Single-pole switch wiring diagram

Light: on

Flip the switch to open and close the hot. Tap a conductor to replay the current path. The copper highlight is the live path, not a wire colour.

A single-pole switch controls one light from one place by making and breaking the hot conductor. The supply feeds the switch, the switch passes the hot on to the fixture when it is closed, and the lamp returns on the neutral. Open the switch and the path is broken before the fixture, so the light goes out.

Trace it yourself

Find the supply. The upper terminal is the hot; the lower is the neutral. Put a finger on the hot.
Follow it to the first terminal of the switch. Look at the blade: lying on the second terminal, go through; tilted away, stop here — everything past this point is dead from the supply side.
Through the switch, follow the switched hot up and over to the lamp's top terminal.
Cross the lamp. It is the only place the hot side and the return side meet, so it is the only thing that lights.
Follow the neutral from the lamp's bottom terminal back to the supply. The loop is complete. Now open the switch in your head and find where the finger has to stop.

How this circuit works

There are three players: the hot coming in, the switched hot going out, and the neutral that completes every lighting circuit. The switch sits in the hot line. When its blade is closed there is an unbroken path from the supply, through the switch, to the lamp and back on the neutral, so current flows and the lamp lights. When the blade opens, that path is cut at the switch and no current can flow, so the lamp is dark.

Notice what the neutral does not do: it never gets switched. It is the standing return, always connected to the fixture, waiting for the hot side to be made. That is the pattern under nearly every lighting circuit you will meet — the neutral is a constant, and the switching all happens on the hot.

Where it goes from here

Master this and the rest follow. Add a second switch and two travellers and you have a 3-way. Feed the supply at the fixture instead of the switch and you have a switch loop. The single-pole is the seed all of them grow from.

What goes wrong

The switch put in the neutral

Symptom: the light switches on and off exactly as it should — and the fixture is live with the switch off.

Move the blade to the return side in your head. Opening it still breaks the loop, so the lamp goes dark and nobody notices. But the hot now runs unbroken from the supply to the lamp, and every terminal on the fixture sits at line voltage while the room is dark. The circuit works; the safety does not. That is the entire reason the blade goes in the hot, and it is a mistake you cannot see from the light. A meter across the fixture with the switch off finds it in a second.

Sur le chantier

The simplest circuit on the site is also the one you will wire the most, so the habits you build here are the ones that follow you to every other box. The rule that matters is the one the diagram shows: the blade breaks the hot, and nothing else.

Common mistakes

Switching the neutral. The light works, the fixture stays live with the switch off. Nobody sees it until someone changes a lamp with wet hands.
Landing the neutral on the switch. A single-pole switch has two screws and neither is for a neutral. The neutral passes the switch box by, spliced through — or, where the book wants one in the box for a future smart switch, it is capped and waiting.
Confusing the bonding screw with a terminal. The green screw is for the bond and only the bond. A hot on it is a fault waiting for a hand.
Mounting it upside down. Up is on. Trivial, and the first thing the customer notices.
Back-stabbing a switch that feeds a real load. The push-in holes hold a conductor with a spring; a screw under a clamp holds it with a bolt. Which is permitted is the device's listing; which one comes back loose is experience.

What the inspector looks for

That the switched conductor is the hot — a meter across the fixture with the switch off tells the story in one reading.
What is in the switch box. The book says which conductors must be present there; the inspector checks the box against the book, not against the diagram.
The bond to the switch strap and to the box, if it is metal.
How the returning switched hot is identified where it shares a cable with a conductor that looks like a neutral — a code rule, and a common finding.
A cover on, the device straight, the yoke screwed to the box and not floating on the plaster.

Check it before you call it done

Power off at the breaker. Prove it dead at the switch with the live-dead-live check before your fingers go in.
Continuity across the two switch screws: closed in one handle position, open in the other. A switch that reads closed both ways is welded; open both ways is broken.
Energise. Switch off: read hot-to-neutral at the fixture. It should be nothing. If it reads full supply, the blade is in the return — the fault above.
Switch on: full supply at the fixture, lamp lit. Switch off, lamp dark. Do it twice; a switch that sticks shows itself on the second try.
Put the plate on and flip it once more with the plate on. A plate screw that pinches a conductor shows up now, not later.

Questions fréquentes

What makes a switch "single-pole"?

One pole means it opens and closes one conductor — the hot. It has a single moving blade with an on and an off position, so it controls a light from exactly one place. Everything more elaborate, from 3-ways to dimmers, is built on top of this idea.

Does the switch go on the hot or the neutral?

A light switch interrupts the ungrounded conductor — the hot — so the fixture is de-energized when the switch is off. Switching the neutral would turn the light off but leave the fixture live, which is the opposite of what you want. This diagram breaks the hot for that reason.

Why is the fixture still live at the lamp when the switch is off?

It is not, in this arrangement — the open switch sits before the lamp, so the whole fixture drops dead when the blade opens. Toggle the diagram off and watch the copper disappear from the fixture. That is exactly why the switch belongs on the hot side.

Voir aussi

Switch loop wiring — same job, supply fed at the light instead.
3-way switch wiring diagram — control one light from two places.
Common wiring devices — the switch itself: screws, strap, bonding screw, and how to tell it from a 3-way.
All wiring diagrams

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