A double pole switch simultaneously interrupts two ungrounded (hot) conductors using a single mechanical toggle. For a standard US residential 240V circuit (such as a baseboard heater or heavy-duty window AC), the direct answer for your materials is a 20A double pole switch (like the Leviton 5295 or Hubbell HBL5295), paired with 12 AWG copper conductors (Black and Red hots), and protected by a 20A double pole breaker. Unlike single pole switches, a double pole switch has no "Line" and "Load" distinction printed on the yoke; the brass terminals are interchangeable in pairs, and the neutral conductor is never switched.
Terminal Mapping and Schematic Symbols
Before tracing the circuit, you must understand the physical device. A standard double pole switch features four brass terminal screws for the hot conductors and one green terminal screw for the equipment grounding conductor. On a schematic diagram, the symbol for a double pole switch looks like two single pole switches drawn side-by-side, with a dashed or solid mechanical linkage line connecting their toggle arms, indicating they open and close simultaneously.
Below is the exact terminal mapping for a 240V load application. Note that because both legs are 120V out of phase, there is no designated "Line" or "Load" side on the switch itself—you can feed power into the top pair and send it to the load from the bottom pair, or vice versa.
| Terminal ID / Color | Physical Location | Wire Color (240V Load) | Wire Color (Dual 120V) | Function & Notes |
|---|---|---|---|---|
| Brass Screw 1 | Top Right | Black (Hot Leg A) | Black (Circuit 1 Hot) | Accepts first ungrounded conductor. Torque to 14 in-lbs. |
| Brass Screw 2 | Top Left | Red (Hot Leg B) | Red (Circuit 2 Hot) | Accepts second ungrounded conductor. Must be on opposite phase. |
| Brass Screw 3 | Bottom Right | Black (Load Leg A) | Black (Load 1 Hot) | Feeds first hot leg to the appliance or fixture. |
| Brass Screw 4 | Bottom Left | Red (Load Leg B) | Red (Load 2 Hot) | Feeds second hot leg to the appliance or fixture. |
| Green Screw | Bottom Center / Yoke | Bare or Green | Bare or Green | Equipment Grounding Conductor (EGC). Bonds switch yoke to ground. |
Assumptions for this table: Copper conductors, 75°C termination ratings, standard US 120/240V split-phase residential power. If using a 12/2 NM-B cable for a 240V load, the white wire must be permanently re-identified with red or black tape at both ends per NEC 200.7(C).
Node-by-Node Trace: 240V Baseboard Heater Circuit
To understand the wiring diagram for a double pole switch in practice, let us trace a 240V, 20A baseboard heater circuit from the panel to the load and back. This trace assumes 12 AWG THHN conductors in EMT conduit.
- Source (Main Panel): A 20A double pole breaker connects to Phase A (Black) and Phase B (Red) on the panel busbars. The neutral busbar is unused for this specific 240V-only load. The ground busbar connects to the bare copper Equipment Grounding Conductor (EGC).
- Feed to Switch Box: The Black, Red, and Bare wires travel through the conduit to the switch gang box.
- Switch Line Terminals: The Black feed wire lands on Brass Screw 1. The Red feed wire lands on Brass Screw 2. The toggle mechanism now controls both legs simultaneously.
- Switch Load Terminals: A second set of Black and Red wires (the load conductors) connect to Brass Screw 3 and Brass Screw 4, respectively. These exit the switch box and travel to the heater.
- The Load (Heater): The Black and Red load wires connect to the heater's internal thermostat or terminal block. When the switch is ON and the thermostat calls for heat, 240V potential is applied across the heating element.
- The Ground Path (Explicit): The EGC (bare copper) never passes through the switch terminals. In the switch box, the incoming EGC and the outgoing EGC are spliced together with a wire nut. A 12 AWG bare copper pigtail is run from this splice to the green grounding screw on the switch yoke. If using a metal gang box, an additional pigtail must bond the splice to the box itself using a 10-32 grounding screw.
If your load requires 120V for control circuitry (like a digital thermostat on a 240V heater), you will have a neutral (white) wire in the box. This neutral is spliced through the box with a wire nut and sent directly to the load. It does not terminate on the double pole switch. The switch only breaks the ungrounded (hot) conductors.
Step-by-Step Installation and Multimeter Verification
Follow these bench-tested steps to terminate the switch and verify the connections before energizing.
- Prep the Conductors: Strip exactly 3/4 inch of insulation from the 12 AWG THHN wires. Do not nick the copper. If using stranded THHN, twist the strands tightly or apply a ferrule to prevent fraying under the screw head.
- Make the Ground Splice: Twist the incoming and outgoing bare ground wires together with a green wire nut. Add a 12 AWG pigtail. Connect the pigtail to the green screw on the switch. Torque to 14 in-lbs.
- Terminate the Hots: Form a clockwise shepherd's hook on the Black and Red wires. Hook them onto the brass screws so the loop closes as the screw tightens. Torque all four brass screws to 14 in-lbs using an insulated torque screwdriver. (Loose connections on 240V circuits cause high-resistance arcing and melted yokes).
- Continuity Test (Power OFF): Set your multimeter to Continuity (Ω). Place one probe on Brass Screw 1 and the other on Brass Screw 3 (same side). Flip the toggle. It should read "OL" (Open Loop) when OFF, and less than 0.5 Ω when ON. Repeat for Screws 2 and 4.
- Isolation Test (Power OFF): With the switch ON, probe Brass Screw 1 to Brass Screw 2. The meter must read "OL". If it beeps, you have a dead short between phases and will trip the breaker violently upon energizing.
- Voltage Verification (Power ON): Safely restore power. Set meter to AC Voltage (V~). Probe Black Line to Red Line: expect 240V (acceptable range 228V-252V). Probe Black Line to Ground: expect 120V. Probe Red Line to Ground: expect 120V. If you read 0V Line-to-Ground but 240V Line-to-Line, your ground path is broken.
Edge Cases: Multiwire Branch Circuits and Handle Ties
A wiring diagram for a double pole switch is also frequently used in Multiwire Branch Circuits (MWBC). An MWBC uses a shared neutral (White) with two hot legs (Black and Red) on opposite phases to supply two independent 120V circuits, such as a split-receptacle kitchen counter.
Under NFPA 70 (National Electrical Code) Article 210.4(B), all ungrounded conductors of an MWBC must be opened simultaneously. If you are switching both halves of an MWBC from a single location, you must use a double pole switch. If you are using two separate single pole switches side-by-side in the same yoke, NEC 210.4(B) requires a recognized handle tie to ensure both circuits are de-energized together for maintenance safety.
| Scenario | Switch Type Required | Neutral Handling | Code Reference |
|---|---|---|---|
| 240V Baseboard Heater | Double Pole Switch | Not present or spliced through | NEC 422.31 |
| MWBC Split Receptacle | Double Pole Switch OR Handle-Tied Single Poles | Shared neutral must be pigtailed, never daisy-chained through device | NEC 210.4 |
| Two Independent 120V Lights | Two Single Pole Switches (Same yoke) | Separate neutrals or pigtailed shared neutral | NEC 404.8 |
When wiring an MWBC to a double pole switch, ensure the Black and Red wires originate from a double pole breaker or handle-tied single pole breakers in the panel. If they are on the same phase leg (e.g., two adjacent single pole breakers without a tie), the shared neutral will carry the additive current of both circuits (up to 40A on a 12 AWG wire), which will melt the neutral conductor and cause a fire. Always verify 240V between the Black and Red hots at the switch box before connecting the load; if you read 0V, they are on the same phase and must be corrected at the panel immediately.
