A Double Pole Double Throw (DPDT) switch features six terminals: two inner "Commons" and four outer "Throws." When learning how to wire a dpdt switch for a 120V AC dual-source transfer (switching a hardwired load between Utility grid power and a Backup Generator/Inverter), the rule is simple: The Load wires always land on the two middle Common terminals, while the Utility and Generator source wires land on the outer Throw terminals. This ensures complete isolation between the two power sources, preventing dangerous backfeeding and satisfying NEC requirements for switching both the hot and neutral conductors of a separately derived system.

Decision Tree: Picking the Exact DPDT Switch for Your Load

Not all toggle switches are built for mains voltage. A switch rated for 20A DC will arc and weld its contacts if used on a 120V AC inductive load. Use this decision matrix to select the correct hardware.

Decision Criteria If Your Application Is... Then You Need...
Voltage & Current 120V AC, up to 15A/20A continuous load 20A, 120/277V AC rating (Horsepower rated preferred)
Switching Action Selecting between two persistent power sources Maintained contact (stays in position when released)
Poles Required 120V single-phase (must switch Hot and Neutral per NEC 230.83) 2 Poles (DPDT)
Enclosure Type Indoor, standard wall or junction box mount Standard single-gang strap or panel-mount depending on box
The Concrete Pick: Based on the matrix above, the default, code-compliant choice for a standard indoor 120V/20A transfer circuit is the Leviton 3032 (20A, 120/277V AC, DPDT Maintained Toggle Switch). It features heavy-duty silver-alloy contacts designed to handle the inrush current of motors and transformers without welding. You can verify its specifications on the official Leviton 3032 product page.

Tools, Materials, and Device Ratings

Do not mix wire gauges on a 20A circuit. The Leviton 3032 terminals accept up to 10 AWG, but for a standard 120V/20A branch circuit, 12 AWG is the correct, code-compliant size.

  • Switch: Leviton 3032 (20A, 120/277V AC, DPDT)
  • Wire: 12 AWG THHN/THWN-2 in Black, White, Red, Gray, and Green. (Using Gray for the generator neutral is a pro-tier NEC 200.6(D) workaround to distinguish it from the utility white neutral).
  • Box: Deep single-gang metal device box (minimum 22.5 cubic inches to accommodate six 12 AWG conductors plus grounds).
  • Tools: Klein 11055 wire strippers, Wiha torque screwdriver (set to 14 in-lbs), Fluke 117 Digital Multimeter, Non-Contact Voltage Tester (NCVT).
  • Connectors: Ideal 341 Orange wire nuts (rated for three 12 AWG wires) for the ground pigtail.

Mains Safety and Lockout Procedure

CRITICAL HAZARD: DUAL-SOURCE SHOCK RISK
This procedure involves TWO independent mains voltage sources (Utility and Generator/Inverter). Turning off the main utility breaker is NOT enough. The generator or inverter can backfeed lethal voltage into the panel.
1. De-energize the Utility branch circuit breaker.
2. De-energize the Generator/Inverter output breaker.
3. Apply Lockout/Tagout (LOTO) to both breakers.
4. Verify dead at the wire ends using a tested NCVT, then confirm 0V AC with a multimeter between all hot conductors and ground.
Note: NEC-style guidance is provided here; your local AHJ (Authority Having Jurisdiction) has final authority on transfer switch installations.

Step-by-Step Wiring Procedure

For this installation, we are switching both the Hot and the Neutral. This is mandatory if your generator/inverter is a "separately derived system" with its own neutral-to-ground bond, as it prevents neutral current from looping through the utility ground path. For a comprehensive breakdown of separately derived system rules, refer to this EC&M guide on NEC transfer switch requirements.

  1. Grounding First: Connect the bare copper Equipment Grounding Conductors (EGC) from the Utility, Generator, and Load together in the back of the metal box using an Orange wire nut and a green grounding pigtail to the box itself. The DPDT switch does not switch the ground; grounds remain continuously bonded.
  2. Utility Hot: Strip 3/4" of insulation from the Utility Hot (Black) wire. Terminate it under the Top Left screw terminal (Throw 1). Torque to 14 in-lbs.
  3. Generator Hot: Strip 3/4" from the Generator Hot (Red) wire. Terminate it under the Bottom Left screw terminal (Throw 2). Torque to 14 in-lbs.
  4. Load Hot: Strip 3/4" from the Load Hot (Black) wire. Terminate it under the Middle Left screw terminal (Common 1). Torque to 14 in-lbs.
  5. Utility Neutral: Strip 3/4" from the Utility Neutral (White) wire. Terminate it under the Top Right screw terminal (Throw 3). Torque to 14 in-lbs.
  6. Generator Neutral: Strip 3/4" from the Generator Neutral (Gray) wire. Terminate it under the Bottom Right screw terminal (Throw 4). Torque to 14 in-lbs.
  7. Load Neutral: Strip 3/4" from the Load Neutral (White) wire. Terminate it under the Middle Right screw terminal (Common 2). Torque to 14 in-lbs.
Torque Matters: Per NEC 110.14(D), terminals must be tightened to the manufacturer's specified torque. The NFPA explicitly highlights that under-torqued connections cause thermal failures under continuous load. Do not guess; use a calibrated torque screwdriver.

Verify and Test Sequence

Before applying mains power, perform a dead-circuit continuity test to ensure the mechanical linkage is routing power correctly.

  1. Continuity Check (Power OFF): Set your Fluke 117 to Continuity/Ohms. Place probes on the Left Common (Load Hot) and Top Left (Utility Hot). Toggle the switch UP. Expected reading: < 1.0 Ohm. Toggle DOWN. Expected reading: OL (Open Loop).
  2. Crossover Verification: Place probes on Left Common and Bottom Left (Gen Hot). Toggle UP. Expected: OL. Toggle DOWN. Expected: < 1.0 Ohm.
  3. Energize Utility: Remove LOTO from the Utility breaker only. Turn it ON. Toggle the DPDT switch UP. Measure between Load Hot and Load Neutral at the device end. Expected reading: 114V to 126V AC.
  4. Energize Generator: Turn OFF the Utility breaker. Start the Generator/Inverter and turn on its output breaker. Toggle the DPDT switch DOWN. Measure at the load. Expected reading: 114V to 126V AC.

The Most Common Botch (And How to Fix It)

The Botch: Wiring the Source feeds (Utility and Generator) to the middle Common terminals, and wiring the Load to the outer Throw terminals.

The Symptom: The switch appears to do nothing, or worse, it creates a direct short or backfeeds the generator when the utility is active. Because the Commons are internally tied to the toggle mechanism, feeding power into the Commons means power is always present on one set of Throws. If the Load is on the Throws, you lose the ability to isolate the sources, and the switch simply acts as a confusing routing matrix rather than a transfer mechanism.

The Fix: Remember the phrase "Load in the Middle, Sources on the Outside." The Commons (middle screws) must always feed the destination (the Load). The Throws (outer screws) receive the incoming options (Utility and Generator). If you have already wired it backward, de-energize both sources, pull the switch out of the box, and swap the Load Black/White wires with the Utility Black/White wires to restore proper isolation.