Wiring diagram
Correct wiring
From three places
Two-way switches by the door and by the sofa, an intermediate switch in the middle, and the cable as a chain: consumer unit → door 3×1.5, door → middle and middle → sofa 5×1.5, sofa → light 3×1.5. In the 5×1.5 cables black and grey are the strappers, blue is neutral passing through, and brown is left spare: from the door, black connects terminal 1 of the two-way switch to 1 of the intermediate, grey connects 2 to 2; towards the sofa, black runs from 3 of the intermediate to 1 of the two-way switch, grey from 4 to 2. Swapping the cores within one pair is not a mistake: the light still changes over from any place, 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 → Middle | 5×1.5 mm²green-and-yellow, blue, brown, black, grey |
| Middle → Sofa | 5×1.5 mm²green-and-yellow, blue, brown, grey, black |
| 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
- Intermediate switch1 pcs
- Frame, 1-gang3 pcs
- Deep back box, 60 mm3 pcs
- Lever connector for 2 wires6 pcs
- 3×1.5 mm² cable — one run2 pcs
- 5×1.5 mm² cable — one run2 pcs
How an intermediate switch works
An intermediate switch has four terminals in two pairs. Two strapper cores from one two-way switch arrive at the input pair, and two cores leave the output pair for the next switch. The rocker connects the pairs either straight (1 to 3, 2 to 4) or crossed (1 to 4, 2 to 3), so it swaps the strapper cores over. As a result the live arrives at the other strapper terminal of the last two-way switch, and the light changes over whatever position the other switches are in.
So the layout is always the same: two-way, intermediates, two-way. The two-way switches are in the two end positions, with as many intermediate switches between them as you like, one for each extra place: one for three places, two for four. It does not matter which pair of the intermediate switch you treat as the input: it is symmetrical. What does matter is that both cores from one two-way switch go to the same pair.
Terminal markings vary between makers. In the diagram above the input pair is marked 1 and 2, the output pair 3 and 4. On the Legrand 2 810 08 intermediate switch one pair is 1 and 3, the other 2 and 4. UK BG switches mark the terminals L1, L1, L2, L2; Dowsing & Reynolds use L1 L2 for one two-way switch and L3 L4 for the other; on the Berker 3037 the top pair goes to one two-way switch and the bottom pair to the other. So find the pairs from the diagram on the device, not from the numbers.
Wiring as a chain through back boxes
The cable runs from the consumer unit to the back box of the first two-way switch, from there to the intermediate switch, on to the next switch, and from the last two-way switch to the light. The live from the consumer unit arrives at the common terminal of the first two-way switch, passes through all the intermediate switches on the pair of strapper cores, and leaves from the common terminal of the last two-way switch on the brown to the light. Neutral and earth are not connected to the switches: in each back box they pass through on lever connectors and carry on to the light.
That way every back box has a neutral, which comes in handy if you later want a smart switch or a sensor. There are fewer joints, and you use about the same amount of cable. So that the switch and the lever connectors fit without being forced, deep 60 mm back boxes are used. 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.
What to buy and how not to confuse it with a two-way switch
For three places you need two two-way switches and one intermediate; for four, two two-way switches and two intermediates. Buy them from one range so that the frames and rockers match. From the outside an intermediate switch looks just like a two-way switch, so check the name and the terminals: a two-way switch has three, an intermediate four. Catalogues call it an intermediate switch or a crossover switch, and under IEC 60669-1 it is pattern 7 (a two-way switch is pattern 6).
You will also need a deep 60 mm back box and a frame for each place, two lever connectors per back box for neutral and earth, and cables: 3×1.5 from the consumer unit and to the light, 5×1.5 between the switches.
Cables, core colours and the MCB
From the consumer unit to the first two-way switch, and from the last two-way switch to the light, runs a 3×1.5: brown is live, blue is neutral, green-and-yellow is earth. Between the switches, a 5×1.5: black and grey are the pair of strappers, blue is neutral passing through, green-and-yellow is earth, and brown is left spare — it is not connected to anything, and its ends are insulated. Blue is neutral in every cable, so nothing needs re-marking.
Colours are set by HD 308: live is brown, black or grey, neutral is blue, and green-and-yellow is earth only — never live, neutral or a strapper. A 4×1.5 cable with earth has no blue core (brown, black, grey, green-and-yellow), but the neutral has to run along the chain to the light, so five-core cable is used between the switches. Earth is taken through every back box, even if the switch has no terminal for it: in Germany the standard requires this, and with pass-through wiring it happens anyway.
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.
Intermediate switches or an impulse relay
Each new place means another intermediate switch and another 5×1.5 cable with a pair of strappers. For three places this is simple. For four places or more, and especially in a long hallway, an impulse relay is more convenient.
The relay goes in the consumer unit, or a compact one in a back box, and retractive push-buttons are fitted instead of switches. All the push-buttons are connected in parallel, and you can fit as many as you like. A push-button needs only the live and a wire to the relay, with no pair of strappers. The limit is the length of the line to the push-buttons — up to 100 m on a Busch-Jaeger relay, for example. Ordinary switches are not connected to an impulse relay; it has to be push-buttons.
Wiring diagrams
From three places
Two-way switches by the door and by the sofa, an intermediate switch in the middle, and the cable as a chain: consumer unit → door 3×1.5, door → middle and middle → sofa 5×1.5, sofa → light 3×1.5. In the 5×1.5 cables black and grey are the strappers, blue is neutral passing through, and brown is left spare: from the door, black connects terminal 1 of the two-way switch to 1 of the intermediate, grey connects 2 to 2; towards the sofa, black runs from 3 of the intermediate to 1 of the two-way switch, grey from 4 to 2. Swapping the cores within one pair is not a mistake: the light still changes over from any place, only “on” ends up in a different combination of rocker positions.
From four places, staircase
A staircase in a house: two-way switches by the front door and in the upstairs hallway, intermediate switches at the bottom and top of the stairs, the consumer unit by the door, the light above the stairs. The cable as a chain: consumer unit → door 3×1.5, then door → bottom → top → hallway, each on 5×1.5 (black and grey are the strappers, blue is neutral, brown is spare), hallway → light 3×1.5. In the diagram, between the intermediate switches and at the hallway the cores of a pair cross over, for example black from 3 of the bottom switch to 2 of the top one: this suits the layout better and is also correct — all that matters is that the whole pair runs from one switch to the next. Each further place means another intermediate switch and another 5×1.5 cable, so from four places on it is worth comparing this circuit with an impulse relay.
Common mistakes
Two-way switch instead of an intermediate
A two-way switch was fitted in the middle: it has three terminals, but four strapper cores arrive. It was wired up as best it could be: the grey from the door to the common terminal L, the black and grey going to the sofa to 1 and 2, while the black from the door was left unconnected. While the switch by the door is in one position, the light does not come on at all. In the other position the light is controlled only from the middle and the sofa, like an ordinary pair of two-way switches.
Why this is wrong
There is no danger, but the light cannot be controlled from three places. Two-way and intermediate switches look the same from the front; they differ in name, number of terminals and the diagram on the device. Only an intermediate switch goes in the middle: it has four terminals, a pair for the strapper cores of each neighbouring switch. Two-way switches go only in the two end positions.
Terminal pairs mixed up
On the intermediate switch the pairs got mixed: from the door, black went to 1 and grey to 3; towards the sofa, black from 2 and grey from 4 — although on this switch the input pair is 1 and 2 and the output pair is 3 and 4. In one position the intermediate switch connects the two door strappers to each other and, separately, the two sofa strappers: the live never reaches the sofa, and no switch can turn the light on. In the other position the door and the sofa change the light over like a pair of two-way switches, but it cannot be controlled from all three places.
Why this is wrong
There is no danger, but the light does not work from every place. Both cores from one two-way switch must sit in one terminal pair of the intermediate switch, and the cores from the other in the other pair. Which terminals form a pair depends on the maker: in the diagram above it is 1 and 2, but on the Legrand 2 810 08 intermediate switch the pairs are 1 and 3, and that is the correct way there. So do not go by the numbers: find the pairs from the diagram on the device.
How to wire an intermediate 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 all the back boxes and at the light: between each other and each one to earth.
Bring in the cables and mark the cores
Two cables come into each back box: at the first two-way switch, from the consumer unit and to the intermediate; at the intermediate, from the previous switch and to the next; at the last two-way switch, from the intermediate and to the light. Label which cable comes from where. In the 5×1.5 cables, black and grey are the pair of strappers: note which pair goes to which neighbouring switch.
Join neutral and earth to pass through
In each back box, join the blues of both cables with one lever connector and the green-and-yellows with another. Neutral and earth are not connected to the switches. Leave the spare brown in the 5×1.5 cables free and insulate its ends.
Connect the two-way switches and the light
On the first two-way switch, the brown from the consumer unit goes to the common terminal (L, COM or the one with a coloured mark), the black to the intermediate to 1, the grey to 2. On the last two-way switch, the black and grey from the intermediate go to 1 and 2, and the brown from the common terminal goes to the light. At the light: L — brown, N — blue, PE — green-and-yellow.
Connect the intermediate switches
Find the two terminal pairs from the diagram on the device. Put the black and grey from the previous switch on one pair, and the black and grey to the next switch on the other; in the diagram above these are 1, 2 and 3, 4. The order of the cores within a pair does not matter; what matters is that the whole pair belongs to one neighbouring switch.
Check it works
Switch the MCB on and go round all the places several times, flipping one switch at a time: every flip in any place must change the light over. If the light does not come on at all in one position of an intermediate switch, its pairs are mixed up. 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
How do I control a light from three places?
Fit two-way switches in the two end positions and an intermediate switch in the middle. The cable runs as a chain: from the consumer unit to the first two-way switch 3×1.5 mm², between the switches 5×1.5, from the last two-way switch to the light 3×1.5. The cores from the first two-way switch go to one terminal pair of the intermediate, and the cores to the second leave from the other. The lighting circuit is protected by a B10 MCB.
How is an intermediate switch different from a two-way switch?
A two-way switch has three terminals: the rocker changes the common terminal over from one strapper terminal to the other. An intermediate has four, in two pairs: the rocker connects them straight or crossed. So the end positions always have two-way switches, and intermediates go only between them.
What cable do I need for an intermediate switch?
To and from the intermediate switch, 5×1.5 mm²: black and grey are the strappers, blue is neutral passing through to the light, green-and-yellow is earth, brown is spare. A 4×1.5 cable is not enough: it has no blue core, and the green-and-yellow must not be used as a strapper or neutral.
How many intermediate switches can go between two two-way switches?
As many as you like, one for each extra place. But each one adds a back box and a 5×1.5 cable. From four places on, and in long hallways, an impulse relay is more convenient: push-buttons are connected to it in parallel, as many as you need.