guide explains which factor bit you. For a long radial where voltage drop rather than capacity is the limit, work the run through the voltage drop calculator before you order the drum.

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07 · Installation Guide

Circuit Protection Requirements for Radial Circuits

Every radial circuit must be protected by an overcurrent protective device at the distribution board. The device must disconnect the circuit within the required time in the event of a fault. The key requirements under BS 7671 are:

Overload protection (Regulation 433) — the rated current of the protective device (In) must be greater than or equal to the design current (Ib) and less than or equal to the current-carrying capacity of the cable (Iz). The conventional operating current (I2) must not exceed 1.45 times Iz.
Automatic disconnection of supply (Regulation 411.3.2.2) — this is the regulation that decides which disconnection time your radial has to meet, and it catches people out. Regulation 411.3.2.2 holds the maximum disconnection times, and Table 41.1 applies to two categories of final circuit: those rated up to 63 A that include one or more socket-outlets, and those rated up to 32 A supplying only fixed connected current-using equipment. So a 20 A radial feeding fixed equipment is not a 5-second circuit — it is 0.4 s on a TN system at 230 V, the same as a socket-outlet circuit. The 5-second time is for distribution circuits, and for final circuits above those thresholds. The earth fault loop impedance (Zs) at the furthest point must not exceed the maximum for the device type and rating.
RCD protection (Regulation 411.3.3) — 30mA RCD additional protection is required for socket-outlets with a rated current not exceeding 32A. The threshold is the rating of the socket-outlet, not the rating of the circuit. The exception for a documented risk assessment is not available for a dwelling. This is achieved using either an RCD upstream of the MCB or an RCBO combining both functions.
Surge protection (SPD) — the risk assessment method has gone. BS 7671:2018+A4:2026 deleted Regulation 443.5 and Annex A443. Under the redrafted Regulation 443.4.1, protection against transient overvoltages shall be provided where the consequence of the overvoltage could result in serious injury or loss of human life, or significant financial or data loss; for all other cases it shall still be provided unless the owner of the installation declares that the loss or damage would be tolerable and accepts the risk. An SPD installed at the origin of the installation shall be Type 1 or Type 2 (Regulation 534.4.1.1) — Type 1 where the building has a lightning protection system.

The most common protective devices for domestic radial circuits are Type B MCBs (for resistive loads such as sockets and lighting) and Type C MCBs (for circuits with higher inrush currents such as motors or large transformers). RCBOs combine overcurrent and RCD protection in a single device and are the standard choice in modern RCBO consumer units.

08 · Installation Guide

Common Radial Circuit Applications in Domestic Installations

In a typical UK domestic installation, the following circuits are always radial:

Lighting Circuits

1.0mm² or 1.5mm² cable, 6A MCB. Each lighting circuit typically serves one floor of a house. LED loads have reduced the current demand significantly, but the cable size remains the same for voltage drop and fault current disconnection times.

Cooker Circuits

6mm² cable, 32A MCB. Dedicated radial to a cooker control unit. Diversity factor applies: first 10A at 100% plus 30% of the remainder, plus 5A if the control unit has a socket outlet.

Electric Shower Circuits

6mm² to 16mm² cable depending on kW rating. Dedicated radial with a pull-cord switch or ceiling-mounted double-pole isolator. 32A to 50A MCB depending on shower rating.

Immersion Heater Circuits

2.5mm² cable, 16A MCB. Dedicated radial to a double-pole switch adjacent to the hot water cylinder. A 3kW immersion heater draws approximately 13A at 230V.

Elec-Mate's AI circuit designer can generate a complete circuit schedule for a domestic installation, specifying radial or ring circuits for each area based on the load, floor area, and your design preferences.

09 · Installation Guide

Testing a Radial Circuit

Testing a radial circuit is simpler than testing a ring circuit because there is only one current path. The tests follow the sequence set out in Chapter 64 of BS 7671 and GN3:

Continuity of protective conductors. Using the R1+R2 method, measure the resistance of the line conductor and CPC from the distribution board to the furthest point. This confirms the circuit is continuous and gives the R1+R2 value needed for the Zs calculation.
Insulation resistance. Test between line and neutral, line and earth, and neutral and earth at 500V DC. The minimum acceptable value is 1MΩ for a 230V circuit, but readings above 2MΩ are expected for a healthy installation.
Polarity. Verify correct polarity at every outlet, switch, and connection point. The line conductor must be connected to the switch contact and the correct terminal at socket outlets.
Earth fault loop impedance (Zs). Measure at the furthest point on the circuit. The measured value must not exceed the maximum Zs for the protective device type and rating — Table 41.2 of BS 7671 for fuses at 0.4 s, or Table 41.3 for circuit-breakers. (Table 41.4 is the 5 s fuse table and does not apply to a final circuit covered by Regulation 411.3.2.2.) Apply the 0.8 correction factor if comparing with tabulated values at a higher conductor temperature.
RCD operation. If the circuit is RCD-protected, test using an alternating current test at the rated residual operating current (IΔn — 30mA for domestic circuits), as required by BS 7671:2018+A4:2026 Regulation 643.8. Note: the previous Appendix 3 Table 3A time/current criteria (300ms at 1× IΔn, 40ms at 5× IΔn) have been deleted in A4:2026 — the revised requirement is an AC test at IΔn regardless of RCD type (AC, A, F, B).

Record all test results on the schedule of test results, which forms part of the electrical certificate (EIC for new circuits, EICR for existing circuits, or Minor Works Certificate for small additions).

10 · Installation Guide

Common Mistakes to Avoid with Radial Circuits

Undersized cable for the installation method. A cable clipped direct to a joist has a different current-carrying capacity to the same cable enclosed in insulation. Always check the correct reference method and apply the corresponding correction factors.
Ignoring voltage drop on long runs. A 32A radial on 4mm²cable over 30 metres may exceed the 5% voltage drop limit under full load. Always calculate voltage drop for the actual cable length and design current.
Overloading a 20A radial. A 20A radial on 2.5mm² cable is not suitable for heavy loads. If the area has multiple high-power appliances (kettles, heaters, tumble dryers), consider a 32A radial or a ring circuit.
Missing RCD protection on socket circuits. Socket-outlets with a rated current not exceeding 32A must have 30mA RCD additional protection, and in a dwelling there is no risk-assessment exception. This applies to radial circuits as well as ring circuits.
Not checking Zs at the furthest point. The earth fault loop impedance at the last outlet on a long radial can be significantly higher than at the distribution board. If Zs exceeds the maximum for the protective device, the circuit will not disconnect within the required time in the event of a fault.

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Check voltage drop and Zs before you install

Elec-Mate's voltage drop calculator and cable sizing tool check your radial circuit design against BS 7671 limits before you run a single metre of cable.

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Frequently Asked Questions About Radial Circuits

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Radial Circuit Explained: When to Use Radial vs Ring

What cable and MCB size does a radial circuit need?

Key takeaways

What Is a Radial Circuit?

Get the BS 7671 A4:2026 Cheat Sheet — free

How Radial Circuits Work

When to Use a Radial Circuit

Radial Circuits Explained: Cable & MCB Sizes (BS 7671)

Radial vs Ring Final Circuit: A Direct Comparison

Radial Circuit

Ring Final Circuit

Cable Sizing for Radial Circuits

Standard Radial Circuit Cable Sizes

High Protective Conductor Current: Radial Socket-Outlet Circuits

Size the Cable for Your Radial Circuit

Cable Sizing Calculator

Current Specification

Try Elec-Mate free for 7 days

Circuit Protection Requirements for Radial Circuits

Common Radial Circuit Applications in Domestic Installations

Lighting Circuits

Cooker Circuits

Electric Shower Circuits

Immersion Heater Circuits

Testing a Radial Circuit

Common Mistakes to Avoid with Radial Circuits

Check voltage drop and Zs before you install

Frequently Asked Questions About Radial Circuits

What is the maximum floor area for a 20A radial circuit?

Can I use a radial circuit instead of a ring circuit for sockets?

What cable size do I need for a radial circuit?

Do radial circuits need RCD protection?

How do I test a radial circuit?

Is a spur allowed on a radial circuit?

What is the advantage of radial circuits over ring circuits?

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Cable Sizing Calculator

Voltage Drop Calculator

Spur Socket Regulations

Cooker Circuit Guide

Cable Sizing Guide BS 7671

Distribution Board Wiring

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