Single-Pole Switch Diagram: How to Wire an SPST On/Off Light Switch
This is a free printable single pole switch diagram: download the diagram as SVG or open it and print to paper or PDF.
A single-pole switch is a single-pole single-throw (SPST) device with two terminals that interrupts the hot (live) conductor in a lighting circuit to control a light fixture from one location only.
A single-pole switch is the simplest and most common light switch in residential and commercial buildings worldwide. It has exactly two terminals and one function: connect or disconnect a single conductor. In mains wiring, it is always wired into the hot (live) conductor — never into the neutral — so that when the switch is open, the light fixture is at zero potential on both sides, which is the safe condition for lamp replacement and maintenance.
In US residential wiring (NEC), the single-pole switch wiring is straightforward. The incoming cable from the supply panel carries a hot (black), neutral (white), and ground (bare) conductor. The hot conductor connects to one of the switch's two terminals; the switched hot runs from the other terminal to the light fixture. The neutral bypasses the switch entirely and connects directly to the fixture's neutral terminal. The ground runs from the supply, to the switch's ground screw, and to the fixture's ground.
In a switch-loop configuration (older US wiring practice where only a 2-conductor cable runs to the switch), both conductors in the cable are hot conductors — the white must be re-identified with black tape to indicate it is not a neutral. The black wire carries incoming hot from the ceiling box; the re-identified white carries switched hot back to the fixture. This practice is permitted for existing switch loops under NEC but new switch loops must include a neutral conductor (NEC 2011 and later, Article 404.2).
In UK wiring (BS 7671), the live conductor (brown or, in older wiring, red) connects to the switch terminal on one side, and the switched live (brown or re-identified grey/black with a brown sleeve) runs to the luminaire. The circuit protective conductor (CPC, green/yellow) bonds the switch plate if metal.
A single-pole switch controls a load from one location only. For two-location control, use a pair of 3-way switches (called 2-way switches in the UK). This is the fundamental distinction between single-pole and multi-way switching.
All mains wiring work must be performed by a licensed electrician and must comply with applicable codes.
How to wire single pole switch diagram
- Isolate the circuit and verify dead Switch off the circuit breaker for the lighting circuit. Use a non-contact voltage tester to verify the switch box is dead — test at all wires, including both sides of the switch loop. Apply lock-out/tag-out to the breaker. Do not rely on the existing switch position to assume a circuit is safe.
- Remove the old switch and identify wiring Remove the cover plate and switch mounting screws. Pull the switch from the box. Photograph the existing wiring configuration. Identify whether it is a direct-feed (neutral in box) or switch-loop (no neutral in box, re-identified white wire) configuration. Label each wire before removing it.
- Prepare conductor ends Strip approximately 19 mm (3/4 inch) of insulation from each conductor. Form a clockwise J-hook on each stripped end for wrapping around the terminal screw. Use a wire stripper set to the correct gauge to avoid nicking the conductor.
- Connect the ground wire Connect the bare copper or green ground wire to the switch's green ground screw. If the box is metal, also pigtail a ground wire to the metal box's ground screw or threaded grounding hole. The ground does not connect to the switching circuit.
- Connect the hot and switched hot wires Connect the incoming hot wire (black) to one terminal screw and the switched hot wire (black, or re-identified white from a switch loop) to the other terminal screw. On a single-pole switch, both terminals are functionally equivalent — but by convention, connect incoming hot to the top terminal. Tighten both screws firmly.
- Re-identify any white wire used as switched hot If a white wire in the switch box is used as a switched hot (switch loop configuration), wrap the exposed end with black electrical tape at the switch terminal and at the fixture box. This re-identification is required by code (NEC Article 200.7, BS 7671) to warn future workers that this white wire is not a neutral.
- Install, restore power, and test Fold conductors neatly into the box and push the switch in. Secure with mounting screws and install the cover plate. Restore power at the breaker. Test the switch in both positions — the light should illuminate in one position and be completely off in the other. Verify the switch label (ON/OFF) corresponds correctly to the light state.
Specifications
| Switch type | SPST (Single-Pole Single-Throw), 2-terminal |
|---|---|
| US standard rating | 15 A, 120/277 V AC |
| UK standard rating | 6 A or 10 A, 250 V AC (BS 3676) |
| Australian standard rating | 10 A, 250 V AC (AS/NZS 3000) |
| Control function | Single-location on/off control only |
| US circuit cable (15 A) | 14/2 AWG NM-B with ground |
| UK circuit cable | 1.5 mm² twin and earth (CPC included) |
| Applicable codes | NEC/NFPA 70 Article 404 (US), BS 7671 Chapter 55 (UK), AS/NZS 3000 (AU/NZ) |
Safety warnings
- Mains electrical switch installation is dangerous work involving lethal voltages. Always isolate the circuit at the breaker and verify absence of voltage at all conductors in the switch box before touching any wires. Lock out and tag the breaker. This work must comply with NEC/NFPA 70 (US), BS 7671 (UK), AS/NZS 3000 (AU/NZ), or the applicable local electrical code, and may require a permit and inspection.
- Always wire the single-pole switch to interrupt the hot (live) conductor, not the neutral. Switching the neutral leaves the fixture and lamp holder live even with the switch open, creating a shock hazard during lamp replacement or fixture maintenance.
- In the UK and countries using IEC wiring colour conventions (brown = live, blue = neutral), ensure that you use the new harmonised colours. In older installations with red (live) and black (neutral), have the installation inspected and updated to current standards by a licensed electrician before carrying out any work.
- Never use back-stab (push-in) terminals on switches where screw terminals are available. Backstab connections have high failure rates and cause arcing faults.
- Smart switches and dimmers require a neutral wire in the switch box to power their electronics. Verify whether a neutral is present in the switch box before purchasing a smart switch. Installing a smart switch without a neutral in a switch-loop box will result in malfunction or damage to the switch.
Tools needed
- Non-contact voltage tester
- Digital multimeter
- Flathead and Pozidrive screwdrivers
- Wire strippers
- Long-nose pliers
- Electrical tape (for conductor re-identification)
- Lockout/tagout device
- Flashlight or work light
Common mistakes
- Wiring the switch into the neutral conductor instead of the hot, leaving the fixture live when the switch is open and creating a hidden shock hazard.
- Failing to re-identify white switch-loop wires used as switched hot conductors. An unmarked white wire looks like a neutral to any subsequent electrician, creating a dangerous ambiguity.
- Using undersized wire (16 AWG) for a 15 A or 20 A lighting circuit. Always match wire gauge to the circuit breaker rating.
- Not connecting the ground wire to the switch's ground screw. While a plastic switch body is not conductive, the ground provides a path for fault current on the box hardware and any future metal switch plates.
- Installing a standard single-pole switch in a location that requires a 3-way switch (e.g. top or bottom of a staircase). A single-pole switch cannot provide two-location control.
Troubleshooting
- Light does not turn on in either switch position
- Cause: Breaker tripped; loose wire at switch or fixture; broken switch; open neutral at fixture Fix: Reset the breaker if tripped. With power off, check all terminal screws for tightness. Test the switch with a multimeter in continuity mode — it should show continuity in ON position and open circuit in OFF position. Replace switch if faulty. Check neutral connection at fixture.
- Light is permanently on and switch does nothing
- Cause: Switch wired into neutral instead of hot; or switch short-circuited internally Fix: Shut off circuit. Verify switch is wired into the hot conductor, not the neutral. Disconnect the switch and test it with a multimeter — if the circuit is still completed (light on) with the switch removed and conductors separated, the switch is bypassed somewhere. Trace the hot conductor through the circuit.
- Light flickers when switch is toggled
- Cause: Loose wire connection at switch terminal; corroded switch contacts; switch rated for fluorescent or inductive loads used with LED lamps Fix: With power off, retighten all terminal screws and check wire nut connections in the box. If the switch is old and the contacts are pitted or corroded, replace the switch. Verify the switch is rated for the load type (LED, incandescent, or mixed).
- Receiving a shock when touching the switch cover or plate
- Cause: Ungrounded metal switch plate or box; hot wire contact with metal enclosure; ground conductor not connected Fix: Immediately shut off the circuit. Do not touch the switch again. Inspect for a hot conductor contacting the metal box or switch body. Verify the ground wire is connected to the switch ground screw and to the metal box. Replace any damaged wiring.
- Smart switch or dimmer turns on briefly when switched off or buzzes
- Cause: No neutral wire in switch box; smart switch requires neutral to power internal electronics; or dimmer incompatible with LED lamp type Fix: Verify neutral availability in the switch box. Use a smart switch model designed for no-neutral wiring if neutral is not available. For dimmer buzzing with LEDs, replace dimmer with one specifically listed as compatible with LED loads.
Frequently asked questions
What is the difference between a single-pole and a 3-way switch?
A single-pole switch (SPST) has two terminals and controls a light from one location only — it is either on or off. A 3-way switch (SPDT) has three terminals and is used in pairs to control one light from two locations. Single-pole switches have 'ON' and 'OFF' labelled on the toggle; 3-way switches do not, because their position alone does not determine circuit state.
Which wire connects to which terminal on a single-pole switch?
A single-pole switch is non-polarised in the sense that either terminal can accept either wire — the switch just opens or closes the circuit. However, the correct practice is to wire the hot (incoming live) to one terminal and the switched hot (going to the load/fixture) to the other. Never wire a single-pole switch into the neutral conductor.
Why should a switch always interrupt the hot wire and not the neutral?
Switching the neutral leaves the fixture's wiring energised at line voltage even when the switch is open. Anyone replacing a bulb and touching the energised lamp socket or wiring would receive a shock. Interrupting the hot removes live voltage from the fixture, making it safe to work on.
Do I need a neutral wire at the switch box in modern wiring?
In the US, NEC 2011 and later (Article 404.2) requires a neutral conductor to be present in the switch box for new wiring, even if it is not used by a simple toggle switch. This is to accommodate smart switches, dimmers, and occupancy sensors that require a neutral to power their electronics. Smart switches installed without a neutral will not function correctly.
Can I use a single-pole switch with a dimmer?
Yes, if the dimmer is a single-pole dimmer switch. Dimmers are available in single-pole and 3-way versions. The dimmer must be compatible with the lamp type (LED, CFL, or incandescent). Many LED lamps require a specifically listed LED-compatible dimmer to dim smoothly without flicker or buzzing.
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