Two-way switch wiring, explained with a simulator
How a two-way (three-way) switch pair controls one light from two places, what an intermediate (four-way) switch adds for a third place, how each is drawn on the plan, and a simulator to flick.
A two-way switch has three terminals: a common and two others. Two of them, joined by a pair of wires called travellers or strappers, control one light from two places, a stair light from the hall and the landing say. The lamp is on when both levers select the same traveller, and either switch changes it. In the US the same device is a three-way switch; the count is of terminals there and of positions in Europe. The wiring is identical.
Try it
Tap the switches. In the first panel one switch does the job. In the second, two two-way switches share the lamp, and the live traveller is shown in orange. In the third, an intermediate switch between them crosses the travellers or not, which is how a third place joins in.
How the pair works
The supply arrives at the common terminal of the first switch. Its lever connects the common to one of the two travellers. The travellers run to the second switch, whose lever connects one of them to its own common, and that common feeds the lamp.
So the lamp is on only when both levers point at the same traveller. Flip either switch and the levers disagree, and the lamp goes off. Flip either again and they agree, on the other traveller this time, and the lamp is back on. There is no up or down that means on: the position of one switch means nothing without the other.
Three places: the intermediate switch
For a third place, a long landing or a hall with three doors, an intermediate switch goes in the middle of the travellers. It has four terminals and two positions: straight, where each traveller passes through, and crossed, where they swap. Either position still lets exactly one traveller reach the second switch, so the pair keeps working, and the intermediate adds a third way to change which one.
Any number of intermediates can sit between the pair. In the US the intermediate is a four-way switch, S4 on the plan; between two three-ways it does the same job.
How it is drawn on the plan
The plan does not show the travellers. It shows the light and the switches that control it, with a line from each switch to the next and to the light, so the reader knows the three belong together. The IEC mark for a two-way switch is a circle with two strokes , and the intermediate has a crossbar . US drawings write S3 and S4 .
The wiring diagram, the drawing with the terminals and the travellers, is a separate sheet, and the residential plan article explains the difference. A plan that draws a two-way pair as two one-way switches, each to the light, is wrong in a way a quick check catches: with two one-way switches in series neither works alone, and in parallel neither can turn the light off. The checking article has the switch simulation that shows it.
The words
| The thing | Europe (IEC) | North America |
|---|---|---|
| A light switched from one place | one-way switch | single-pole switch, S |
| One of a pair switching from two places | two-way switch | three-way switch, S3 |
| The switch between the pair, for a third place | intermediate switch | four-way switch, S4 |
| The two wires between the pair | strappers, travellers | travelers |
| The terminal the supply or the lamp connects to | common (COM) | common |
Common mistakes
- Two one-way switches drawn for a two-way job. They cannot do it. The plan should show two two-way marks wired to each other.
- A dimmer in a two-way pair. An ordinary dimmer is a one-way device. A two-way dimmer exists, but it is a different product, and the plan should say so.
- Only one traveller. With one wire between the switches, one switch can only ever break the circuit, not restore it. The pair needs two.
- The intermediate at the end. It belongs between the two two-way switches, never at the end of the chain.
Questions
Can I use a two-way switch as a one-way switch?
Yes. Leave one of the two travellers' terminals unused and it behaves as a one-way switch. Electricians often fit two-way switches everywhere for that reason.
Does a two-way switch need special cable?
It needs one more conductor between the switches than a one-way switch does, for the second traveller. Which cable, and how it is earthed and identified, is a matter for your regulations and a qualified person; this article explains the principle, not the installation.
How many switches can control one light?
Two two-way switches and as many intermediates between them as you need. Three places is common on a landing, four in a large open room.
Is a three-way switch the same as a two-way switch?
Yes. It is the same device with a different name on each side of the Atlantic. The IEC vs US article has the rest of the vocabulary.