A Single Pole Double Throw (SPDT) switch wiring diagram routes one incoming power source to one of two separate outgoing loads. Unlike a standard single-pole switch (which simply breaks one line) or a residential 3-way setup (which uses two SPDT switches in tandem to control one load from two locations), a standalone SPDT switch is your dedicated tool for A/B selection. If you are wiring a workshop dust collector to serve two different drop hoses, switching a porch between a bright floodlight and a dim accent light, or selecting between a shore-power and battery feed in an RV, you need an SPDT. For standard 120V/15A home and shop wiring, the default, code-compliant pick is the Leviton 5634-2W 15A Single Pole Double Throw Toggle Switch.

Decoding the SPDT Diagram Symbols and Physical Terminals

In electrical schematics, the SPDT symbol is drawn as a single straight line (the pole) hinged at a fixed pivot point, with the free end pointing toward one of two distinct dots (the throws). The line represents the movable internal contact, while the dots represent the two stationary output terminals. When you look at a physical SPDT toggle switch, you will not find the standard 'LINE' and 'LOAD' markings found on basic switches. Instead, you will see three brass terminal screws (plus one green ground screw).

Understanding which physical screw maps to which schematic node is where most DIYers make their first mistake. Here is the exact terminal mapping for a standard 120V AC toggle switch:

Terminal Name Schematic Symbol Physical Appearance Wiring Function
Common (Pole) The hinged pivot line Odd-colored screw (often black or dark brass), sometimes marked 'C' Connects to the incoming ungrounded (hot) source conductor.
Throw 1 (Output A) Top stationary dot Standard light-brass screw Connects to the switched hot conductor for Load A.
Throw 2 (Output B) Bottom stationary dot Standard light-brass screw Connects to the switched hot conductor for Load B.
Ground Not shown in logic diagram Green hex screw on metal mounting strap Connects to the equipment grounding conductor (bare or green).
Bench Tip: If your switch has three identical brass screws and no 'C' marking, flip the toggle to the center (if it's a center-off model) or use your multimeter's continuity setting to identify the Common terminal before pulling wires through the box.

Node-by-Node Trace: 120V AC Dual Load Wiring

Let's trace a complete 120V AC circuit from the breaker panel to two separate hardwired loads (e.g., an overhead shop light and a localized task light). We are using 14 AWG NM-B (Romex) cable on a 15A breaker.

Step 1: The Hot Feed (Source to Switch)
The black (hot) wire from the breaker panel enters the switch box. This wire terminates on the Common terminal of the SPDT switch. This is your single input pole. The white (neutral) wire from this same feed cable does not land on the switch; it gets spliced directly to the neutral pigtails of both loads using a wire nut or Wago connector.

Step 2: The Switched Hots (Switch to Loads)
You will run two separate 14 AWG black wires out of the switch box. The first black wire lands on Throw 1 and routes to the black wire of Load A. The second black wire lands on Throw 2 and routes to the black wire of Load B. When the toggle is flipped up, the internal contact bridges Common to Throw 1, energizing Load A. When flipped down, it bridges Common to Throw 2, energizing Load B.

Step 3: The Neutral Path (Bypassing the Switch)
NEC-style guidance strictly forbids switching the neutral conductor on a standard single-phase branch circuit. All white neutral wires (the feed neutral, Load A neutral, and Load B neutral) must be bonded together in the back of the box. The switch only interrupts the ungrounded (hot) conductors.

Step 4: The Ground Path (Equipment Bonding)
All bare copper ground wires (feed, Load A, Load B) are spliced together and a 14 AWG pigtail is run to the green ground screw on the switch strap. If you are using a metal junction box, a second pigtail must bond the ground cluster to the box itself using a green grounding clip or a #10-32 grounding screw.

Verification: How to Test Every Connection with a Meter

Before you push the wires back into the box and energize the breaker, you must verify the switch logic and ensure no dead shorts exist. Grab a reliable digital multimeter, like a Fluke 117 or equivalent, and follow this sequence.

  1. De-energize and Lock Out: Turn off the 15A breaker at the panel. Verify the panel is off by testing a known live circuit first, then test your work area.
  2. Continuity Test (Switch Logic): Set your meter to continuity (the diode/sound wave symbol). Place the red probe on the Common terminal and the black probe on Throw 1. Flip the toggle. You should hear a beep in one position, and silence in the other. Move the black probe to Throw 2. The beep should now occur in the opposite toggle position. This confirms the internal mechanical wiper is routing correctly.
  3. Short Circuit Check: Keep the meter on continuity. Place one probe on the Common terminal (hot in) and the other probe on the bundled neutral wires. The meter must read 'OL' (Open Loop) or infinity. If it beeps, you have a dead short between hot and neutral—find it before turning the breaker on.
  4. Ground Verification: Place one probe on the switch's green ground screw and the other on the bare copper ground bundle. It should beep, confirming your equipment bonding path is intact.
  5. Live Voltage Test: Once wired, capped, and safely buttoned up, turn the breaker on. Set your meter to AC Voltage. Measure between the Common screw and the ground box (should read ~120V). Measure between Throw 1 and ground (should read ~120V when toggled to A, 0V when toggled to B).

Decision Tree: Choosing the Right SPDT Switch for Your Build

Not all SPDT switches are created equal. The internal contact geometry and arc suppression capabilities vary wildly depending on whether you are switching 120V AC lighting, 12V DC battery banks, or 5V logic signals. Use this decision matrix to lock in your exact part number.

Application Scenario Voltage / Current Required Feature Concrete Part Pick
120V AC Home/Shop Lighting (Dual Load) 120V AC / 15A UL Listed, fits standard Decora or toggle wall plate Leviton 5634-2W (15A) or 5635-2W (20A)
12V/24V DC RV or Marine Source Selection 12-24V DC / 50A+ Heavy DC arc suppression, tin-plated copper, ignition protected Blue Sea Systems 9001e (Heavy Duty Battery Switch)
5V/12V DC Arduino or Breadboard Logic 5V DC / <1A PCB mount or breadboard friendly, low contact resistance C&K 7101J16ZQE2 (Miniature PCB Toggle)
240V AC Heavy Appliance Routing 240V AC / 30A Double Pole Double Throw (DPDT) required to switch both hots Hubbell 2623 (30A DPDT Toggle)
Safety Warning: Never use a standard 120V AC wall toggle switch (like the Leviton 5634) to switch DC loads like solar strings or battery banks. DC arcs do not cross zero and will weld the internal contacts of an AC-rated switch shut, creating a severe fire hazard. Always use DC-rated switches with specialized arc chutes for battery/solar work.

Edge Cases, Motor Loads, and NEC Code Caveats

While routing resistive loads (like incandescent lights or heaters) through an SPDT switch is straightforward, inductive loads introduce severe edge cases that can destroy your hardware.

The Motor Horsepower (HP) Rating Trap:
If your 'Load A' is a 1/2 HP dust collector motor or a sump pump, a standard 15A toggle switch is not sufficient, even if the motor's running amperage is well under 15A. When an AC motor starts, it draws Locked Rotor Amperage (LRA) which can be 5 to 7 times the running current. Standard lighting switches are not designed to break this inductive arc and will fail prematurely. According to switch theory and NEC Article 404.14, switches controlling motor loads must carry a specific Horsepower rating. If you are switching motors, you must upgrade to a heavy-duty, HP-rated toggle switch or use the SPDT switch to trigger the coil of a properly sized contactor.

Break-Before-Make vs. Make-Before-Break:
Most standard wall toggle SPDT switches are 'break-before-make'. This means as you flip the toggle, the connection to Throw 1 breaks completely before the connection to Throw 2 is established. This is safe for independent lighting loads. However, if you are using an SPDT switch to select between two audio amplifiers or sensitive data buses, this momentary open circuit can cause voltage spikes or signal drops. In those specific low-voltage electronics scenarios, you must source a 'make-before-break' (shorting) SPDT switch, which briefly connects both throws to the common pole during the transition.

By mapping your physical terminals correctly, verifying your neutral and ground paths bypass the switch mechanism, and selecting a part number rated for your specific AC/DC arc environment, your SPDT wiring will operate reliably for decades. Always defer to your local Authority Having Jurisdiction (AHJ) for final code compliance on permanent branch circuit modifications.