Wiring Diagram for Light Switch — Single-Pole Installation
This is a free printable wiring diagram for light switch: download the diagram as SVG or open it and print to paper or PDF.
A wiring diagram for a light switch shows how a single-pole switch interrupts the hot (black) wire between the circuit breaker and a light fixture, giving you control over the light from one location. This is the most common electrical wiring task in residential construction and renovation. Understanding the correct connection order — breaker, switch, fixture, neutral return — prevents shock hazards and code violations.
A single-pole light switch controls one light or group of lights from a single location using two terminals and one toggle or rocker lever. In standard US residential wiring per the National Electrical Code (NEC), the switch must interrupt only the hot (black) wire — never the neutral. This is a fundamental safety requirement: when the switch is off, the light fixture wiring should have no live voltage, so a person changing a bulb is not exposed to voltage. The switch is always installed in the hot line between the breaker panel and the fixture. The cable from the panel to the switch box contains three conductors: black (hot), white (neutral), and bare copper or green (ground). At the switch, the black wire connects to one switch terminal and a second black wire (or black-taped white wire) runs from the other switch terminal to the fixture's hot terminal. The white neutral wire runs from the panel directly through to the fixture neutral — it does not connect to the switch. In switch loop wiring (older installation method where cable runs from fixture to switch rather than from panel to switch), a white wire carries the switched hot — this white wire must be re-identified with black tape or paint at both ends per NEC 200.7. The ground wire connects to the switch ground terminal (if present on a metal-screw switch) and continues to the fixture and panel ground bus. Modern switches require a neutral for electronic functions (dimmers, smart switches) — check manufacturer specs before installing. Single-pole switches have two brass-colored screw terminals on the same side of the device body. The toggle or rocker is marked ON (up) and OFF (down) — connecting wires reversed swaps this marking but does not affect function.
UK Light Switch Terminals: L1, L2, and Loop-In Wiring Explained
In UK wiring (BS 7671 / 18th Edition), a single-gang one-way light switch has two active terminals labelled COM and L1. COM (common) receives the permanent live from the supply; L1 is the switched live that continues to the lamp. Some manufacturers label these terminals differently — you may see C/1 or simply two screw terminals without labels — always check the back of the switch faceplate for the printed terminal map.
A UK two-way switch has three terminals: COM, L1, and L2. In a two-way switching circuit (what Americans call a 3-way circuit) the permanent live feeds COM on the first switch. Two strapper wires — typically the brown and grey cores of a 3-core-and-earth cable — run between the L1 terminals of both switches and between the L2 terminals of both switches. The switched live exits from COM on the second switch and runs to the lamp. Swapping which core lands on L1 versus L2 at one of the switches only changes which physical toggle position is ON; it creates no safety fault.
Hot Wire Top or Bottom: Does It Matter?
For a standard US single-pole switch the line hot (black wire) can connect to either terminal screw — the switch has no internal polarity and the NEC does not specify top versus bottom. By convention many electricians place the line hot on the bottom screw and the switch-leg return (to the fixture) on the top screw, but both orientations are electrically identical. On a 3-way switch the rule is different: the line hot must land on the COM terminal (the single dark-coloured screw), not on a traveler screw. The two traveler screws are interchangeable but COM is not.
In a UK installation the convention mirrors the function: the permanent live wire goes to COM, which may be positioned at the top or bottom depending on the switch model. The position of the COM terminal varies by manufacturer, so confirm by reading the terminal label rather than relying on physical position.
Light switch wiring diagrams vary based on the type of load (standard bulb, ceiling fan with light kit), voltage (120 V / 230 V / 12 V), and application (domestic, commercial, low-voltage landscape). The cable entering the switch box may carry the supply directly or arrive via the light fitting, changing the terminal connections at the switch. Whether you are wiring a house switch, a 12 V camp light, or a ceiling-fan combination switch, you can draw and export any layout free at circuitdiagrammaker.com.
How to wire wiring diagram for light switch
- De-energize and verify Turn off the correct circuit breaker. Use a non-contact voltage tester on all wires in the switch box. Verify zero voltage on both the incoming cable and the switch loop cable before touching any wires.
- Identify and sort wires Pull wires from the box. Identify: incoming hot (black), incoming neutral (white — does not connect to switch), switch leg hot (black or black-taped white going to fixture), and ground (bare copper or green).
- Connect switch terminals Form a small clockwise loop at the end of each hot wire with needle-nose pliers. Hook one hot wire clockwise under each brass terminal screw. Tighten screws firmly until wire cannot rotate.
- Connect ground If metal box: pigtail the ground to box ground screw and switch ground screw (green). If plastic box: connect ground directly to switch ground screw. Use 6-inch pigtail wire for the connection.
- Test before closing box Restore breaker power. Verify switch ON turns light on and OFF turns light off. Check with voltage tester that switch terminals are live only when switch is on. Turn off breaker before pushing switch into box and attaching faceplate.
Specifications
| Switch rating | 15A 120V AC (residential standard) |
|---|---|
| Hot wire | Black, 14 AWG (15A circuit) |
| Neutral | White, 14 AWG (not switched) |
| Ground | Bare copper or green, 14 AWG |
Safety warnings
- Turn off the circuit breaker feeding the switch circuit before any work — use a non-contact voltage tester to verify zero voltage on all wires before touching them.
- The switch must interrupt only the hot wire — switching the neutral wire is a code violation and leaves the fixture energized when the switch is off, creating an electrocution risk during bulb changes.
- Connect the ground wire to the switch ground screw — an ungrounded metal switch plate is a shock hazard if the hot wire contacts the plate or switch body.
Tools needed
- Non-contact voltage tester
- Flathead and Phillips screwdrivers
- Wire stripper (14 AWG and 12 AWG settings)
- Needle-nose pliers for wire loops on screw terminals
Common mistakes
- Connecting the neutral wire to a switch terminal — the neutral must pass through to the fixture without interruption; switching it leaves the fixture live when the switch appears off.
- Failing to identify the switch leg (white wire used as switched hot) with black tape — an unidentified white wire in a switch box confuses the next person working on the circuit.
- Not pigtailing the ground from a metal switch box — the ground wire must connect to both the box and the switch ground terminal; a single connection to the box alone leaves the switch ungrounded.
- On a UK one-way switch, connecting the permanent live to L1 instead of COM — the circuit will never complete and the lamp will not light.
- On a US single-pole switch in a switch-loop installation, failing to re-identify the white wire (switch-leg return) with black tape, leaving an unmarked hot conductor that could mislead future electricians.
- Assuming the top terminal is always the line-hot terminal — switch orientation in the box and the position of COM varies by manufacturer and installation.
Troubleshooting
- Light does not turn on at all
- Cause: Blown circuit breaker, open wire connection at switch or fixture, or failed bulb Fix: Check breaker first. Then test voltage at switch terminals with switch ON. If no voltage with switch ON, the incoming hot has no power. If voltage present with switch ON, trace to fixture and check bulb.
- Light flickers when switch is toggled
- Cause: Loose screw terminal connection at the switch or fixture Fix: Turn off breaker. Check tightness of all screw terminals at switch and at fixture wire nuts. Remake any connections where wire is not fully under the screw head.
- Switch feels warm to the touch
- Cause: Overloaded switch (total load exceeds switch ampere rating), or a loose high-resistance connection generating heat Fix: Calculate total wattage on the switch circuit. A 15A switch handles 1800W at 120V. If overloaded, split the load to a second circuit. Also check all connections for tightness and clean wire ends.
- UK light switch has no L1/L2 markings — wires connected but light does not work
- Cause: The permanent live was connected to L1 instead of COM, so the switch never completes the circuit to the lamp. Fix: Check the back of the switch body for a printed terminal diagram. Move the permanent live to the terminal marked COM (C or common). The switched live to the lamp goes on L1.
Frequently asked questions
Which wire connects to which terminal on a single-pole light switch?
A single-pole switch has two brass screw terminals on the same side. Connect the incoming hot (black) wire to one terminal and the outgoing switched hot (going to the fixture) to the other. There is no polarity requirement on a single-pole switch — either terminal can be the incoming or outgoing hot. The ground connects to the green screw at the bottom of the device.
Can I wire a light switch without a ground wire?
You can, but it is not up to current code in new construction. Older homes with two-wire cable (no ground) are grandfathered for repairs but should be upgraded when possible. A grounded switch is safer — metal faceplates and switch bodies that could contact a loose wire are protected. If replacing an old ungrounded switch, use a plastic faceplate to reduce shock risk until grounding can be added.
What is the difference between a single-pole and 3-way light switch?
A single-pole switch controls a light from one location only and has two terminals plus ground. A 3-way switch controls a light from two locations (like top and bottom of a staircase) and has three terminals: one common and two travelers. Single-pole switches are labeled ON/OFF on the toggle; 3-way switches are not labeled because the ON position depends on the other switch's state.
Why does my light stay on when the switch is in the OFF position?
The most common cause is that the neutral wire was accidentally connected to the switch instead of the hot wire. If the switch interrupts the neutral, the light fixture remains energized (hot through the bulb) even when the switch is open — the bulb may glow dimly or stay fully on. Turn off the breaker and verify the black (hot) wire connects to the switch, not the white (neutral).
Can I install a dimmer switch using the same wiring?
Yes, in most cases. A standard dimmer replaces the single-pole switch and connects the same way — two hot wires (line and load) to the dimmer leads or terminals. However, modern dimmers often require a neutral wire for their electronics — check the dimmer instructions. LED dimmers must be rated for LED loads; incandescent dimmers cause LED flickering. Always verify dimmer compatibility with the specific LED bulbs you are using.
What do L1 and L2 mean on a UK light switch?
On a UK one-way switch only COM and L1 are present: COM is the permanent live input and L1 is the switched live output to the lamp. On a UK two-way switch, L1 and L2 are the two strapper terminals that link matching terminals across the pair of switches. COM at the first switch receives the permanent live; COM at the second switch sends the switched live to the lamp.
Does the hot wire go on the top or bottom screw of a light switch?
For a US single-pole switch, neither position is code-required — the switch has no polarity and either terminal accepts the line hot. By convention many electricians place the line hot on the bottom screw. For a 3-way switch the hot must go to the COM terminal (the dark-coloured screw), which may be at the top or bottom depending on the switch orientation in the box.
Can I use a two-way UK switch as a one-way switch?
Yes. Connect the permanent live to COM and the switched live to L1. Leave the L2 terminal unused. The switch will operate as a standard on/off, though the toggle direction that equates to ON may differ from adjacent switches depending on which terminal is used.
What is the wiring diagram for a house light switch?
In a standard loop-at-switch method, the supply cable (live and neutral) enters the switch box. Live connects to the switch common; the switched live leaves the switch and returns to the light fitting. Neutral runs unswitched directly to the fitting. In a loop-at-fitting method, supply enters the ceiling rose and a switch drop cable runs down to the switch — the cable's neutral is sleeved as a switched live at both ends.
What is the wiring diagram for a light switch controlling a ceiling fan?
For independent control of a fan motor and a light kit, run a 3-conductor cable (14/3 or 12/3) between the switch location and the fan. Use one switch for the black (fan) wire and a second switch for the red (light) wire; the white neutral is continuous. If only one switch is used, both the fan and light share the same switched live. The fan's internal terminal block separates the motor and light-kit feeds.
What is the wiring diagram for a 12 volt light switch?
A 12 V DC light switch diagram shows the battery positive through a fuse to the switch common, the switched output running to the lamp positive terminal, and the lamp negative returning to the battery negative (or chassis ground). Polarity must be observed with LED fixtures. Wire sizing depends on current draw: for example a 50 W lamp at 12 V draws about 4.2 A, requiring at minimum 1.5 mm² (AWG 14) cable for short runs.
What is the electrical wiring diagram for a light switch?
An electrical light switch diagram shows a single-pole switch with two terminals: line (incoming supply) on one terminal and load (switched feed to lamp) on the other. The neutral wire bypasses the switch entirely and goes directly to the lamp's neutral terminal. Earth wires connect to metal switch plates and light fittings. This is the most basic switched-lighting circuit and forms the basis of all more complex switching arrangements.
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