4-way switch wiring diagram

Light: on

Toggle any of the three switches. The 4-way in the middle either runs the travellers straight through or crosses them — that is the whole trick. Tap a conductor to replay the current path. The copper highlight is the live path, not a wire colour.

A 4-way circuit controls one light from three or more places. It keeps the two 3-way switches at the ends and drops a 4-way (intermediate) switch between the travellers. The 4-way either passes the travellers straight through or crosses them, so throwing it flips the light like any other switch.

Trace it yourself

Hot from the supply to the common of switch 1. Its blade puts the hot on one of the two travellers — say the upper one.
The travellers run into the 4-way. Straight, the upper traveller comes out on the upper side; crossed, it comes out on the lower. Follow the live one through and note which side it leaves on.
At switch 2, its blade is listening to one traveller. Same traveller the live one is on? The common of switch 2 is live and the lamp is on. Different? Dead end, light off.
Now flip only the 4-way. The live traveller swaps sides between the 4-way and switch 2, so whatever switch 2 was hearing, it now hears the opposite. That is why the middle switch toggles the light without ever touching a common.
Add a second 4-way in your head, between the first and switch 2. Same rule: straight or crossed. Any number of them in a row, any one of them flips the light.

How this circuit works

Start with a plain 3-way: two switches, two travellers, light on when both ends agree on a traveller. Now cut the travellers in the middle and insert the 4-way. In its straight position it reconnects them as they were, so the circuit behaves exactly like the 3-way. In its crossed position it swaps the two travellers, which is the same as flipping one of the end switches.

That is the whole device: a pair of travellers in, a pair out, and a blade that chooses straight or crossed. Toggle the 4-way above and watch the copper path jump from one traveller to the other. Because every switch on the run either picks a traveller or swaps the pair, every switch changes whether the path completes — which is why any of them turns the light on or off.

Need six-way control? There is no such switch. You add another 4-way. Every additional location between the ends is one more intermediate switch on the travellers, and the rule above never changes.

What goes wrong

The 4-way's pairs mixed up

Symptom: the light works from the two end switches but the 4-way does nothing — or it works from the 4-way and one end only.

A 4-way has two pairs of terminals: one pair takes the travellers coming in, the other pair sends them out. Land one incoming traveller and one outgoing traveller on the same pair and the switch is no longer passing or crossing anything — in one position it shorts two travellers together, in the other it opens one of them. The end switches may still appear to work, which is how the fault survives to the walk-through. Which screws are a pair is marked on the device; the drawing above shows the pairs as the left side and the right side of the middle box.

En la obra

A 4-way is a 3-way with a switch dropped into the travellers. Everything on the 3-way page still applies; what is new is one device with four screws in pairs, and one more box to keep straight.

Common mistakes

Landing across the pairs. The four screws are two pairs: travellers in on one, travellers out on the other. Put an incoming and an outgoing on the same pair and the fault above appears — and the end switches may still seem to work.
Putting a 3-way in the middle. A 3-way has three screws and a common; a 4-way has four and no common. If you find yourself looking for the odd screw on the middle switch, you have the wrong device.
Using a 4-way at an end. The ends are always 3-ways. A 4-way has nowhere to land a hot or a lamp feed.
Losing track of which traveller pair is which once two cables enter the middle box. Both travellers from the first 3-way go to one pair; both travellers to the last 3-way come from the other.
Assuming a neutral in the middle box. Two traveller pairs, a bond, and nothing else unless the book or the device asks for one.

What the inspector looks for

The right device in the right box: 3-ways at the ends, 4-ways between.
That it is the hot being switched — 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.
Each traveller pair in one cable, and the cable protected and supported the way the book says for the wiring method.
Re-identification on any conductor that looks like a neutral and carries a hot, the way the book calls for.
Bonds on every strap and every box, and box fill within what the book allows — the middle box holds two cables' worth.

Check it before you call it done

Breaker off. Live-dead-live in every switch box and at the fixture.
On the 4-way, continuity: in one handle position each screw on the left pair connects straight across to the screw beside it; in the other, they cross. The device is marked which screws are a pair — read the marking, then confirm with the meter.
Map the commons on both 3-ways the way the 3-way page describes. Hot to one common, lamp feed to the other.
Energise and work every handle in turn from every starting position. Every flip of any of the three switches must change the lamp. One switch that only works "sometimes" is the crossed-pair fault.
Light off: read from a lamp terminal to the bond, not across the lamp — nothing means the hot is what the switches break. Close to full supply means the switching landed in the neutral and the fixture is live with every handle off. Then covers on.

Preguntas frecuentes

What does the 4-way switch actually do?

It sits between the two travellers and either passes them straight through or swaps them over. That is its only job. Because it can reverse the two travellers, throwing it changes which traveller is live at the far switch, which flips the light — exactly like the end switches do.

How do I control a light from four or five places?

Keep the two 3-way (SPDT) switches at the ends, and add one 4-way (intermediate) switch between the travellers for every extra location. Two ends plus three 4-ways gives you five switches on one light. The rule never changes: the ends pick a traveller, each 4-way passes or crosses.

Why does every switch still toggle the light?

Each switch either changes which traveller the end switches are using, or swaps the pair in the middle. Any of those changes flips whether the path is complete, so any switch flips the light regardless of the others.

What is the difference between a 3-way and a 4-way switch?

A 3-way (SPDT) has three terminals and always sits at an end of the run. A 4-way (intermediate) has four terminals and always sits in the middle, between travellers. You never use a 4-way on its own — it only makes sense between two 3-ways.

Ver también

3-way switch wiring diagram — start here; the 4-way is built on top of it.
Glossary: traveller
Common wiring devices — the 4-way's four screws in pairs, beside the 3-way's three.
All wiring diagrams

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