The Direct Answer: Wiring an SPST (Single-Pole) Switch

In electrical theory, an SPST (Single Pole, Single Throw) switch is the simplest form of a switch: it has two terminals and makes or breaks a single circuit path. In residential 120V AC wiring, the SPST switch is universally known as a standard single-pole light switch. It controls a lighting load from one physical location.

To wire a standard 120V AC SPST switch, you will connect the incoming hot (black) wire to one brass terminal, the outgoing switched hot (black) wire to the other brass terminal, and the ground (bare/green) wire to the green grounding screw. The neutral (white) wires are spliced together and bypass the switch entirely. For a standard 15-amp residential lighting circuit, use a 15A/120V rated switch (like the Leviton 1451-W) and 14 AWG copper wire.

Decision Tree: Sizing Your Switch, Wire, and Breaker

Before stripping any wire, you must match your switch rating to your circuit breaker and wire gauge. The National Electrical Code (NEC) requires that the switch rating be equal to or greater than the load it serves, and the wire ampacity must be protected by the breaker. Use this decision path to select your exact materials:

Circuit Breaker Size Wire Gauge (Copper NM-B) Required Switch Rating Recommended Part Number
15 Amp 14 AWG 15A / 120V AC Leviton 1451-W
20 Amp 12 AWG 20A / 120V AC Leviton 1653-W
Pro-Tip for 2026 LED Loads: Modern LED lighting draws remarkably little current (a 15A circuit can technically handle over 1,800 watts of LED lighting). However, if your switch box also contains a receptacle (outlet), you are legally required to use a 20A switch on a 20A circuit, even if the lighting load itself is under 5 amps.

Tools and Materials Checklist

Do not start the job until you have the following specific tools and materials on your workbench. Using the right connectors and testers prevents the most common residential electrical fires and shock hazards.

  • Switch: Leviton 1451-W (15A) or Leviton 1653-W (20A) single-pole toggle.
  • Wire: 14 AWG or 12 AWG NM-B (Romex), matched to your breaker.
  • Connectors: Wago 221-412 lever nuts (rated for up to 12 AWG solid/stranded) or purple/yellow Wire-Nuts.
  • Tester 1: Klein Tools NCVT-1 Non-Contact Voltage Tester (for initial dead-circuit verification).
  • Tester 2: Fluke 117 True-RMS Multimeter (for definitive voltage verification and post-install testing).
  • Hand Tools: Klein 11055 wire strippers, Klein 32288 multi-bit screwdriver (includes #2 Phillips and 1/4" slotted).

Mains Safety Protocol (Do Not Skip)

CRITICAL SHOCK HAZARD: Working on a 120V AC circuit can be lethal. You must de-energize the circuit at the main service panel before opening the switch box.
  1. Turn OFF the corresponding breaker at the panel.
  2. Apply a lockout/tagout (LOTO) device or place a piece of tape over the breaker to prevent someone from accidentally turning it back on while you are working.
  3. Test your non-contact voltage tester on a known live circuit (like an extension cord) to verify the tester's battery is working.
  4. Insert the tester into the switch box. It must remain silent and unlit.
  5. Verify dead with a multimeter set to AC Voltage: place one probe on the black wire and one on the bare ground wire. The reading must be exactly 0.0V.

NEC-style guidance: If you are unsure which breaker controls the circuit, or if the panel is a known hazard (e.g., Federal Pacific or Zinsco), stop and hire a licensed electrician. Your local Authority Having Jurisdiction (AHJ) has final say on permit requirements.

Step-by-Step: How to Wire an SPST Switch

With the circuit verified dead, follow these exact termination steps. Standard SPST switches do not differentiate between 'Line' (power in) and 'Load' (power out) on their brass screws; either black wire can go to either brass terminal.

  1. Prep the Wires: Strip exactly 3/4 inch of insulation from the black and white wires using your Klein 11055 strippers. Strip the bare copper ground wire to about 3/4 inch as well. If the wires are old and oxidized, snip the ends and re-strip to ensure clean, bright copper.
  2. Terminate the Ground (Bare/Green to Green Screw): Form a tight 'shepherd's hook' (J-bend) in the bare copper ground wire using the needle-nose pliers on your wire strippers. Hook it around the green grounding screw on the bottom of the switch so that the wire wraps clockwise. Tighten the screw firmly until the wire is seated flat against the metal strap. (If your box is metal, you must also run a 6-inch bare copper pigtail from the switch's green screw to the metal box's ground clip).
  3. Terminate the Line/Hot (Black to Brass Screw #1): Identify the incoming always-hot black wire. Form a clockwise shepherd's hook and wrap it around one of the brass terminal screws. Tighten until the wire is securely pinned beneath the screw head. Ensure no bare copper is exposed outside the terminal washer.
  4. Terminate the Load/Hot (Black to Brass Screw #2): Identify the outgoing black wire that runs to the light fixture. Form a clockwise hook and attach it to the remaining brass terminal screw. Tighten securely.
  5. Bypass the Neutral (White to White): The SPST switch does not use the neutral wire. Strip 1/2 inch off the incoming white neutral wire and the outgoing white neutral wire. Insert both into a Wago 221-412 lever nut and snap the levers down. Tuck this splice neatly into the back of the electrical box.
  6. Seat the Switch: Carefully fold the wires in a Z-pattern (grounds in back, neutrals in middle, switch in front). Push the switch into the box and secure it with the two 6-32 mounting screws. Ensure the switch is perfectly plumb using a small torpedo level before final tightening.

Verify and Test: Meter Readings and the 'Switched Neutral' Botch

Do not screw on the wall plate until you have verified the circuit operates correctly and safely. Restore power at the breaker panel.

Expected Multimeter Readings

Set your Fluke 117 to AC Voltage (V~). With the switch in the OFF position:

  • Probe Line (Brass #1) to Ground: ~120V (Nominal range 114V-126V)
  • Probe Load (Brass #2) to Ground: 0V

Flip the switch to the ON position:

  • Probe Line (Brass #1) to Ground: ~120V
  • Probe Load (Brass #2) to Ground: ~120V
  • The light fixture should illuminate.

The Most Common Botch: The Switched Neutral

The most dangerous mistake when wiring an SPST switch is interrupting the neutral wire instead of the hot wire. This happens when a DIYer misidentifies the wires in the box and puts the white wires on the brass switch terminals, while splicing the black wires together.

The Symptom: The light will still turn on and off when you flip the switch. However, because the switch is breaking the neutral return path rather than the hot supply path, the light fixture remains fully energized at 120V even when the switch is 'off'. If you attempt to change a lightbulb or work on the fixture later, touching the socket threads and a grounded surface will result in a severe or lethal shock. Furthermore, a non-contact voltage tester will beep at the light fixture even when the switch is turned off.

The Fix: If your NCVT beeps at the fixture when the switch is off, turn the breaker back off immediately. Open the switch box, remove the whites from the switch, splice them together, and ensure the blacks are on the brass screws. Always switch the hot, never the neutral. For a deeper understanding of NEC grounding and switching requirements, refer to the NFPA 70 (National Electrical Code) guidelines and OSHA's electrical safety standards for residential and commercial wiring.

For dedicated lighting circuits in modern homes, standardizing on 15A breakers, 14 AWG NM-B wire, and 15A rated SPST switches (like the Leviton 1451-W) provides the most cost-effective, code-compliant, and physically easy-to-wire solution. Reserve 12 AWG and 20A switches strictly for circuits that include receptacles or high-draw appliance loads.