A Guide to Two-Way Switching

Two-way switches are used to operate a single component (usually a light bulb) with two different switches. You’ve definitely encountered two-way switches many times in your life. Typical examples are:

• The landing light being switchable whether you’re upstairs or downstairs
• A light switch in your bedroom by the door and another one by the bed (hotels often have these, or you may have a switch cord coming from the ceiling)
• Long hallways with two entrances, so you can turn the lights on and off whichever entrance you use

With a two-way switch, flicking either switch changes the state of the light, regardless of where the other one is. You can’t have “on” or “off” makers (e.g. writing or red markings) on a two-way switch, because it can be on or off in either position.

Let’s have a look at how they work, and what to do when changing a two-way light switch.

A note on usage

Just a quick note – in Europe (including the UK), we call this kind of switch a two-way, but in the United States, this is called a three-way switch. The European version refers to the fact that you can divert the current along two different routes, whereas the US version refers to the three terminals on each switch. Since we’re UK-based, we’ll stick with “two-way”, but if you’re looking for advice, you should note which country the adviser is from.

How a two-way switch works

If you understand how a simple lighting circuit works, you’ll know that you need a negative terminal (think of a battery), which links to a wire, which links to a bulb, then a wire, then back to the positive terminal. You can insert a switch in one of the wires, which will join or break the circuit to turn the light on or off.

In a two-way switch system, a switch doesn’t simply break the circuit, it moves it from one terminal to another. Whether or not that move creates or breaks the circuit depends on which terminal the other switch is touching. What is guaranteed is that whichever switch you toggle, you will switch the light to its opposite state.

Here’s a basic diagram of a two-way switch:

As you can see, there’s one wire coming in and two wires going out. The grey line represents the switch itself, and it can move between L1 and L2. That shows how the circuit can be switched from one terminal to the other.

Wiring a two-way switch

There are actually several ways to wire a two-way switch.

The simplest way is where the Live wire goes from the consumer unit to the Common terminal of switch A, and the Neutral wire goes to the Common terminal of switch B, then to the bulb and on to the consumer unit. Then, two wires (assumed to be live) run (i) from L1 on switch A to L1 B, and (ii) from L2 on A to L2 on B. Now, if both switches are connected to L1 or both to L2, the light will be on, but if one is on L1 and the other is on L2 (it doesn’t matter which is which), it will be off. The lamp cannot be connected to either of the L→L terminals 

Another way is to run a Live wire to both L1s and a Neutral wire to both L2s, then run a single wire between the two Common terminals. The lamp should be on the Neutral wire, between the consumer unit and where the wire splits to meet the switches. Now, if both switches are in the same position (both L1 or both L2), the light will be off, as all that will be happening is that the Live or Neutral wires will be looping back on themselves – i.e. no circuit. If one is on L1 and the other is on L2, the circuit will be completed, as both a Live and a Neutral terminal are connected, and the two common terminals are connecting them together.

There are other ways of wiring a two-way switch but they are a bit more complicated. They all essentially do the same thing, though – they cause the circuit to change its state from on to off, or from off to on, depending on what it currently is.

Here’s a video that explains it better than words! 

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Wiring your own two-way switch?

If you’re wiring a two-way switch from scratch, we cannot emphasise this enough: this is a job for a professional electrician. With so many criss-crossing wires, there is a lot that can possibly go wrong here, and only a qualified, professional electrician with the correct knowledge, tools and safety gear should tackle this job. In fact, you may even be breaking the law if you try to wire it yourself. This isn’t just about your safety – it’s that of your friends and family, as well as visitors and any future inhabitants of your home.

If there’s an existing two-way switch, we still would not recommend a complete amateur should tackle it, for the same reasons. Don’t forget, two-way switches are often also double switches (i.e. there’s a two-way for the landing and a regular switch for the hall, all housed in the same  unit) or even triple. That adds up to a lot of wires to mix up.

If you are confident and competent, have a look at our guide to changing a light switch and follow the procedure, taking great care to make sure the power to the whole property is off, and that you have made sure there’s no current in the circuits before you start. Remember that upstairs and downstairs lighting are often on different circuits, yet an upstairs/downstairs switch could be connected to both. Pay very close attention to which terminals the wires are connected to, and if two wires are connected to each other, make sure they stay connected.

Find beautiful two-way switches at ACAS

If you’re a pro electrician, you’re probably not reading this, but if you’re a homeowner or interior designer looking for the complete range of high quality two-way switches, look no further.