Helpful video reference. Brian the electrician's "Level 1 Electrical - Wiring And Testing A Radial Socket Outlet" covers this as a first-year training task in the UK, which is exactly the level of detail a careful homeowner needs. It covers the connections at both ends and the tests you should carry out before energising.
1. Decide whether a radial is the right circuit type
A radial socket circuit runs from the consumer unit to one or more sockets, with the final socket being a dead end. It does not loop back. This is fine for a single room, a garage, a garden room or anywhere the cable run is too long to complete a ring economically.
A 20 A radial on 2.5 mm² cable serves a floor area up to 50 m² for a single appliance outlet, though in practice most single-room radials use a 32 A MCB with 4 mm² cable to give more flexibility. If you are adding two or three sockets in one room, a 20 A radial on 2.5 mm² is usually plenty.
2. Size the MCB and cable correctly
The MCB must not exceed the current-carrying capacity of the cable it protects. Two common options for domestic radial socket circuits:
- 20 A Type B MCB with 2.5 mm² twin and earth - suits a single room or small workshop.
- 32 A Type B MCB with 4 mm² twin and earth - suits a larger area or a dedicated circuit for high-current appliances.
Never connect 2.5 mm² cable to a 32 A MCB. The MCB will not protect the cable from overload. This is a common mistake and a fire risk.
Also check the method of installation. Cable buried in an insulated wall loses heat less easily than cable clipped to a joist. BS 7671 Appendix 4 correction factors reduce the allowable current for buried or grouped cables - if in doubt, go up a cable size.
3. Plan the cable route
Cable buried in a wall must run in the permitted zones: vertically up or down from an accessory, or horizontally within 150 mm of the ceiling or floor, or within 150 mm of an internal corner. Outside these zones, cable must be mechanically protected (conduit or trunking) or be armoured.
Plan the route before you start. Work out how the cable enters the consumer unit enclosure, how it reaches the first socket, and whether any notifiable structural work (chasing, drilling joists) is involved alongside the electrical work.
4. Fit the back box
Flush-fit a metal back box into a masonry wall or fit a surface-mount pattress. Depth matters: 25 mm is the minimum for a standard single socket, 35 mm gives more room to dress the cables. Mark the position so the faceplate will be level, score around the outline and chisel out the recess. Fix the box with two 3.5 mm woodscrews or frame fixings appropriate to the wall material.
5. Run the cable
With the main switch at the consumer unit off and confirmed dead, feed the cable from the consumer unit end to the socket position. Pull through enough cable to reach all terminals with a little slack. At the socket end, allow 150 mm beyond the face of the wall so you have plenty to work with.
Protect any surface run in oval conduit or mini-trunking. Clip buried cable at maximum 400 mm centres if it is visible before plastering.
6. Connect at the socket
Strip the outer sheath back 50 mm. Sleeve the bare copper CPC (circuit protective conductor) in a length of green-and-yellow sleeving. Strip each conductor 8-10 mm.
- Brown conductor to the L terminal.
- Blue conductor to the N terminal.
- Green-and-yellow sleeved CPC to the E terminal.
Tighten each terminal screw to the torque specified on the accessory (usually 0.5-0.8 Nm). Tug each conductor - none should pull out. Fold the cable neatly into the back box and fit the faceplate.
7. Connect at the consumer unit
With the main switch still off, prepare the cable end at the consumer unit. Fit a new MCB of the correct rating into a spare way. Connect the brown conductor to the MCB output terminal, the blue conductor to the neutral bar and the sleeved CPC to the earth bar. Dress the cables neatly inside the board.
If there is no spare way, stop. You need either a larger consumer unit or a small secondary board. Do not use a bridged spare way or remove protection from another circuit.
8. Test before switching on
Before restoring the main switch, carry out:
- Continuity of the CPC using a low-resistance ohmmeter. The reading end to end should be below 1 ohm for a typical house circuit.
- Insulation resistance at 500 V DC between live and neutral (L-N), live and earth (L-E), and neutral and earth (N-E). Each should read above 1 MOhm.
With the main switch back on, use a socket tester to confirm correct polarity and earth loop impedance. A flashing red on an RCD-protected circuit is normal on some testers - use the MFT to measure Ze and Zs properly.
When to call us
A new radial circuit is a notifiable job. Richard is registered with NAPIT, which means the work comes with a Part P completion certificate that building control and conveyancers accept. You do not need to notify building control separately.
Need a new socket circuit in Sandwich?
Richard designs, installs and certifies new circuits at a fair local rate. The completion certificate is included.
Contact Richard