How to Select and Wire a Switch Double Pole Single Throw (DPST)

A switch double pole single throw (DPST) connects or disconnects two independent circuits simultaneously with a single actuator. If you are switching a standard 120V/240V residential 20A resistive load manually, use a 20A/250V AC rated DPST toggle switch like the Carling 611121. If you are automating a high-inrush motor or building a control panel, step up to a DPST electromechanical relay or contactor like the Omron G7J-2A-B DC24 to safely handle the inductive kick and coil-side control logic.

The DPST Architecture: Manual Switches vs. Electromechanical Relays

In electrical terminology, a 'switch' isn't just a mechanical toggle on a wall. Electromechanical relays and contactors are also switches—they just use a magnetic coil to throw the contacts instead of a human finger. A DPST configuration means there are two distinct current paths (poles) that open or close together (single throw). Both poles are electrically isolated from each other, making DPST ideal for switching both the Line and Neutral of a 120V circuit, or the two hot legs (L1 and L2) of a 240V split-phase circuit.

Understanding whether you need a manual DPST toggle/rocker or an automated DPST relay dictates your wiring strategy, specifically regarding coil voltage and flyback protection.

DPST Rating Table: Which Column Governs Your Load?

The most common mistake DIYers make is looking only at the nominal amperage. A switch rated for '20A' might handle 20A of heating elements perfectly, but will weld its contacts shut if used to start a 20A compressor. Here is how to read the datasheet and determine which rating column governs your specific application.

The Rule: For resistive loads, the standard Contact Rating governs. For inductive and motor loads, the Breaking Capacity (the ability to extinguish the arc when opening under load) or the specific HP rating governs. Never use a standard resistive-rated switch for a motor without a massive derating factor (usually 50% to 80% reduction in allowable current).

Coil vs. Contact Side Wiring (and DC Flyback Protection)

When using an electromechanical DPST relay or contactor, you are dealing with two completely separate circuits: the control circuit (coil) and the load circuit (contacts).

Wiring the Load Side (Contacts)

The contact side handles the heavy lifting. Terminals are typically labeled L1/L2 (Line in) and T1/T2 (Load out). Use wire sized for the load's ampacity (e.g., 12 AWG THHN for a 20A load) and torque the terminal screws to the manufacturer's specification (usually 12-15 in-lbs for standard 10-32 screws) to prevent resistive heating.

Wiring the Control Side (Coil)

The coil side is typically labeled A1 and A2. This is a low-current electromagnet. You can wire A1 to your control voltage (e.g., 24VDC from a PLC or thermostat) and A2 to the control ground. Polarity generally does not matter for AC coils, but for DC coils, A1 is usually positive and A2 is negative.

Load-Specific Selection Decision Path

Use this decision tree to select the exact component for your workbench or jobsite. Follow the logic down to your concrete part pick.

Note: The Omron G7J series is the industry workhorse for general-purpose automated DPST switching. If your application falls into a gray area, this is the safest default recommendation due to its robust 25A contact rating and integrated arc barriers.

Testing Dead and Live: Troubleshooting DPST Switches

When a circuit fails, you need to determine if the DPST switch is the culprit. Grab your multimeter and follow this sequence.

1. Testing Dead (De-energized Continuity)

Lock out the breaker and verify zero voltage. Set your multimeter to Continuity or Ohms (Ω).

• Open State (OFF): Place probes across L1 and T1, then L2 and T2. The meter must read 'OL' (Open Loop) or infinite resistance. Any reading below 1 MΩ indicates internal carbon tracking or moisture.
• Closed State (ON or Coil Energized): Actuate the switch (or manually press the relay contactor armature with an insulated tool). The meter should read < 0.5 ohms across both pole pairs. If you read > 2 ohms, the contacts are pitted, oxidized, or failing.

2. Testing Live (Energized Voltage Drop)

If continuity tests pass but the load still fails, test for voltage drop under actual operating load. Set your meter to AC or DC Volts (matching the load).

• Place one probe on L1 and the other on T1 while the circuit is running.
• A healthy switch will show a voltage drop of < 0.1V.
• If you read a voltage drop of 1V to 5V+ across the closed contacts, the switch is dissipating power as heat (P = V × I). A 3V drop at 15A means the switch is burning 45 watts internally. It is failing and must be replaced immediately.

Repair vs. Replace: When to Swap the Component

There is a persistent myth in older trade circles that you can 'repair' a pitted contactor by filing down the contacts. Do not do this.

Modern DPST relays and contactors use silver-alloy or silver-cadmium oxide contact tips. These materials are specifically chosen to resist arc erosion and prevent welding. If you take a file or sandpaper to a pitted contact, you strip away the thin silver alloy layer, exposing the base copper. Copper oxidizes rapidly under arcing conditions, creating a high-resistance joint that will overheat and potentially cause an enclosure fire.

When a DPST switch shows signs of contact welding (fails to open when de-energized), severe pitting, coil burnout (smells like ozone/melting plastic), or fails the live voltage drop test, discard it. When in doubt, replace the entire component with the Omron G7J-2A-B DC24 for panel automation or the Carling 611121 for manual enclosures. Never attempt to repair sealed electromechanical switches.

For deeper reading on relay contact materials and arc suppression, refer to the All About Circuits guide on relays, and always ensure your permanent wiring complies with the latest NFPA 70 (National Electrical Code) standards for your jurisdiction.