How to Wire a Two Way Light Switch
Updated: Oct 9
A two way switch has three terminals, COM, L1 and L2, and is linked to the second switch by two strappers, so one light can be controlled from two positions. You isolate the lighting circuit, prove it dead, connect the feed to COM at one switch and the switched live to COM at the other, and run the two strappers between the L1 and L2 terminals. Older installations vary, so trace and test every core before changing anything.
We always advise caution with this kind of job. In England, electrical work in dwellings must satisfy Part P of the Building Regulations, and the wider installation work should comply with BS 7671. Part P guidance also distinguishes between non-notifiable maintenance or alterations on existing circuits outside special locations, and notifiable work such as installing a new circuit, replacing a consumer unit, or altering a circuit in a special location like a bathroom.
What a two way light switch actually does
A two way switch does not simply turn a light on and off in the same way as a standard one way switch. It is designed so the circuit can be changed from either switch position, which is why it is commonly used on stairs, landings, hallways and larger rooms with more than one entrance. On the device itself, manufacturers typically mark the terminals as C for common, L1 and L2, and product data for UK wiring accessories notes that two way switches can be wired either one way or two way using those terminal markings.
In practice, that sounds straightforward, but real properties are rarely that simple. Older homes can contain mixed cable colours, borrowed neutrals, undocumented alterations, or previous repairs that do not match what the front plate suggests. That is one reason we recommend testing and inspection rather than relying on colour alone or copying a diagram from a different setup. Older properties often contain a mix of old and updated cable colours, which our guide to what colour the live wire is explains, so identification and testing matter more than assumptions.
First: isolate the circuit and prove it dead
Before any switch plate comes off, switch off the lighting circuit at the consumer unit and lock it off (or, if it cannot be locked, label it and keep the consumer unit closed) so nobody switches it back on. Then prove it dead at both switch positions with an approved two-pole voltage indicator, testing the indicator on a known live source before and after. A non-contact tester pen is not enough. Two way circuits can pick up a feed from more than one place, so test every conductor in both boxes.
Replacing a two way switch like-for-like, or converting a one way light to two way on an existing lighting circuit outside the zones around a bath or shower, is not notifiable under Part P. Approved Document P still expects it to be done to BS 7671, inspected and tested. Any new circuit, or any addition or alteration within those zones, is notifiable and must be done by a registered electrician who can self-certify it, or notified to building control before work starts.
Two way switch terminals: COM, L1 and L2
Every two way switch has three terminals per switch position:
COM (common, sometimes C): the terminal the rocker always connects to. At one switch it takes the feed; at the other it sends the switched live to the light.
L1 and L2: the two traveller terminals (also called strappers). The rocker connects COM to either L1 or L2, depending on which way it is flicked.
Markings vary between brands (the common may be COM, C or L), so always check the diagram on the back of the switch rather than assume. A two way switch also works as an ordinary one way switch: connect the switch cable to COM and L1 and leave L2 empty. That is why most UK switches are two way even when used one way.
Is a 3-way switch the same as a two-way switch?
Yes. "3-way switch" is the American name for what the UK calls a two way switch: one light, two switch positions. In the UK, "three way" usually means a single plate with three separate switches (a 3 gang switch), which is a different thing again.
How to install a two way light switch, step by step
Isolate the lighting circuit at the consumer unit, lock it off and prove it dead at both switch positions.
Fit or confirm the cables: a twin and earth switch drop from the ceiling rose to the first switch, and a 3 core and earth between the two switches.
At switch one, connect the permanent feed to COM and the two strappers to L1 and L2.
At switch two, connect the same strappers to L1 and L2 and the switched live core to COM.
Join the switched live back to the rose, sleeve every core that carries line voltage brown and every earth green/yellow, and connect the earths.
Fix the switches, restore the power and test continuity of the earth, insulation resistance and polarity, then check the light works from both positions.
Each connection is set out in detail below.
Wiring a two way switch with 3 core and earth
The conventional UK method uses 3 core and earth cable between the two switches. In modern cable the cores are brown, black and grey, plus a bare earth (CPC). In cable installed before the 2004 colour change they are red, yellow and blue. All three coloured cores carry line voltage in this circuit (none of them is a neutral), so black and grey (or yellow and blue in old cable) should be sleeved or taped brown at each end, and the bare earth sleeved green/yellow.
Here is the conventional layout, described as a wiring diagram:
At the ceiling rose: a twin and earth switch cable drops from the loop-in rose to the first switch. Its brown core is the permanent feed from the rose's loop terminal; its blue core returns the switched live to the light and should be sleeved brown at both ends.
At switch one: the brown feed from the twin goes to COM. The black core of the 3 core goes to L1 and the grey core to L2.
Between the switches: the 3 core runs from switch one to switch two.
At switch two: black to L1, grey to L2, and the brown core of the 3 core to COM. This brown now carries the switched live.
Back at switch one: the brown core of the 3 core is joined to the blue (brown-sleeved) core of the twin, in a connector inside the back box or on a spare "loop" terminal if the switch has one. That sends the switched live back to the rose and the light.
Earths: every CPC is sleeved green/yellow and connected to the earth terminal of both switches, and to the back box if it is metal.
Whichever core colours you use between the switches, keep the travellers on L1 and L2 at both ends and the COM connections on COM. Swapping L1 and L2 at one switch only reverses which way the rocker is "on"; moving a traveller onto COM is what breaks the circuit. If you find a different layout (some installers make the joins at the ceiling rose or a junction box rather than in the switch box), trace and test every core before changing anything, and do not mix the two approaches.
The switch feed from the ceiling rose
In a loop-in ceiling rose, the switch cable's brown goes to the loop (permanent live) terminal block with the other circuit browns, and its sleeved blue goes to the switched-live terminal with the brown of the pendant flex. If the blue core of your switch cable is not sleeved brown, sleeve it: it is live whenever the light is on.
Converting a one way switch to two way
Turning a one way light into a two way light needs one new cable: a 3 core and earth run from the existing switch position to the new second position, usually chased into the wall. A cable buried less than 50 mm deep in a wall must run in a safe zone (straight up, down or across from an accessory, or within 150 mm of the top of the wall or a corner) and must also have 30 mA RCD protection unless it is mechanically protected, for example in earthed metal conduit (BS 7671 regulations 522.6.202 and 522.6.203). Swap the existing one way switch (wired as in our guide to how to wire a one-way light switch) for a two way switch, then wire both as described above. The twin cable from the rose stays where it is.
Because this is an addition to an existing lighting circuit, it is not notifiable outside the zones around a bath or shower, but it must be tested once it is done (continuity of the CPC, insulation resistance and polarity at a minimum). If the lighting circuit has no RCD protection, that needs sorting first, and adding RCD protection usually means consumer unit work for a registered electrician.
Adding a third switch: intermediate switching
To control one light from three or more places, an intermediate switch goes between the two two way switches, on the traveller cores. It has four terminals, and flicking it crosses the two travellers over. You can add as many intermediates as you like, one per extra position. Our guide to intermediate switch wiring covers the terminals and cable runs step by step.
Two way switch not working?
Most faults are a mix-up at the terminals:
The light only works when the other switch is in one position: the COM and a traveller have been swapped at one switch. Find the core that goes to COM and move it.
One switch does nothing at all: a loose or missed traveller, or the switch has been wired as one way (L2 empty).
The breaker trips when a switch is flicked: a traveller is touching earth or neutral, or a core has been damaged when the plate was pushed back.
Dimmer or smart switch will not work two way: only use dimmers and smart switches rated for two way use. Many need a matching secondary switch, and many smart switches need a neutral, which most UK switch boxes do not have.
If the wiring in the box does not match either method above, stop there and get it traced.
Why this is not a job to guess your way through
The biggest risk with two way switching is assuming every switch drop is wired the same way. It is not, and we see the results of guesswork: nuisance tripping, loose terminations, failed testing and lighting circuits that behave unpredictably. That is why our lighting installation and repairs service is built around safe diagnosis and proper installation rather than quick guesses. We are registered with a Competent Person Scheme and work to BS 7671.
When a two way light switch becomes part of a bigger issue
Sometimes the switch itself is not the real problem. Flickering, buzzing, burning smells, tripping, or an installation that has not been updated for decades are all signs the wider lighting circuit needs checking.
The same applies if the consumer unit is older and lacks modern protection, because older fuse boards often have no RCD. If there is any doubt about the condition of the system, an EICR report is often the sensible next step before more changes are made.
Two way switch questions, answered
What cable do you need for a two way switch?
A 3 core and earth cable between the two switches (brown, black and grey cores plus a bare earth in modern cable), and the existing twin and earth switch drop from the ceiling rose to the first switch.
Can any light switch be used as a two way switch?
Most UK light switches are two way. Used one way, they take the switch cable on COM and L1 with L2 left empty, so check the terminals on the back before you buy a new one.
Do I need an electrician to add a second switch?
Converting a one way light to two way on an existing lighting circuit outside the zones around a bath or shower is not notifiable, but it must be done to BS 7671 and tested. A new circuit, or work within those zones, is notifiable.
Getting two way lighting fitted
Two way switching depends on the exact circuit in front of you, and the right process starts with safe isolation, proper identification, testing and installation to BS 7671, not trial and error. If you would rather hand it over, we fit and repair two way and intermediate lighting across Shifnal, Wolverhampton, Telford and the wider West Midlands, and test and certify the work when it is done.
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