Single-Pole Light Switch Diagram

Single-Pole Light Switch Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable single pole light switch diagram: download the diagram as SVG or open it and print to paper or PDF.

A single-pole light switch is a two-terminal device that controls a light fixture from one location by interrupting only the hot (line) conductor of a 120 V or 277 V branch circuit.

In North American electrical terminology, a single-pole switch is equivalent to what UK and Commonwealth electricians call a one-way switch. Both describe the same topology: a single-pole, single-throw (SPST) mechanism that opens or closes the circuit at one location only. Understanding the two naming conventions avoids confusion when referencing international wiring diagrams.

Under the National Electrical Code (NEC / NFPA 70), a single-pole switch must interrupt only the ungrounded (hot) conductor — typically a black wire. The neutral (white) and equipment grounding conductor (green or bare copper) must not be switched. The switch creates a single break in the hot conductor path between the panel breaker and the light fixture.

Modern NEC practice distinguishes between two common wiring arrangements:

1. Switch loop (legacy and still common): The cable runs from the panel to the fixture first, then a two-conductor cable drops down to the switch. The white wire in the switch loop is re-identified with black tape at both ends to show it is being used as an ungrounded (hot) conductor, not a neutral. This arrangement has no neutral at the switch box — a limitation for smart switch installation.

2. Feed-through wiring (modern, NEC 2011+ preferred): The cable runs from the panel to the switch box first, then continues to the fixture. This brings a true neutral through the switch box, enabling smart switch installation without a neutral adapter.

The equipment grounding conductor (EGC) — bare copper or green insulated — must be connected to any metal switch box, to the ground terminal on smart switches or GFCI devices, and to the grounding terminal in the panel. Plastic boxes do not require an EGC termination at the box itself, but the EGC must be continuous through the circuit.

Switch ratings in North America are typically 15 A at 120/277 V AC, which accommodates standard lighting branch circuits. Dimmers are rated separately and have specific minimum/maximum load requirements — always check the dimmer's compatibility list before installation.

A single-pole switch is identified by two screw terminals (brass-coloured) and a green screw for the equipment grounding conductor. It has a clear ON/OFF marking on the toggle.

How to wire single pole light switch diagram

Turn off the circuit breaker and verify dead Identify the correct circuit breaker in the panelboard and switch it to OFF. Apply a lock-out/tag-out clip or a warning label. At the switch location, use a non-contact voltage tester and a two-pole voltmeter to confirm no voltage is present at any wire in the box. Test hot-to-neutral, hot-to-ground, and neutral-to-ground.
Remove the old switch (if replacing) and document the wiring Take a photo of the existing connections before disconnecting anything. Note which screw each wire is connected to. In a switch loop, expect two black wires (or a black and a white re-identified as black with tape) on the brass screws; in feed-through wiring, you may see a black and a white with a neutral capped off in the box.
Connect the hot conductor to one brass screw Form a clockwise J-hook in the stripped wire end. Hook it around the brass screw so that tightening the screw draws the wire into the screw's contact. The hot (black or re-identified white) conductor goes to either brass screw — the two brass screws are electrically equivalent on a single-pole switch.
Connect the switched hot to the other brass screw The wire returning to the light fixture (also black, or a re-identified white in a switch loop) connects to the second brass screw using the same J-hook technique. Tighten firmly — a loose screw connection arcs and is a fire hazard.
Connect the equipment grounding conductor to the green screw Strip and form the bare or green ground wire and attach it to the green grounding screw on the switch. If a metal box is used, also connect a pigtail from the box ground screw to the same ground wire cluster using a wire connector (wire nut).
Fold wires carefully into the box and mount the switch Fold the wires in a Z-pattern so they stack neatly behind the switch body without being pinched. Tighten the mounting screws to hold the switch level in the box. Install the faceplate cover plate.
Restore power and test Turn the circuit breaker back on. Operate the switch to confirm the light works in both ON and OFF positions. Verify that the ON position corresponds to the toggle pointing up. Use a non-contact tester on the fixture to confirm the light is fully de-energised in the OFF position.

Specifications

Standard switch rating (North America)	15 A, 120 V / 277 V AC
Conductor size (15 A circuit)	14 AWG copper (NEC Table 310.16)
Conductor size (20 A circuit)	12 AWG copper (NEC Table 310.16)
Minimum box volume (one device)	18 in³ (NEC 314.16, one device fill = 2 × conductor fill)
Switch pole configuration	Single-pole, single-throw (SPST)
Applicable standard	NEC / NFPA 70 (USA), UL 20 (switch listing standard)
AFCI requirement (USA)	Required on bedroom, living room, dining room, and most habitable space lighting circuits (NEC 210.12)

Safety warnings

All electrical work on fixed wiring in a residence must comply with the National Electrical Code (NFPA 70) and local amendments. Many jurisdictions require a licensed electrician to perform or inspect the work, and a permit may be required. Check local requirements before proceeding.
Always verify the circuit is dead using both a non-contact voltage tester AND a two-pole voltmeter before touching any wires. Non-contact testers can give false negatives near certain LED driver frequencies — use both tools for confirmation.
Never switch the neutral conductor with a single-pole switch. If the neutral is opened while the hot is connected, the fixture is still energised and a person replacing the lamp can receive a shock.
In the USA, AFCI (arc-fault circuit interrupter) protection is required for most bedroom and living area lighting circuits under NEC 210.12. Verify your installation meets current code requirements.
When working in a panel or sub-panel, treat all conductors as live even after switching off the relevant breaker — the bus bars and supply conductors remain energised. Only a licensed electrician should work inside a panelboard.

Tools needed

Non-contact voltage tester (NCV tester)
Two-pole voltmeter with CAT III or CAT IV rated probes
Flathead and Phillips-head screwdrivers (insulated handles)
Wire strippers suitable for 12 AWG and 14 AWG conductors
Long-nose (needle-nose) pliers for forming J-hooks
Diagonal cutters / side cutters
Circuit breaker lock-out/tag-out device

Common mistakes

Re-identifying the white wire in a switch loop with white tape at only one end — NEC 200.7 requires re-identification at both ends to indicate the wire is serving as an ungrounded conductor.
Using 14 AWG wire on a 20 A circuit — 14 AWG is rated only for 15 A. Always match the wire gauge to the breaker: 12 AWG for 20 A circuits.
Over-stripping the conductors, leaving excessive bare copper exposed outside the terminal screw — the maximum exposed conductor length should be just enough to form a complete loop around the screw shank.
Connecting the neutral (white) to the switch instead of splicing it through the box — results in the light being permanently on.
Installing the switch upside down so that the ON position is toggle-down, violating NEC 404.6 and confusing users.

Troubleshooting

Light stays on when switch is in the OFF position: Cause: The neutral has been connected through the switch instead of the hot conductor; the switch is opening the neutral rather than the hot. Fix: Isolate the circuit, remove the switch, and trace both conductors with a non-contact tester while the breaker is on. The conductor that is live with the switch in any position is the incoming hot — connect that to one brass screw and the other conductor to the second brass screw.
Breaker trips immediately when the switch is turned on: Cause: A short circuit exists in the wiring between the switch and the light fixture, or within the fixture itself. Fix: Disconnect the switch leg cable at the switch box. Restore power and flip the breaker. If it holds, the fault is downstream in the cable or fixture. If it still trips, the fault is in the supply wiring or panel. Perform an insulation resistance test to locate the fault.
Smart switch turns the light on briefly when the toggle is in the OFF position: Cause: The smart switch is using the light bulb as a return path for its standby current — common in no-neutral smart switch installations with certain LED driver types. Fix: Ensure the LED fixture is on the smart switch manufacturer's compatibility list. Alternatively, add a small bypass capacitor or resistor module (sometimes supplied with the smart switch) across the load, or run a neutral wire to the switch box.

Frequently asked questions

What is the difference between a single-pole and a three-way switch?

A single-pole switch controls a light from one location and has two screw terminals. A three-way switch (US terminology) controls a light from two locations — at the top and bottom of a staircase, for example — and has three terminals: one common and two travellers. Three-way is the US equivalent of two-way switching in UK/Australian practice.

Why does my smart switch require a neutral wire and what can I do if I do not have one?

Smart switches need a continuous low-current supply to power their radio module and electronics even when the light is off. Without a neutral at the switch box, there is no return path for that current. Solutions include running a new three-conductor cable, using a smart switch with no-neutral technology (which passes a small current through the light), or using a relay module at the fixture.

Can a single-pole switch be installed upside down?

Mechanically yes, but the NEC and standard wiring practice require that when the switch is in the ON position the toggle points UP. If installed upside down, the toggle points down for ON — this can confuse occupants and violates the intent of NEC 404.6. Some decorative designs obscure the marking; check the manufacturer's orientation markings.

Do I need a ground wire on my switch if the existing wiring has no ground?

NEC 404.9(B) allows replacement of a non-grounded switch with a new switch that has a grounding terminal only if grounding means are available. If no ground exists, you may install a non-grounding-type replacement or, where required, upgrade the wiring. Do not connect the ground terminal to the neutral — this creates a shock hazard.

What is the maximum load a standard 15 A single-pole switch can control?

At 120 V, a 15 A switch can theoretically control 1 800 W. However, the NEC continuous load rule limits sustained loads to 80% of the breaker rating, and tungsten-filament (incandescent) ratings on switches are typically lower than their general-purpose rating due to the high inrush current of filament lamps. Always check the switch's tungsten-filament current rating if controlling incandescent loads.