Wiring diagram
Correct wiring
In a room: by the door and by the sofa
A 3×1.5 runs from the consumer unit to the back box by the door: brown to the common terminal L of the first two-way switch, blue and green-and-yellow to lever connectors and straight on. Between the two-way switches runs a 5×1.5: black connects terminals 1 and 1, grey connects 2 and 2, blue and green-and-yellow carry neutral and earth, and brown is left spare. From the second two-way switch to the light runs a 3×1.5, with the switched live from its common terminal L on the brown. Swapping 1 and 2 on one switch is not a mistake: either switch still changes the light over, only “on” ends up in a different combination of rocker positions.
What happens
Light: on
Live
Cables
| From → to | Cores × size |
|---|---|
| Panel → Door | 3×1.5 mm²green-and-yellow, blue, brown |
| Door → Sofa | 5×1.5 mm²green-and-yellow, blue, brown, black, grey |
| Sofa → 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
- Two-way switch2 pcs
- Frame, 1-gang2 pcs
- Deep back box, 60 mm2 pcs
- Lever connector for 2 wires4 pcs
- 3×1.5 mm² cable — one run2 pcs
- 5×1.5 mm² cable — one run1 pcs
How a two-way switch works
A two-way switch has three terminals: a common and two strapper terminals. The rocker connects the common to one strapper terminal or the other, like a set of railway points. Two two-way switches are joined by two strapper cores: the live comes in on the common terminal of the first and leaves from the common terminal of the second to the lamp. The light is on when both switches point to the same core, and goes off when you flip either of them.
Terminal markings vary between makers: L · 1 · 2 on Legrand, COM/L1/L2 on UK switches, a red mark on the common terminal on Berker, and L with two arrows in German diagrams. In the diagram above the terminals are marked L, 1, 2; for your own switch, check the diagram on the device.
An ordinary switch cannot replace a two-way switch, but a two-way switch can be fitted in place of an ordinary one: you then connect the common terminal and one strapper terminal.
Cables, core colours and the MCB
The two-way switches are wired as a chain: consumer unit → first two-way switch → second two-way switch → light. From the consumer unit to the first switch, a 3×1.5: brown is the live to the common terminal, blue is neutral, green-and-yellow is earth. Between the switches, a 5×1.5: black connects terminals 1 and 1, grey connects 2 and 2, blue carries neutral, green-and-yellow carries earth, and brown is left spare. From the second switch to the light, a 3×1.5 again: brown is the switched live from the common terminal, blue is neutral, green-and-yellow is earth. Neutral and earth are joined in each back box with lever connectors and do not go to 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.
The five-core cable between the switches is there for the neutral: it reaches both back boxes, so if you later need a smart relay or another device that needs a neutral, the wiring does not have to change. Blue stays neutral everywhere, so no re-marking is needed. Green-and-yellow is used only as earth, never as live or neutral. Earth is taken to every back box, even if the switch has no terminal for it: in Germany the standard requires this.
Lighting is wired as a separate circuit at the consumer unit: a B10 MCB on 1.5 mm² cable. In Spain, 10 A for lighting is mandatory. In Germany, B16 is also used on 1.5 mm², but only where the standard’s table rates the cable for 16 A (in conduit in a wall or clipped direct to a wall), and not in a thermally insulated wall.
Two-gang two-way switch: two groups from two places
If you need to control two groups of lights from two places, for example the two groups of a chandelier, fit two-gang two-way switches. This is two independent two-way switches in one unit: each rocker has its own common and two strapper terminals, six in all. The common terminals are usually separate: that is how the Berker 3038 is made, and on UK BG switches each rocker has its own COM. On the first switch, where the live arrives from the consumer unit, they are linked together; on the second, each common goes to its own group.
Between the switches run four strapper cores plus neutral and earth — too many for one 5×1.5. The simplest option is two cables: a 5×1.5 for the first rocker, as for an ordinary two-way switch (black on 1, grey on 2, blue as neutral, brown spare), and a 4×1.5 for the second (black on 1, grey on 2, brown spare, green-and-yellow as earth). A single 7×1.5 cable will also do, but it is harder to find, its cores are usually black with numbers, and they are easier to mix up. From the consumer unit to the first switch, a 3×1.5; from the second to the chandelier, a 5×1.5, as for an ordinary two-gang switch: black and brown to the groups, blue as neutral, grey spare. The back boxes need to be deep: each one takes three cables.
The interactive diagram has no two-gang two-way switch, but each of its rockers works exactly like the “In a room” option.
Three places or more
Two two-way switches control a light from two places only. For a third place and beyond, an intermediate switch is fitted between the two-way switches, one for each extra place, with the same 5×1.5 between neighbouring switches. When there are many places, an impulse relay in the consumer unit with push-buttons is simpler: you can fit as many ordinary push-buttons in parallel as you like.
Wiring diagrams
In a room: by the door and by the sofa
A 3×1.5 runs from the consumer unit to the back box by the door: brown to the common terminal L of the first two-way switch, blue and green-and-yellow to lever connectors and straight on. Between the two-way switches runs a 5×1.5: black connects terminals 1 and 1, grey connects 2 and 2, blue and green-and-yellow carry neutral and earth, and brown is left spare. From the second two-way switch to the light runs a 3×1.5, with the switched live from its common terminal L on the brown. Swapping 1 and 2 on one switch is not a mistake: either switch still changes the light over, only “on” ends up in a different combination of rocker positions.
On a staircase
The same circuit as in a room, only the consumer unit is upstairs: the 3×1.5 from the consumer unit comes to the two-way switch at the top of the stairs, a 5×1.5 runs down to the switch at the bottom, and from there a 3×1.5 goes to the light halfway up the flight. The cores are the same: black on 1, grey on 2, blue and green-and-yellow passing through, brown spare. If the light is closer to the top switch, swap their roles — supply to the bottom one, light from the top one — and the cables stay the same.
Common mistakes
L and 1 mixed up
On the two-way switch by the door, the live went to terminal 1 and the black core to the sofa went to the common terminal L. The light only comes on with one combination of switch positions and stays off with all the others: it can no longer be switched freely from both places.
Why this is wrong
There is no danger, but the switch no longer works as a two-way switch. Makers label the common terminal differently: L on Legrand, COM on UK switches, a red mark on Berker, and L next to the arrows in German diagrams. Find it from the diagram on the device: the live or the wire to the lamp goes on it, and the strappers go on the other two.
Neutral through the two-way switches
At the consumer unit the cable cores were swapped: the brown went on the neutral and the blue on the MCB. The neutral now runs through the two-way switches, while the live on the blue core passes straight through both back boxes to the light. The switches work as they should, but when the light is off, the lampholder stays live.
Why this is wrong
When you change a lamp with the light off, it is easy to get a shock, and the blue cores in both back boxes, which everyone takes for neutral, are live as well. Two-way switches, like an ordinary switch, go in the live: the brown core from the consumer unit to the MCB, the blue to the neutral bar, and at the first two-way switch the brown to the common terminal. If the colours in old wiring are unclear, find the live with a tester and mark it.
How to wire a two-way 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 both back boxes and at the light: between each other and each one to earth.
Bring the cables into the back boxes
Into the deep back box of the first two-way switch, bring the cable from the consumer unit (3×1.5) and the cable to the second switch (5×1.5); into the second, the same 5×1.5 and the cable to the light (3×1.5). Label the cables and stick to one rule at both ends of the 5×1.5: black to terminal 1, grey to 2.
Join neutral and earth
In each back box, join blue to blue and green-and-yellow to green-and-yellow with lever connectors: neutral and earth carry straight on. 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. Insulate the brown core of the 5×1.5 at both ends and leave it spare.
Connect the two-way switches
On the first switch, the brown from the consumer unit goes to the common terminal (L, COM or the one with a coloured mark), black to 1, grey to 2. On the second, black to 1, grey to 2, and the brown to the light to the common terminal.
Connect the light
Connect the brown core from the second two-way switch to terminal L of the light, the blue to N and the green-and-yellow to PE.
Check it works
Switch the MCB on and try all four combinations of switch positions: each switch must change the light over regardless of the other. If the light only comes on with one combination, the common terminal is mixed up on one of the switches. Switch the light off and check the lampholder contact with a tester: there must be no live there.
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 two-way switch?
From the consumer unit to the first two-way switch, 3×1.5 mm²; between the switches, 5×1.5: black and grey are the strappers, blue is neutral passing through, green-and-yellow is earth, brown is spare. From the second switch to the light, 3×1.5. The lighting circuit is protected by a B10 MCB.
How is a two-way switch different from an ordinary one?
An ordinary switch has two terminals, and the rocker simply breaks the live. A two-way switch has three: the rocker changes the common terminal over from one strapper terminal to the other, so two of them control one light from two places. A two-way switch can also replace an ordinary one if you connect the common and one strapper terminal.
What happens if I mix up 1 and 2 on a two-way switch?
Nothing bad: either switch still turns the light on and off, only the “on” positions change places. The real mistake is mixing up the common terminal with a strapper terminal: then the light only comes on with one combination of switch positions.
How do I control a light from three places?
Fit an intermediate switch between the two two-way switches, one for each extra place, with the same 5×1.5 between neighbouring switches. If there are many places, an impulse relay in the consumer unit with ordinary push-buttons is more convenient: you can fit as many as you like.