Switch loop wiring diagram

Light: on

Both conductors in the drop to the switch are in the hot family — one down, one back up — and there is no neutral in that box. Flip the switch and watch the returning conductor go live. The copper highlight is the live path, not a wire colour.

A switch loop controls a light when the supply arrives at the fixture rather than the switch. A two-wire cable drops to the switch: one conductor carries the feed hot down, the switch makes or breaks it, and the other conductor carries the switched hot back up to the lamp. The neutral stays at the fixture.

Trace it yourself

Start at the supply. Both the hot and the neutral arrive at the light box — the box at the top, not the switch box.
Follow the hot into the light box. It does not go to the lamp. It turns down one leg of the loop and drops to the switch's common.
Look at the blade. Closed, the hot comes straight back up the other leg of the loop as the switched hot. Open, that second leg is dead.
The returning leg lands on the lamp's top terminal. Cross the lamp.
The lamp's other terminal returns on the neutral that never left the light box. Now look at the switch box: name every conductor in it. There is no neutral there — both are the hot side of the same loop.

How this circuit works

Trace the hot from the fixture box. It runs down one leg of the loop to the switch's common. If the switch is closed, it comes straight back up the other leg as the switched hot and feeds the lamp; the lamp returns on the neutral that never left the fixture. Open the switch and the returning leg goes dead — watch the copper drain out of it in the diagram.

The thing to carry away is that both conductors in that drop belong to the hot side of the circuit. There is no neutral in the switch box at all. That surprises people the first time, and it is the whole reason the arrangement has its own name.

What goes wrong

The loop mistaken for a hot and a neutral

Symptom: a "neutral" in the switch box that reads line voltage — or a device that needs a neutral, fitted in a box that has none.

Two conductors in a box look like a hot and a neutral because that is what two conductors usually are. In a switch loop they are the hot going down and the hot coming back. Treat the second one as a neutral and you will land it somewhere a neutral belongs, put line voltage where it does not belong, and wonder why the breaker went. The habit that prevents it is the one this whole site drills: trace the circuit, name each conductor by its job, and let the meter confirm. How the returning conductor must be identified in the box is a code rule; the physics of why it is not a neutral is above.

Sur le chantier

A switch loop is where habit gets people. Two conductors in a box, one of them wearing the colour a neutral usually wears, and no neutral anywhere near. Everything below comes back to naming each conductor by its job before you land it.

Common mistakes

Treating the returning hot as a neutral. It goes down to the switch as a hot and comes back as a switched hot. Land it where a neutral belongs at the fixture and you have put line voltage on the neutral side of every lamp on that box.
Not re-identifying it. The book says the conductor that looks like a neutral but is used as a hot must be marked. Skipping the marking is the finding that gets written up most on these.
Breaking the wrong conductor at the switch. The switch belongs between the incoming hot and the switched hot. Put the fixture's neutral through it and the light "works" while the fixture stays live.
Assuming there is a neutral for the smart switch. There is not, unless one was run. A powered switch or sensor that needs a neutral cannot live in a classic loop box — check its own diagram before you buy it.
Leaving the fixture box overfilled. The loop puts extra splices in the fixture box, and the book sets how much can be in there.

What the inspector looks for

The re-identification on the conductor that is a hot but looks like a neutral — at both ends, the way the book says.
That the switch box holds what the book says a switch box must hold. Whether a loop with no neutral is still permitted where you are is the book's answer, not this page's.
The splices at the fixture: hot to the loop down, switched hot to the fixture, neutral straight through to the fixture — and a bond on everything metal.
A meter reading at the fixture with the switch off: from either lamp terminal to the bond, nothing. Full supply there is the swapped-conductor fault — the switch is breaking the neutral and the lamp sits live with the handle off.
Fill and support at the fixture box, because the loop adds two conductors and a splice to a box that was already busy.

Check it before you call it done

Breaker off. Live-dead-live at the fixture box, then at the switch box — both boxes, because the loop puts line voltage in both.
At the switch box, with the switch open, one conductor should read continuity back to the fixture's hot splice and the other back to the fixture's switched-hot terminal. That names them without trusting a colour.
Mark the returning conductor as a hot now, before the cover goes on and before you forget.
Energise. Switch off: nothing across the lamp. Switch on: full supply across the lamp. Switch off again and read from the lamp's hot terminal to ground — nothing. If it reads live with the switch off, the switch is in the wrong conductor.
Note it on the panel schedule or the fixture box if the loop is unusual for the building — the next person opening that switch box will thank you.

Questions fréquentes

What is a switch loop?

It is the arrangement where the supply and neutral arrive at the light fixture, and only a two-wire drop runs down to the switch. One of those two wires carries the hot down to the switch; the other carries the switched hot back up. The switch itself never sees a neutral.

Why is there no neutral at the switch?

Because the supply landed at the fixture, not the switch. The pair going down to the switch is there only to loop the hot out and bring the switched hot back — both are in the hot family. Older installs left it at that; how the returning conductor must be identified today is a code matter, and the diagram deliberately does not assign colours.

How is this different from a standard single-pole?

The switching is identical — one blade, breaking the hot. The only difference is where the supply enters. Feed it at the switch and it is the textbook single-pole; feed it at the fixture and you get a switch loop. The lamp behaves the same either way.

Voir aussi

Single-pole switch wiring — the same switching, supply fed at the switch.
Glossary: neutral vs identified conductor
Common wiring devices — which devices need a neutral in the box, and which do not.
All wiring diagrams

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