The double pole double throw switch symbol represents a 6-terminal electromechanical component that controls two independent circuits simultaneously, with each circuit capable of switching between two distinct paths. In US ANSI/NEMA schematics, it is drawn as two parallel single-pole double-throw (SPDT) switches mechanically linked by a dashed line. In international IEC standards, it appears as two stacked rectangular contact blocks. Whether you are designing a DC motor reversing circuit or wiring a dual-source transfer switch, understanding the exact pinout and regional symbol variations is critical to avoiding dead shorts and equipment damage.
The Complete DPDT Symbol & Pinout Reference Table
Before wiring any switch, you must map the physical terminals to the schematic symbol. The most common physical layout for a standard maintained DPDT toggle switch (such as the industry-standard Carling V-Series or Eaton 7351K) uses a 6-pin configuration. The table below maps the physical pins to their IEC designations and functional roles.
| Terminal Pin | IEC Designation | Function (Position A) | Function (Position B) | Common Practical Application |
|---|---|---|---|---|
| 1 | 11 (Throw 1) | Connected to Common (2) | Open | Motor Forward / Source A Line 1 |
| 2 | 12 (Common 1) | Receives Input/Output | Receives Input/Output | Power Supply Positive / Load Line 1 |
| 3 | 13 (Throw 2) | Open | Connected to Common (2) | Motor Reverse / Source B Line 1 |
| 4 | 21 (Throw 1) | Connected to Common (5) | Open | Motor Forward / Source A Line 2 |
| 5 | 22 (Common 2) | Receives Input/Output | Receives Input/Output | Power Supply Negative / Load Line 2 |
| 6 | 23 (Throw 2) | Open | Connected to Common (5) | Motor Reverse / Source B Line 2 |
Note: The physical pin numbering (1-6) is standard for North American toggle switches, but always verify with a multimeter. The dashed line in the schematic symbol linking Pole 1 (pins 1-2-3) and Pole 2 (pins 4-5-6) indicates mechanical linkage, meaning both poles change state at the exact same time. It does not indicate an electrical bond between the poles.
Regional Standard Variants & Faded Marking Protocols
Schematic symbols for the DPDT switch vary significantly depending on the regional standard governing your documentation. Misinterpreting these symbols is a primary cause of wiring errors in imported machinery or legacy control panels.
| Standard | Region | Visual Representation | Key Identification Feature |
|---|---|---|---|
| ANSI / NEMA | United States, Canada | Two parallel knife-switch or zig-zag contacts | Dashed line linking the two switch arms |
| IEC 60617 | Global, EU, UK (Current) | Two stacked rectangular boxes with internal lines | Mechanical link shown as a dotted line crossing boxes |
| BS 3939 | United Kingdom (Legacy) | Two parallel lines with crossed diagonal throws | Often lacks the explicit mechanical link line |
Safe Interpretation When Markings are Faded or Missing:
On older industrial panels or marine environments, the physical pin numbers stamped into the switch housing often wear away or corrode. Never guess the pinout based on physical position alone, as manufacturers like Carling, Eaton, and Schurter use different internal wafer orientations.
Rows People Get Wrong & Real-World Wiring Pitfalls
When translating the double pole double throw switch symbol into physical wiring, specific rows and concepts from the reference table cause the most field failures.
Misidentifying the Commons (Pins 2 and 5)
The most frequent error is wiring the load to the throw terminals (1, 3, 4, 6) and the source to the commons (2, 5), then assuming the switch disconnects the load entirely when centered (if using a center-off momentary variant). In a standard maintained DPDT, the Commons must be your pivot point. If you are switching two independent 120V AC circuits, the Commons should be the Line (hot) feeds, and the Throws should go to the respective loads. Reversing this feeds voltage back into the switch's internal wiper mechanism, which can cause arcing across the pole barrier if the dielectric breakdown voltage is exceeded.
The Motor Reversing "X" Cross-Over Short
When using a DPDT switch to reverse a DC motor, you must cross the throws: Pin 1 jumps to Pin 6, and Pin 3 jumps to Pin 4. The power source connects to Pins 2 and 5, and the motor connects to Pins 1 and 3. If you fail to cross the throws and instead wire 1-to-4 and 3-to-6, the motor will simply turn off in one position and run in one direction in the other. Worse, if you miswire the source and load on a center-off DPDT, toggling the switch can create a direct dead short across your DC bus, instantly destroying the switch contacts and potentially causing a lithium battery pack to vent if not protected by a properly sized Class-T fuse.
Ignoring Inductive Load Derating
A switch rated for "20A 125VAC" on its bezel is rated for resistive loads (like heaters). If you are using that same DPDT switch to control a 120V AC induction motor (an inductive load), the inrush current and the back-EMF arc upon opening will destroy 20A contacts in weeks. According to Carling Technologies specifications, a 20A resistive toggle typically derates to 3A or 5A for motor (inductive) loads. Always check the datasheet for the specific HP (horsepower) or inductive amp rating, or use the DPDT switch solely as a low-current pilot signal to trigger heavy-duty contactors.
For a deeper dive into how switch types interact with different load profiles, the All About Circuits textbook chapter on switch types provides excellent foundational theory on contact bounce and arc suppression. Always ensure your physical wiring matches the schematic symbol's mechanical linkage assumption, and verify continuity before energizing any newly wired DPDT circuit.
