Helpful video reference. The video above is from Gary Hayers of GSH Electrical, a UK college lecturer and former installation electrician whose channel is used on City and Guilds and EAL training courses. His diagram walk-through "Wiring Diagram Lighting Circuit Joint Box Method — Great for Wiring to LED Downlights (Spotlights)" shows the terminal connections at the box clearly, including how the switch drop cores must be re-identified. Worth watching before you start.
1. Isolate the circuit and confirm dead
Switch off the correct lighting MCB at the consumer unit. If your board uses rewireable fuses, remove the cartridge and pocket it. Use a calibrated voltage tester at the switch and at the ceiling to confirm the circuit is dead. A neon screwdriver is not reliable enough for this test.
Take a photograph of any existing connections before disturbing them. If something looks wrong or unfamiliar once you open it up, stop there.
2. Choose and mount the junction box
The junction box must remain accessible after installation. A four-terminal or six-terminal round plastic box screwed directly to a joist works well, as does a box bracket spanning between joists in a loft void. The key point: if someone needs to check or alter the connection in ten years, they must be able to reach it without dismantling a ceiling or other permanent structure.
If you want to bury the joint in a ceiling void, use a certified maintenance-free enclosure such as a Wago Box fitted with the appropriate Wago connectors. Standard junction boxes with choc block terminals cannot be hidden from view.
3. Run and connect the supply cable
Feed 1mm² two-core and earth cable from the previous connection point on the circuit (often the consumer unit via a spur from another fitting, or from a loop-in rose elsewhere on the circuit). Connect the brown core to the line terminal in the box, the blue core to the neutral terminal, and the bare earth core, sleeved green and yellow, to the earth terminal. Do not overtighten and crush the conductors.
4. Wire the switch drop
Run a length of 1mm² two-core and earth from the junction box down the wall to the switch. This cable carries the line out to the switch on the brown core, and the switched live back on the blue core.
At the junction box: connect the brown core to the same line terminal as the supply. The blue core returns as switched live, so sleeve it red or brown at this end and connect it to a separate terminal (often labelled SL or switched live). At the switch plate, the blue core also needs to be sleeved or marked to show it carries the switched live. Bare earth sleeved green and yellow connects to the earth terminal at both ends.
5. Wire to the light fitting
Run a further length of 1mm² two-core and earth from the junction box to the fitting. At the box, connect brown to the switched live terminal (so the fitting is only live when the switch is on), blue to the neutral terminal, and sleeved earth to the earth terminal. At the fitting, follow its manufacturer's labelling for L, N and E.
Metal fittings must have the earth connected. Plastic fittings with no metal parts and a double-insulation symbol do not need an earth at the fitting itself, but the earth core should still be left in the ceiling, sleeved and folded back, rather than cut short.
6. Check conductor marking before closing up
Walk through the box and confirm: every bare earth conductor has green and yellow sleeving, and every re-identified core (particularly the blue switch return) is clearly marked brown or red at both ends. Count your terminals and make sure each one has only the correct conductors in it. No twisted bare wires pushed into a shared terminal.
Secure the junction box lid. Do not overtighten screws on plastic boxes.
7. Restore power and test
Switch the MCB back on at the consumer unit. Test the light from every switch position. If the circuit trips the MCB or RCD immediately, switch it back off and recheck for a conductor touching the wrong terminal or a bare wire bridging a gap. If the light does not come on at all, check the switched live connection at the box and at the switch.
When to call us
Adding a downlight or extending a lighting circuit is usually straightforward in a modern house. Older properties in Sandwich and the surrounding villages sometimes have wiring from the 1960s or 70s where the cable colours differ and the earthing may not be continuous. If you open up a ceiling and find anything that does not match what you expected, give Richard a call before proceeding.
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Richard installs and extends lighting circuits using compliant junction boxes, from adding a single downlight to running new circuits through a loft. Small local jobs start at £10 per 10 minutes.
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