Diagrama de cableado de un interruptor de 3 vías

Light: on

Flip either switch up or down and watch the light and the energized path change with it. Tap a conductor to replay the current path. The copper highlight is the live path, not a wire colour.

A 3-way circuit uses two switches to control one light from two places. Each switch is single-pole, double-throw: a single common terminal that connects to one of two travellers. The light is on whenever the two switches select the same traveller, so flipping either one changes the light.

Trace it yourself

Do this with a finger on the drawing before you touch a button, then flip one switch and do it again. Both switches start up.

Find the hot at the supply and follow it to the common of switch 1 — the terminal now labelled on the drawing.
Look at the blade of switch 1. Up, it lies on the upper traveller; down, the lower. That traveller is now live all the way across to switch 2. The other traveller is dead.
At switch 2, look at its blade. It is listening to one traveller — the one it lies on. Is that the live one?
If yes, the common of switch 2 is live: that is the switched hot, and it feeds the lamp. Cross the lamp and follow the neutral home. Light on.
If no, the path dead-ends at switch 2's open side. Notice the live traveller is still copper right up to that terminal — the circuit is broken at the switch, not dead at the panel. Light off.
Now say out loud what one flip of either switch will do, and use the predict-then-check line under the switches to find out.

How this circuit works

Follow the hot from the supply. It lands on the common of the first switch. From there it takes one of the two travellers across to the second switch, depending on which way that first switch is thrown. At the second switch, the common is the switched hot that feeds the lamp; the lamp's other side returns on the neutral.

So the whole question is: does the traveller the first switch chose line up with the traveller the second switch is listening to? If yes, there is an unbroken path from hot to lamp and the light is on. If no, the path dead-ends at an open blade and the light is off. That is why neither switch has a fixed "on" position — each one just changes which traveller is live, and the light follows whether the two agree.

Toggle the switches above and watch the copper trace. When the light is off, notice that the hot side is still energized right up to the open blade — the circuit is broken at the switch, not dead at the panel. That is the safety lesson the flat pictures never show you.

The other way to wire it

This diagram feeds the supply in at the first switch. The common alternative brings the supply and neutral to the light fixture and drops a switch loop to the switches. The switching topology is identical — two commons, two travellers — but the conductors carrying the supply and the neutral live in different places. Which arrangement is used, and the conductor colours that identify each one, are set by code, not by the circuit.

What goes wrong

Two mistakes account for most 3-way call-backs, and both are diagnosable from the symptom alone once you understand the drawing above.

1. A traveller landed on the common

Symptom: the light only works from one switch, and only while the other switch is in one particular position.

Below, switch 2 has traveller 1 on its common screw and the lamp feed on a traveller screw. Work the switches and find the pattern first. Then read the answer.

Light: on

Work the switches and find the pattern before you read the answer below. The copper highlight is the live path, not a wire colour.

Why it behaves like that

The lamp is only fed when switch 2's blade lies on the screw the lamp feed is on — its "up" position — and traveller 1 is live, which needs switch 1 up too. Switch 1 down puts the hot on traveller 2, which now goes nowhere useful: switch 2 down merely joins the two travellers to each other. So the light is on in exactly one of the four combinations. On the job: the symptom "works from one switch only" points straight at a common screw with the wrong wire on it. The common is the odd-coloured screw on the device; the two travellers are interchangeable, the common never is.

2. The two commons confused for each other

Symptom: the light is on in three positions and off in one — or off in three and on in one.

The incoming hot and the lamp feed both belong on commons, one at each switch. Put the hot on a traveller screw at switch 1 instead and the first switch no longer chooses between travellers — it either connects the hot through or does not — so the symmetry that makes a 3-way work is gone. You do not need a diagram for this one: the fix is the same rule as above. Hot to one common, lamp to the other, travellers between the rest.

En la obra

Every 3-way that comes back to bite you is a common screw with the wrong conductor on it. The device tells you which screw is the common — it is the odd-coloured one — and the meter tells you which conductor is which. Trust those two, not the colour of the insulation.

Common mistakes

A traveller on the common screw. The fault drawn above. Works from one switch, and only sometimes from the other. The common is the odd screw; the two traveller screws are interchangeable, the common never is.
Grabbing a single-pole switch by mistake. A 3-way has no ON/OFF printed on the handle, because it has no on position. A switch with ON/OFF moulded into it is a single-pole, and it has two screws, not three.
Splitting the travellers between two cables. Both travellers ride the same cable between the switches. Send one down a different route and the loop the book wants kept together is broken.
Mistaking the returning travellers for a hot and a neutral at the second box — they are two switched hots. There is no neutral in that pair.
Forgetting the neutral for a smart or dimming 3-way. Many need one at the box; the classic loop does not supply it. The device's own diagram says which.

What the inspector looks for

That it is the hot being switched, not the neutral — with the light off, a lamp terminal to the bond reads nothing if the hot is switched, and close to full supply if the neutral is.
What is in each switch box. The book says which conductors must be present; the inspector reads the box, not your wiring diagram.
How the conductors that look like neutrals but carry a hot are re-identified — the marking the book calls for, at both ends.
Both travellers in the same cable, and the cable protected and supported the way the book says for the method you used.
The bond on both switch straps and both boxes.

Check it before you call it done

Breaker off. Live-dead-live in both switch boxes and at the fixture. Three boxes carry line voltage in this circuit, not one.
Map the common on each switch with a continuity check before you land anything: the common shows continuity to one traveller screw with the handle up and to the other with it down. The two traveller screws never read continuity to each other.
Land the incoming hot on switch 1's common, the lamp feed on switch 2's common, and the travellers across the remaining pairs. Mark what needs marking now.
Energise and run all four combinations: both up, both down, one up one down, the other way round. The lamp must change state on every single flip of either handle. If any flip does nothing, a traveller is on a common.
Light off: read from a lamp terminal to the bond. Nothing means the hot is the conductor being switched. Close to full supply means the switching is in the neutral — the lamp is sitting live with the switch off, and it has to be rewired. Measure to the bond, not across the lamp: with the lamp in place and off, across the lamp reads about nothing either way, because no current flows and the filament ties both terminals to the same potential. Only then, covers on.

Preguntas frecuentes

Why is it called a 3-way switch if there are only two of them?

The name counts the terminals, not the switches. Each switch has three terminals — one common and two travellers — so the trade calls it a 3-way. You always use two of them to control one light from two places.

What are the traveller conductors?

The two conductors that run between the two switches. Each switch connects its common to one traveller or the other; when both switches happen to pick the same traveller, the circuit is complete and the light is on. Toggle either switch and it lands on the other traveller, breaking or making the path.

Why does either switch turn the light on or off?

Because neither switch has a fixed on or off position. The light state depends on whether the two switches agree on a traveller. Flipping one switch always changes that agreement, so it always flips the light — no matter what the other switch is doing.

Is there more than one way to wire a 3-way?

Yes. The common alternative feeds the supply in at the light fixture instead of at the first switch, and runs a switch loop down. The topology of the switching is the same — two commons, two travellers — but where the supply and neutral live changes. This diagram shows the supply-at-the-switch arrangement.

Ver también

4-way switch wiring diagram — control a light from three or more places.
Glossary: traveller — the conductor that does the work here.
Common wiring devices — how to tell a 3-way from a single-pole in your hand.
All wiring diagrams

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