Wiring diagram
Correct wiring
One-gang switch
The simplest circuit: a 3×1.5 cable runs from the consumer unit to the switch back box — live, neutral and earth — and another 3×1.5 runs from the back box to the light. The brown from the consumer unit goes to terminal L, the brown to the light goes to output ↓, and the blues and the green-and-yellows of the two cables are joined in the back box with lever connectors and carry straight on to the light. Blue is neutral everywhere here, so nothing needs re-marking. Use a deep 60 mm back box: two lever connectors have to fit behind the switch.
What happens
Light: off
Live
Cables
| From → to | Cores × size |
|---|---|
| Panel → Door | 3×1.5 mm²green-and-yellow, blue, brown |
| Door → Light | 3×1.5 mm²green-and-yellow, blue, brown |
What to buy
- B10 MCB for the lighting circuitin the consumer unit, if there is no separate lighting circuit yet1 pcs
- One-gang switch1 pcs
- Frame, 1-gang1 pcs
- Deep back box, 60 mm1 pcs
- Lever connector for 2 wires2 pcs
- 3×1.5 mm² cable — one run2 pcs
How a switch works
A switch is a break in the live conductor. The live from the MCB comes in on terminal L and leaves from the output to the light, while neutral and earth go straight to the light, bypassing the switch. So when the switch is off, there is no voltage at the lampholder. An ordinary switch and a two-wire dimmer do not need a neutral, but a relay behind the switch, a motion sensor and many smart switches do.
A two-gang switch is two switches in one housing with a shared input: one live, two outputs, one for each group of lamps. A dimmer takes the place of the switch and, instead of just on and off, changes the brightness smoothly.
Terminal markings vary between makers: Legrand uses L and 1, UK switches COM and L1, German diagrams L and an arrow, and Berker marks the common terminal in red. In the diagram above the output is marked with an arrow ↓. For your own switch, check the diagram on the device.
Why wire through back boxes
In every diagram in this series the cable runs as a chain: from the consumer unit to the switch back box, from there to the light, and, where there are several switching points, from back box to back box first. Neutral and earth pass through each back box: the cores of the two cables are joined with lever connectors, and only the live goes to the switch. This is a common way of wiring in Germany.
The main advantage is that every back box has a neutral. If you later want a smart switch, a motion sensor or a dimmer that needs a neutral, nothing has to be rewired. There are fewer joints and boxes: all the connections sit behind the switch and at the light, and the ones behind the switch are easy to reach by taking the switch off. You use about the same amount of cable, sometimes a little less.
The price is a tight fit: the back box has to be deep, 60 mm, so that the lever connectors and some spare core length fit behind the switch. If your home is wired through junction boxes, the circuit is electrically the same, but different cables arrive at the switch, and there is usually no neutral in the back box.
Cables and core colours
Core colours are set by HD 308: live is brown, black or grey, neutral is blue, and green-and-yellow is earth only, never live or neutral. A 3×1.5 cable has green-and-yellow, blue and brown cores; a 5×1.5 has all five colours.
In diagrams wired through back boxes, blue stays neutral everywhere: live, neutral and earth come from the consumer unit to the switch, and the switched live on the brown, neutral and earth go on to the light. No re-marking is needed. If a blue core does end up carrying the live somewhere, for example in old wiring with no neutral at the switch, mark it at both ends with brown tape or sleeving: this is mandatory in the UK and standard practice elsewhere.
Earth is taken to every back box, even if the switch has no earth terminal: in Germany the standard requires this explicitly. Do not run cable without a green-and-yellow core to a switch.
Lighting on its own circuit
Lighting is wired as a separate circuit with its own MCB, not from a socket circuit: if the socket MCB trips, you are not left in the dark. For 1.5 mm² cable a B10 MCB is used: it protects such cable whatever the installation method, and in Spain 10 A for lighting is an explicit requirement.
In Germany, B16 is often fitted on 1.5 mm². This is acceptable only where the standard’s table rates the cable for at least 16 A (in conduit in a wall or clipped direct to a wall), and not in a thermally insulated wall.
Dimmers for LEDs: what to check
Dimmers work on the leading edge (RL), for wire-wound transformers, or on the trailing edge (RC), for LEDs and electronic transformers. For LED lamps, choose a dimmer with an RC mode or a universal one that detects the load by itself. The lamp must be marked “dimmable”: under EU rules the manufacturer has to state on the packaging if a lamp cannot be dimmed or does not work with all dimmers.
A dimmer has a minimum and a maximum load, which is the total wattage of all the lamps on it. The LED minimum is usually 2–4 W: a single 3 W LED candle or G9 lamp may fall short of it, and the light will flicker at low brightness. Many models have an LED maximum of about 100 W on trailing edge and less on leading edge. If dimmers are fitted side by side or the room is warmer than 25 °C, the permitted load is reduced — the figures are in the instructions.
A two-wire dimmer does not need a neutral, but some models recommend connecting one at low loads — and the back box already has it.
Wiring diagrams
One-gang switch
The simplest circuit: a 3×1.5 cable runs from the consumer unit to the switch back box — live, neutral and earth — and another 3×1.5 runs from the back box to the light. The brown from the consumer unit goes to terminal L, the brown to the light goes to output ↓, and the blues and the green-and-yellows of the two cables are joined in the back box with lever connectors and carry straight on to the light. Blue is neutral everywhere here, so nothing needs re-marking. Use a deep 60 mm back box: two lever connectors have to fit behind the switch.
Two-gang switch, chandelier
Each rocker switches its own group of lamps in the chandelier: the live comes in on the common terminal L, and outputs ↓1 and ↓2 go to the groups. From the consumer unit to the back box it is the same 3×1.5, and from the back box to the chandelier a 5×1.5: black is the first group on ↓1, brown the second on ↓2, blue is neutral, and grey is left spare. In the chandelier, neutral goes to the first group and on to the second with a short link. It does not matter which live core feeds which group, as long as it is the same at both ends.
Dimmer
Wired like a one-gang switch: the same two 3×1.5 cables, live on L, the feed to the lamp from terminal ↓, and the neutral in the back box comes in handy if the dimmer needs one. For LEDs, choose a trailing-edge (RC) or universal dimmer and lamps marked “dimmable”. In the interactive diagram, move the brightness slider: the graph shows how much of each half-cycle the dimmer lets through to the lamp. Then change the lamps and the mode: a single 3 W LED candle lamp is below the minimum load and flickers at low brightness, a non-dimmable LED flickers all the time, 20 lamps of 7 W each overload the dimmer (the LED limit is about 100 W), and leading edge (RL) is not for LEDs.
Common mistakes
Neutral through the switch
At the consumer unit the cable cores were swapped: the brown went on the neutral and the blue on the MCB. The switch now breaks the neutral, while the live runs along the blue core straight through the back box to the light. The light switches on and off as usual, but the lampholder stays live even when the light is off.
Why this is wrong
When you change a lamp or take the light down with the switch off, it is easy to get a shock: you assume that because the light is off, there is no danger. The blue cores in the back box, which everyone takes for neutral, are live too. A switch always goes in the live: the brown core from the consumer unit to the MCB, the blue to the neutral bar. If the colours in old wiring are unclear, find the live with a tester and mark it.
Live on output ↓1
The live from the consumer unit went not to the common terminal L of the two-gang switch but to output ↓1, and the black core of the first group went to L. The first rocker switches the first group, but the second does nothing on its own: the second group only comes on when the first is on too.
Why this is wrong
There is no danger, but the chandelier does not work as intended. A two-gang switch has one common terminal and two outputs, and makers label them differently: L and arrows, while Berker marks the common terminal in red. Find the common terminal from the diagram on the device, connect the live to it, and connect outputs ↓1 and ↓2 to the cores of the groups.
How to wire a switch, step by step
Switch off the lighting MCB
At the consumer unit, switch off the MCB for the lighting circuit, or the main switch if you are not sure which one it is. Tell everyone at home so that nobody switches it back on.
Check there is no voltage
First prove your voltage tester or multimeter on a socket you know is live. Then test every core in the back box and at the light: between each other and each one to earth.
Bring the cables into the back box
Bring two cables into the deep back box: from the consumer unit (3×1.5) and to the light (3×1.5, or 5×1.5 for a chandelier with two groups). Strip the sheath, leave the cores some spare length and label which cable comes from where.
Join neutral and earth
Join the blue from the consumer unit to the blue to the light with a lever connector, and the green-and-yellows of the two cables with a second one. They do not go to the switch. If the switch has an earth terminal, use a lever connector for three wires for the green-and-yellows and run the third wire from it to the switch. No bare copper should show outside the connectors.
Connect the switch
Brown from the consumer unit to the common terminal L, brown to the light to output ↓. On a two-gang switch, the live goes to L, the black to the chandelier to ↓1 and the brown to ↓2; insulate the grey and leave it spare. Fold the cores and connectors in behind the switch and screw it in place.
Connect the light
Connect the brown core from the switch to terminal L of the light, the blue to N and the green-and-yellow to PE. For a chandelier with two groups: black to the first group, brown to the second; if the groups have separate N terminals, link them together.
Check it works
Switch the MCB on and click the switch a few times. Then switch the light off and check the lampholder contact with a tester: there must be no live there, otherwise the switch is breaking the neutral. With a two-wire dimmer or an illuminated switch the tester may glow faintly even when the wiring is correct: a small current flows through their electronics, so always change lamps with the MCB off.
Safety and who may do the work
Only work with the MCB switched off and after testing every core with a voltage tester or multimeter, having first proved the tester on a socket you know is live. Who may carry out this work is set by the law of your country. In England, replacing a switch is not notifiable under Part P, but the work must comply with BS 7671. In Germany, changes to the wiring may only be made by an electrician registered with the network operator; in Spain, by an authorised installer; in Poland, by a person with an SEP qualification. In a number of other countries, work on the wiring is for electricians only, so check the rules where you live.
Frequently asked questions
What cable do I need for a light switch?
For a one-gang switch, two runs of 3×1.5 mm² with a green-and-yellow core: from the consumer unit to the switch back box and from there to the light. For a chandelier with two groups, a 5×1.5 runs from the back box. 1.5 mm² cable on a lighting circuit is protected by a B10 MCB.
Which terminal does the live go to on a switch?
To the common terminal: L, COM, P or the one with a red mark on Berker. On a plain, non-illuminated one-gang switch it makes no electrical difference which of the two terminals is the input, but it is easier to follow the markings. On a two-gang switch, strictly to the common L, otherwise the second rocker only works together with the first. Connect a dimmer strictly by its L and ↓ markings.
What happens if the switch breaks the neutral instead of the live?
The light switches on and off as usual, but with the switch off the lampholder stays live. You can only spot this with a tester, and it is dangerous when you change a lamp or the light fitting. A switch goes in the live.
Why does an LED lamp flicker on a dimmer?
Usually for one of three reasons: the dimmer has no trailing-edge (RC) mode, the lamp is not marked as dimmable, or the total wattage of the lamps is below the dimmer’s minimum load — for LEDs usually 2–4 W, so one small candle lamp may not be enough. Check all three on the packaging and in the instructions.