A Double Pole, Double Throw (DPDT) switch symbol represents a single mechanical actuator controlling two independent circuits, each with two selectable paths. In US schematics (IEEE 315/NEMA), it appears as two Single Pole Double Throw (SPDT) switches linked by a dashed line, while IEC standards often use a multi-pole block or interconnected contact grid. Physically, you are working with 6 terminals: two commons (poles), two normally open (NO), and two normally closed (NC).
DPDT Switch Symbol & Terminal Pinout Reference
The table below maps the physical terminals of a standard 6-pin DPDT toggle or rocker switch (like the common Carling or C&K models) to their schematic representations. This is your master reference for translating a 2D drawing into a 3D wiring job.
| Terminal ID | Function | IEEE 315 / NEMA Symbol | IEC 60617 Symbol | Practical Wiring Role |
|---|---|---|---|---|
| 1 & 4 | Commons (Poles) | Center pivot point of the wiper arm | Input side of the contact block | Connects to power source (Line) or load center. The 'moving' contact. |
| 2 & 5 | Normally Open (NO) | Top or bottom fixed contact (depending on draw) | Upper/Lower output path in block | Load A / Direction 1. Energized only when actuator is thrown to this position. |
| 3 & 6 | Normally Closed (NC) | Opposite fixed contact from NO | Opposite output path in block | Load B / Direction 2. Energized in the default or opposite throw position. |
| N/A | Mechanical Link | Dashed line connecting the two wiper arms | Solid line or shared actuator block | Not a physical wire. Indicates both poles move simultaneously. |
Regional & Standard Variants: NEMA, IEC, and Legacy UK
Reading a DPDT switch symbol correctly requires knowing which drafting standard the engineer used. Misinterpreting a regional variant can lead to wiring a motor in reverse or shorting a control circuit.
- IEEE 315 / NEMA (US & Canada): The most common format in North American residential, commercial, and hobbyist schematics. It uses a literal, pictorial representation. You will see two distinct SPDT switch symbols drawn side-by-side or stacked, with a dashed mechanical linkage line connecting their actuator arms. This standard prioritizes visualizing the physical movement of the contacts.
- IEC 60617 (Europe & Global Industrial): Preferred in PLC programming, industrial automation, and modern European wiring diagrams. IEC symbols abstract the physical switch into a standardized grid or block format. A DPDT switch is shown as a single rectangular block with intersecting lines representing the contact matrix, or as two parallel contact lines with a shared solid actuator line. It prioritizes logical circuit flow over physical geometry.
- BS 3939 (Legacy UK): Largely superseded by IEC standards (specifically BS EN 60617), but you will still encounter BS 3939 symbols when retrofitting or troubleshooting 1970s and 1980s British industrial control panels. It looks similar to early IEC but uses distinct diagonal slash marks to indicate the number of poles, which can easily be confused with hash marks denoting wire counts in modern diagrams.
Symbols and Rows People Get Wrong
The most common bench mistake when reading an IEEE/NEMA DPDT switch symbol is treating the dashed line connecting the two poles as an electrical jumper. That dashed line represents mechanical linkage only. If you solder a wire between the two commons because you thought the dashed line indicated a bus connection, you will tie two independent circuits together, potentially causing a dead short or destroying a sensitive H-bridge motor driver.
Confusing DPDT with DPST: A Double Pole, Single Throw (DPST) symbol only has one fixed contact per pole (4 terminals total). It acts like a standard light switch for two separate lines. If your schematic shows the wiper arm able to pivot to two distinct fixed contacts, it is a DPDT (6 terminals). Always count the physical connection points on the symbol.
Momentary vs. Maintained: A standard DPDT symbol implies a 'maintained' position (the switch stays where you leave it). If the symbol includes a small spring graphic or a return arrow next to the actuator arm, it is a momentary DPDT (like a guitar effect pedal switch). Wiring a momentary switch into a circuit designed for a maintained latch will result in the circuit dropping out as soon as you release the button.
Safe Interpretation: Mapping Faded or Unmarked Terminals
On the jobsite or at the bench, you will frequently encounter DPDT toggle switches where the 1-6 terminal markings have been obscured by heat shrink, solder flux, or decades of grime. Never guess the pinout based on physical position alone, as manufacturers like C&K, Carling, and Switchcraft sometimes mirror their pinouts.
Use a digital multimeter (DMM) in continuity mode to map the switch safely while it is completely disconnected from any power source.
- Set your DMM to continuity (the setting that beeps when probes touch). Verify it works by touching the probes together.
- Identify the Poles (Commons): Place one probe on a random pin. Move the second probe across the remaining 5 pins. Toggle the switch. The pin that shows continuity with your first probe in both switch positions is one of the Commons (Pole 1). Repeat to find Pole 2.
- Identify NO and NC: Keep one probe on Pole 1. Find the pin that beeps when the switch is in Position A. Label this NC (or NO, depending on your preferred default state). Toggle the switch. The pin that now beeps is the opposite throw. Mark them clearly with a paint pen.
- Verify Isolation: Ensure there is zero continuity between Pole 1's cluster and Pole 2's cluster. If there is continuity between the poles, the switch is internally shorted or is actually a specialized crossover switch, not a standard DPDT.
DPDT Switch Symbol FAQ
How do I read a DPDT switch symbol on a complex schematic?
Start by locating the mechanical linkage line (dashed in IEEE, solid/block in IEC). Trace the 'Common' or 'Pole' line back to the power source or input signal. Then, follow the two 'Throw' lines forward to their respective loads. In complex motor control schematics, a DPDT is often used as a polarity reverser; in this case, the NO and NC terminals of Pole 1 will cross over to the NC and NO terminals of Pole 2 to flip the DC voltage across the motor leads.
What is the exact difference between DPDT and DPST switch symbols?
A DPST (Double Pole, Single Throw) symbol shows two independent switches that open or close a single path each (4 terminals total). It is functionally identical to a standard 240V residential double-pole breaker or a heavy-duty appliance switch. A DPDT (Double Pole, Double Throw) symbol shows the wiper arm choosing between two different paths for each pole (6 terminals total). DPST routes power on/off; DPDT routes power between two different destinations or reverses polarity.
What does the dashed line mean in a US DPDT switch symbol?
In IEEE 315 and NEMA standards, the dashed line connecting the two switch poles indicates a mechanical linkage. It tells the technician that both sets of contacts are physically mounted on the same actuator mechanism and will change state at the exact same millisecond. It is strictly a mechanical notation, not an electrical one, and should never be wired as a jumper.
How do I identify a center-off DPDT symbol?
A standard DPDT is an ON-ON switch. A center-off DPDT (ON-OFF-ON) will be drawn with a third, middle position where the wiper arm does not touch either fixed contact. In IEC block diagrams, this is shown as a three-position switch actuator with a distinct 'open' gap in the center of the contact matrix. Physically, these switches have a spring-loaded or detent center position and are heavily used in DC motor control to provide a hard stop between forward and reverse throws.
