A Double Pole Single Throw (DPST) switch is the standard control mechanism for breaking two ungrounded (hot) conductors simultaneously. If you are wiring a 240V baseboard heater, a large motor disconnect, or dual-voltage equipment, you need to break both hot legs to ensure the load is completely de-energized. The direct answer for a standard US 240V residential load is simple: wire the two source hots (Line 1 and Line 2) to the top terminals, wire the two load hots (Load 1 and Load 2) to the bottom terminals, and splice the ground wire completely around the switch.

This guide walks through the exact double pole single throw switch wiring diagram, translating schematic symbols into physical terminal mappings, tracing the current path node-by-node, and providing a concrete decision tree to select the right switch rating for your specific amperage.

The DPST Switch Symbol and Physical Terminal Map

Before pulling wire, you must understand what the schematic symbols represent and how they map to the physical brass or silver screws on the device. In electrical schematics, a DPST switch is drawn as two parallel Single Pole Single Throw (SPST) switches connected by a dashed mechanical linkage line. This dashed line indicates that a single physical actuator (the toggle lever) opens or closes both sets of contacts at the exact same time. There is no 'common' terminal on a DPST switch, unlike a 3-way switch.

On the physical device, manufacturers typically stamp the terminals with LINE and LOAD designations, or use L1/L2 and T1/T2. Here is the exact terminal mapping for a standard 240V split-phase circuit using 12 AWG or 10 AWG THHN/NM-B cable.

DPST Terminal and Pin Mapping Table (US 240V Split-Phase)
Terminal Marking Alternate Marking Wire Color (Source) Wire Color (Load) Function
LINE 1 L1 / IN 1 Black (Hot A) N/A Receives first ungrounded conductor from breaker
LINE 2 L2 / IN 2 Red (Hot B) N/A Receives second ungrounded conductor from breaker
LOAD 1 T1 / OUT 1 N/A Black (Hot A) Sends first ungrounded conductor to the equipment
LOAD 2 T2 / OUT 2 N/A Red (Hot B) Sends second ungrounded conductor to the equipment
Ground EGC / Box Lug Bare / Green Bare / Green Equipment Grounding Conductor (Bypasses switch entirely)

Node-by-Node Wiring Trace: Source to Load

To wire this correctly, you must trace the current path from the panel to the load, ensuring the switch only interrupts the ungrounded conductors. Below is the textual node-by-node trace for a 20A, 240V circuit feeding a resistive load (like a baseboard heater).

  1. Node 1: The Breaker Panel. A 2-pole, 20A breaker supplies 240V. The black wire (Hot A) and red wire (Hot B) exit the breaker. The bare copper ground wire exits the ground bar.
  2. Node 2: The Switch Box (Source Side). The 12/2 NM-B (or 12 AWG THHN in conduit) source cable enters the switch box. The black wire lands on the switch's LINE 1 terminal. The red wire lands on the switch's LINE 2 terminal.
  3. Node 3: The Switch Actuator. When the toggle is flipped to 'ON', the internal mechanical linkage bridges LINE 1 to LOAD 1, and LINE 2 to LOAD 2 simultaneously.
  4. Node 4: The Switch Box (Load Side). A second 12/2 cable exits the box toward the load. The black wire lands on LOAD 1. The red wire lands on LOAD 2.
  5. Node 5: The Load Equipment. The black and red wires connect to the equipment's L1 and L2 terminal block.
Polarity and Ground Path Callout: In a standard US 240V split-phase system, there is no neutral wire, meaning polarity between the two hot legs is irrelevant to the load (swapping black and red at the load will not change the operation of a purely resistive 240V heater). However, the ground path must remain continuous and unswitched. You must splice the source ground wire to the load ground wire using a wire nut or a grounding pigtail to the metal box. Never land a ground wire on a switch terminal, and never allow the switch to interrupt the equipment grounding conductor (EGC). Refer to NFPA 70 (NEC) Article 250 for grounding continuity requirements.

Step-by-Step Installation and Meter Verification

Working with 240V circuits carries a severe shock and arc-flash hazard. Follow these numbered steps to install and verify the circuit safely.

  1. De-energize and Lockout. Turn off the 2-pole breaker at the main panel. Apply a lockout/tagout (LOTO) device if others have access to the panel.
  2. Verify Dead. Use a non-contact voltage tester (NCV) on the cables entering the box. Follow up with a CAT III or CAT IV multimeter set to AC Voltage. Place one probe on the black wire and one on the red wire. The meter must read 0V. Place one probe on black and one on the ground wire; it must also read 0V.
  3. Prep the Wires. Strip exactly 3/4 inch of insulation from the 12 AWG conductors using wire strippers. Do not nick the copper.
  4. Splice the Ground. Connect the source bare copper, the load bare copper, and a 6-inch grounding pigtail (if using a metal box) with a yellow or red wire nut. Land the pigtail on the box's ground screw.
  5. Land the Line Conductors. Hook the source black wire clockwise around the LINE 1 screw and torque to the manufacturer's spec (typically 14 in-lbs for standard 20A switches). Hook the source red wire to LINE 2.
  6. Land the Load Conductors. Hook the load black wire to LOAD 1 and the load red wire to LOAD 2.
  7. Bench Verification (Continuity Test). Before pushing the wires into the box, set your multimeter to Continuity (the diode/beep symbol). Place probes on LINE 1 and LOAD 1. Flip the switch ON. The meter should beep and read < 1 ohm. Flip it OFF; it should read 'OL' (Open Loop / infinite resistance). Repeat for LINE 2 to LOAD 2.
  8. Live Verification (Voltage Test). Once mounted and the breaker is turned back on, carefully measure across LOAD 1 and LOAD 2 with the switch ON. You should read between 228V and 252V (nominal 240V). Measure from LOAD 1 to Ground; it should read ~120V. Measure LOAD 2 to Ground; it should also read ~120V.

Decision Tree: Choosing the Right DPST Switch for Your Load

Not all DPST switches are rated for the same loads. A switch rated for 15A resistive might weld its contacts shut if used to start a 15A inductive motor due to inrush current. Use the decision tree below to select the exact part number you need.

DPST Switch Selection Decision Matrix
Load Type & Amperage Required Switch Rating Concrete Part Pick Why This Pick?
Resistive (Baseboard heater, water heater element) < 16A at 240V 20A, 120/277V AC Leviton 1286-1 Standard double-pole toggle, rated for the exact thermal profile of resistive heating elements.
Inductive (Motor, compressor, heavy transformer) 15A - 30A 30A, 600V AC Motor Rated Hubbell HBLDS3 (or equivalent NEMA 3R disconnect) Inductive loads create massive arcs when contacts open. You need a 600V rating and heavy-duty contacts to prevent welding.
High Amperage (> 30A) or frequent daily switching Use a Contactor + Pilot Switch Eaton C25DND240A (40A Contactor) + 15A SPST pilot switch Manual DPST switches overheat and fail mechanically above 30A. Use a low-amp switch to trigger a high-amp contactor coil.

Default Recommendation: If you are wiring a standard DIY 240V baseboard heater or a small 240V shop tool that draws less than 15 amps, terminate your search and buy the Leviton 1286-1. It is a 20A, 120/277V AC double pole single throw toggle switch that fits in a standard single-gang wall box, accepts up to 10 AWG wire, and meets UL listing requirements for residential resistive loads.

Common Wiring Mistakes and Code Caveats

When interpreting a double pole single throw switch wiring diagram, DIYers frequently make three critical errors that violate the National Electrical Code (NEC) or create lethal hazards.

Mistake 1: Using a DPST switch on a 120/240V appliance circuit. Appliances like electric dryers and ranges use a 120/240V circuit, which includes two hot wires and a neutral wire. NEC Article 404.2(B) strictly forbids switching a grounded neutral conductor unless all ungrounded conductors are simultaneously opened. A DPST switch only has two poles. If you try to switch the two hots and leave the neutral connected, you are violating code if the equipment requires a switched neutral (rare in US residential), but more importantly, you cannot use a DPST to switch one hot and the neutral. For 120/240V loads, the switch must either be a 3-pole switch (switching L1, L2, and Neutral) or you must leave the neutral unswitched and only switch the two hots, ensuring the equipment is designed for this.

Mistake 2: Substituting two Single Pole switches for one DPST. Never wire two separate single-pole switches to control the two legs of a 240V circuit. If one switch fails, or if a user only flips one, the equipment remains energized at 120V to ground, creating a hidden, lethal shock hazard. The NEC requires a single actuator to break all ungrounded conductors of a multi-wire branch circuit or 240V load simultaneously. For deeper reading on switch mechanics and multi-pole requirements, consult the switch types documentation at All About Circuits.

Mistake 3: Backstabbing the terminals. Many cheap single-pole switches allow you to push stripped wire into the back of the device (backstabbing). Most high-quality 20A DPST switches do not have backstab holes because the mechanical tension springs cannot reliably hold 10 AWG or 12 AWG wire under high thermal loads. Always wrap the wire clockwise around the terminal screw and torque it down. A loose connection on a 240V circuit will arc, melt the switch housing, and potentially start an electrical fire inside the wall cavity.