Radial Circuit Diagram: UK Radial Final Circuit Wiring and Ring Final Comparison
This is a free printable radial circuit diagram: download the diagram as SVG or open it and print to paper or PDF.
A radial circuit diagram shows a single cable originating at the consumer unit and serving one or more socket outlets or fixed loads, terminating at the final outlet with no cable returning to the board.
In UK wiring practice (governed by BS 7671, the IET Wiring Regulations), two types of final circuit are used to supply 13 A socket outlets: the ring final circuit and the radial final circuit. Understanding the distinction is fundamental to correctly reading and drawing either circuit type.
A ring final circuit is formed by a single cable that leaves the consumer unit, passes through a series of socket outlets, and returns to the same consumer unit way — forming a complete ring. Both ends of the cable terminate on the same MCB or fuse. Because the ring provides two parallel paths for current, each carrying approximately half the total circuit current, 2.5 mm² cable protected by a 32 A device can serve an unlimited floor area (in practice, a floor area of up to 100 m² is commonly cited in guidance, though BS 7671 does not specify a direct area limit in terms that simple — the designer must verify the actual Zs and volt drop).
A radial final circuit runs from the consumer unit to each outlet in series and terminates at the last outlet. There is no return cable. Because all current flows through the same single cable path, the cable size and protection device must be matched to the expected total load. BS 7671 recognises two standard radial socket outlet circuit arrangements: a 20 A radial circuit using 2.5 mm² cable and a 20 A MCB, serving a floor area up to 50 m²; and a 30/32 A radial circuit using 4 mm² cable and a 30/32 A device, serving a floor area up to 75 m².
In practice, radial circuits in the UK are commonly used for dedicated single loads — a single socket for a washing machine, dishwasher, or refrigerator — and for spur connections from a ring final circuit. A spur is an unswitched or fused connection unit branching off a ring circuit to supply one additional outlet, subject to the condition that the total number of spurs does not exceed the number of socket outlets directly connected to the ring.
A radial circuit must not be confused with the North American circuit topology, where 'radial' simply describes every branch circuit (which is always radial by topology — cables run from the panel to the loads with no return ring).
How to wire radial circuit diagram
- Determine whether a ring or radial circuit is appropriate Assess the floor area to be served and the likely connected load. For a room up to 50 m² or a single dedicated appliance, a 20 A radial circuit is appropriate. For larger areas or where a single cable run is more practical, a ring final circuit or a 30/32 A radial circuit with 4 mm² cable should be considered. Confirm with BS 7671 Appendix 15 guidance and verify Zs and volt drop.
- Plan the cable route Determine the most practical route from the consumer unit way to each socket outlet in sequence. In a new build, cable is buried in walls at a depth compliant with BS 7671 or run in conduit. In an existing building, surface conduit or mini-trunking is common. Identify any areas where mechanical protection is required (floor zones, wall zones near switches and sockets).
- Select the correct cable and protection device For a 20 A radial circuit: 2.5 mm² flat twin and earth (brown live, blue neutral, bare earth sleeved green/yellow) and a 20 A Type B MCB. For a 30/32 A radial circuit: 4 mm² flat twin and earth and a 30/32 A Type B MCB. Confirm that the protective device type (B, C, or D) is appropriate for the load characteristics.
- Install the first socket outlet At the first socket outlet position, fit the back box. Run the cable from the consumer unit to the back box. If this is not the final outlet, a second cable will continue onward to the next outlet — both cables must be terminated at this socket (incoming and outgoing). Strip and connect: brown (live) to the L terminal, blue (neutral) to the N terminal, green/yellow sleeved bare earth to the E terminal.
- Continue the cable run to subsequent outlets Repeat the termination process at each intermediate outlet, connecting both the incoming and outgoing cables. At intermediate outlets, the two live conductors are double-up on the L terminal (or use a terminal block if only one cable is accepted per terminal), similarly for neutral and earth.
- Terminate the final outlet At the last (final) outlet in the radial circuit, only the incoming cable terminates. There is no outgoing cable. This is the defining characteristic of a radial circuit. Connect live, neutral, and earth as normal to the single cable.
- Terminate at the consumer unit and test Terminate the circuit cable at the allocated MCB way. Live (brown) to the MCB, neutral (blue) to the neutral bar, earth (green/yellow) to the earth bar. Perform the tests required by BS 7671 Chapter 64: insulation resistance, polarity, earth continuity, and earth fault loop impedance (Zs). Verify that measured Zs is within the limit for the MCB type and rating.
Specifications
| 20 A radial circuit cable size | 2.5 mm² twin and earth |
|---|---|
| 20 A radial circuit maximum floor area (guidance) | 50 m² |
| 30/32 A radial circuit cable size | 4 mm² twin and earth |
| 30/32 A radial circuit maximum floor area (guidance) | 75 m² |
| Ring final circuit cable size | 2.5 mm² twin and earth |
| Ring final circuit MCB rating | 32 A |
| UK supply voltage | 230 V, 50 Hz |
| BS 1363 socket outlet rating | 13 A |
Safety warnings
- All fixed electrical installation work in the UK must comply with BS 7671 (IET Wiring Regulations, 18th Edition or current edition). In England and Wales, Part P of the Building Regulations requires that most domestic electrical work is carried out by a registered competent person or notified to the building control body.
- Always isolate the consumer unit way for the circuit being worked on and verify dead with a calibrated voltage tester before handling any conductors. If the consumer unit does not have lockable MCBs, lock out or secure the panel against accidental re-energisation.
- Apply green/yellow earth sleeving to all bare earth conductors at every termination point. An unsleeved bare conductor at a socket outlet terminal can be mistaken for an unconnected wire and left unconnected by a future installer.
- Do not exceed the floor area limits for the circuit type. Exceeding the 50 m² limit of a 20 A radial circuit increases the likelihood that the Zs exceeds the limit for the MCB, reducing fault protection effectiveness.
- Do not create a ring final circuit by adding a return cable from the final outlet back to the consumer unit unless the circuit has been designed and tested as a ring. A ring circuit requires confirmation of continuity, polarity, and insulation resistance for both legs before energisation.
Tools needed
- Non-contact voltage tester
- Lockout device for MCB
- Cable stripping knife and wire strippers
- Flat-blade screwdriver for terminal connections
- Continuity and insulation resistance tester (to BS 7671 testing requirements)
- Earth fault loop impedance tester (Zs measurement)
- Fish tape and cable drill for concealed routing
- Green/yellow sleeving and sleeving applicator
Common mistakes
- Wiring a radial circuit and then adding a return cable to the consumer unit without properly testing it as a ring, creating a ring with unverified loop impedance and potentially concealed faults.
- Failing to apply green/yellow earth sleeving to bare earth conductors at socket outlet terminals.
- Using 2.5 mm² cable protected by a 32 A MCB (the rating appropriate only for ring final circuits), creating an under-protected radial circuit.
- Connecting more than one socket outlet on a spur from the radial circuit's final outlet, which is only permissible from a ring circuit under specific conditions.
- Not measuring Zs after installation, leaving it unknown whether the earth fault loop impedance is within the limit for the protective device.
Troubleshooting
- MCB trips when certain sockets on the radial circuit are loaded
- Cause: Total connected load exceeds the 20 A (or 32 A) MCB rating, or an appliance has developed a fault creating an overload or earth fault. Fix: Calculate the actual connected load when the tripping occurs. If overload: redistribute loads to another circuit or install a dedicated circuit for the high-demand appliance. If the MCB trips immediately: test insulation resistance on the circuit to identify a fault to earth.
- One socket outlet on the circuit is dead but others work
- Cause: Open circuit in the live or neutral conductor at that outlet's terminals — typically a loose push-in connection or a pulled terminal screw. Fix: Isolate the circuit. Open the socket faceplate and check all terminal connections. Examine whether the outgoing cable to subsequent outlets (if any) is also loose — a loose connection in the live conductor to an intermediate socket will cause all sockets beyond that point to lose power.
- Measured Zs at a socket outlet fails the limit for the MCB
- Cause: Circuit cable run is too long for the cable size and MCB rating, introducing too much resistance in the earth fault return path. This is particularly common on 20 A radial circuits with long runs to remote parts of a building. Fix: Increase the cable cross-sectional area to reduce resistance, or move the circuit's MCB to a smaller rating (e.g., 16 A) to meet the Zs limit for the lower-rated device. Confirm any solution with a re-measurement of Zs at the furthest outlet.
Frequently asked questions
What is the main difference between a radial and ring final circuit in UK wiring?
A ring final circuit has two cable ends terminating at the same way in the consumer unit, forming a closed loop through the socket outlets. A radial circuit has only one cable end at the consumer unit and terminates at the last outlet. The ring's two-path topology allows a smaller cable to serve a larger area; a radial circuit must use larger cable or smaller area coverage for the same protection device rating.
Can I wire a UK kitchen with a radial circuit instead of a ring final circuit?
Yes. A 20 A radial circuit using 2.5 mm² cable can serve a floor area up to 50 m², which is typically adequate for a kitchen. However, a kitchen also typically requires two small appliance circuits (under Appendix 15 guidance in the 18th Edition), and the designer must verify that Zs (earth fault loop impedance) and volt drop are within limits. Many designers still use ring finals for kitchen socket circuits for practical reasons.
How many sockets can I have on a UK radial circuit?
BS 7671 does not specify a maximum number of socket outlets on a radial circuit — the limit is the floor area and the requirement that the total connected load does not cause overloading of the cable or protective device. In practice, a radial circuit for a single appliance (washing machine, refrigerator) typically has one outlet, while a radial circuit for a room might have multiple outlets provided the floor area and cable sizing requirements are met.
What cable size is used for a UK 20 A radial circuit?
A 20 A radial final circuit uses 2.5 mm² twin and earth (T&E) cable (also written as 2.5 mm²/1.5 mm² — 2.5 mm² live conductors, 1.5 mm² earth conductor) and is protected by a 20 A MCB, serving a floor area up to 50 m² per BS 7671 guidance.
What is the difference between a spur and a radial circuit in UK practice?
A spur is a branch connection from a ring final circuit, supplying one additional unswitched socket outlet or one fused connection unit. It is not a separate circuit — it is part of the ring circuit. A radial circuit is a separate final circuit originating at its own way in the consumer unit. A spur from a ring uses the ring circuit's protection device; a radial circuit has its own dedicated MCB.
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