Double Pole Double Throw Switch Diagram: Terminals, Wiring, and Applications

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A double pole double throw switch diagram shows the six terminals of a DPDT switch — two poles each with a common and two throw positions — used to reverse motor polarity, switch between two supplies, or control two independent circuits simultaneously.

A double pole double throw (DPDT) switch is a six-terminal electromechanical switch that simultaneously operates two independent single-pole double-throw (SPDT) switch sections from a single actuator. Understanding the DPDT diagram requires identifying the three terminals per pole: the common (COM or C), the normally-closed throw (NC or throw 1), and the normally-open throw (NO or throw 2). The switch toggles between connecting each COM to its NC throw (position 1) and connecting each COM to its NO throw (position 2).

The six terminals are arranged symmetrically: the two COM terminals are typically in the centre column, with the NC throws on one side and the NO throws on the other (or upper/lower, depending on the switch orientation). In a toggle or slide DPDT, the mechanical actuator moves both sets of contacts simultaneously, ensuring both poles always switch in exact unison.

The most electrically important property of the DPDT is its ability to reverse current direction through a load by cross-connecting the two poles. This is the basis of the DC motor direction reversal circuit: the motor is connected between the two COM terminals, and the supply positive and negative are cross-wired through the throws so that throwing the switch in one direction feeds current through the motor in the forward direction, and in the other direction reverses the current path and thus the motor rotation. This circuit is the simplest form of H-bridge.

In mains AC wiring, DPDT switches are used as two-way changeover switches to select between two independent supply sources (e.g., main supply and emergency generator feed) for a panel, where simultaneous single-pole switching alone cannot provide safe isolation and changeover. The DPDT ensures both line and neutral switch simultaneously, maintaining correct polarity on the supply.

DPDT relays (where the DPDT contacts are controlled by an electromagnetic coil rather than a manual actuator) are extensively used in control circuits and automation. The coil circuit is independent of the contact circuit, providing galvanic isolation between the control signal and the switched circuit.

DPDT switches are rated by contact voltage and current. For mains AC applications, use switches rated for the AC supply voltage and the load current. For DC switching, note that DC arcs are harder to extinguish than AC arcs at the same voltage, and the switch's DC rating may be lower than its AC rating.

How to wire double pole double throw switch diagram

Identify the six DPDT terminals on the switch Label the two common terminals (COM1 and COM2), the two normally-closed throw terminals (NC1 and NC2), and the two normally-open throw terminals (NO1 and NO2). Use a multimeter in continuity mode with the switch in each position to verify which terminals connect in each position — do not rely solely on silk-screen labels, especially on unbranded components.
Determine the application and draw the wiring diagram For motor reversal: motor between COM1 and COM2; positive supply to NC1 and NO2; negative supply to NC2 and NO1. For supply changeover: supply A line to NC1, supply B line to NO1; supply A neutral to NC2, supply B neutral to NO2; load line from COM1, load neutral from COM2. Draw the full circuit before wiring.
Verify switch ratings against the circuit requirements Check the switch's rated voltage (AC and/or DC), rated current, and duty type (resistive or inductive). For inductive loads (motors, solenoids, relays), apply a derating factor — an inductive load causes a higher arc energy at contact opening than a resistive load at the same current. A switch rated 10 A resistive may be derated to 3 A for inductive loads.
Connect the supply or signal inputs to the throw terminals In supply changeover: connect source A conductors to the NC throws and source B conductors to the NO throws. This means source A is active when the switch is in the default (un-actuated) position, and source B is active when the switch is actuated. Label the sources clearly on the switch panel.
Connect the loads or output signals to the common terminals The common terminals carry the switched output to the load. In a motor reversal circuit, the motor leads connect here. In a supply changeover, the distribution panel feed connects here. Ensure wire gauges and termination methods are appropriate for the current rating.
Test the switching action before applying load current With no supply connected, use a multimeter in continuity mode to verify that each COM connects to the correct throw in each switch position, and that there is no continuity between the two poles (no cross-connection between pole 1 and pole 2 terminals). Apply supply voltage and test the load function in both switch positions.

Specifications

Number of terminals	6 (2 × COM, 2 × NC, 2 × NO)
Number of switch positions	2 (ON-ON) or 3 for centre-off variant (ON-OFF-ON)
Typical AC contact rating	125–250 V AC, 5–15 A (switch-specific)
Typical DC contact rating	28–30 V DC, 3–10 A (switch-specific; always lower than AC rating at the same current)
Contact type options	Break-before-make (BBM) or Make-before-break (MBM) — specify for application

Safety warnings

All DPDT switch installations in mains AC circuits must comply with the applicable national wiring regulations (NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000) and must be carried out by a licensed electrician. Always isolate and verify dead before working on any mains-connected wiring.
Do not use a DPDT switch rated for AC mains voltage to switch high DC voltages without verifying the switch's DC voltage and current rating. DC arcs are more persistent than AC arcs and the DC contact rating is typically lower than the AC rating.
In supply changeover applications, verify that the switch is break-before-make type. A make-before-break switch will momentarily parallel two supply sources during switching, which can cause a short circuit if the sources are out of phase or at different voltages.
For inductive loads, apply derating to the switch current rating. Relay and motor switching generates voltage spikes that accelerate contact erosion. Use spark suppression (RC snubber across contacts for AC loads; flyback diode for DC inductive loads) to extend contact life.

Tools needed

Digital multimeter with continuity and resistance functions
Insulated screwdrivers
Wire strippers and crimping tool
Approved voltage indicator (for mains applications)

Common mistakes

Connecting the motor across one pole's COM and throw terminals instead of across the two COM terminals, which only gives one rotation direction regardless of switch position.
Using a single-pole double-throw (SPDT) switch where the circuit diagram requires a DPDT, resulting in only one of the two poles being switched.
Using a make-before-break DPDT in a supply changeover circuit, causing two supply sources to be momentarily connected in parallel during switching.
Exceeding the switch's DC current or voltage rating by applying the AC rating to a DC circuit without checking the DC specification.
Connecting the normally-closed and normally-open throws incorrectly, reversing the switch action so the motor runs backwards in the 'forward' position.

Troubleshooting

Motor runs in only one direction regardless of switch position: Cause: Motor is wired to one COM and one throw terminal of the same pole, rather than between the two COM terminals. Fix: Isolate the circuit. Verify that the motor leads connect to COM1 and COM2. Rewire if necessary so the motor bridges the two common terminals.
Switch becomes hot during motor operation: Cause: Motor current exceeds the switch's rated contact current for the load type (especially if the motor is inductive and the switch is rated for resistive loads only). Fix: Check the switch's rated current for inductive DC loads. Replace with a higher-rated switch or use the DPDT switch to control a relay or contactor whose contacts are rated for the motor current.
Supply changeover circuit has no output in one switch position: Cause: One set of throw terminals is not making contact in one switch position — worn or dirty contacts, or the switch actuation mechanism is not completing the throw fully. Fix: Use a multimeter to test continuity between the COM and each throw in each switch position. Clean contacts with contact cleaner if accessible, or replace the switch if contacts are worn.

Frequently asked questions

What is the difference between a DPDT and a SPDT switch?

A single pole double throw (SPDT) switch has three terminals: one common and two throws. It switches one circuit between two paths. A DPDT switch is effectively two SPDT switches sharing one actuator — six terminals, two commons and four throws. It switches two independent circuits simultaneously between two paths each, making it capable of motor reversal and supply changeover applications that a SPDT cannot achieve alone.

How does a DPDT switch reverse a DC motor?

Connect the motor between the two COM terminals. Wire the supply positive to one throw on pole 1 and the supply negative to the corresponding throw on pole 2. Cross-wire the remaining throws: supply positive to the throw on pole 2 and supply negative to the throw on pole 1. In position 1, current flows through the motor in one direction; in position 2, both poles switch and current flows in the reverse direction, reversing motor rotation.

Can a DPDT switch be used to switch mains voltage?

Yes, provided the switch is rated for the mains voltage and load current. A mains-rated DPDT switch can simultaneously switch the live and neutral conductors, providing true double-pole isolation of the load. Verify the switch's AC voltage rating, its rated load current, and its short-circuit withstand rating before using it in a mains application.

What is a centre-off DPDT switch?

A centre-off DPDT (sometimes called a 3-position DPDT or ON-OFF-ON switch) has a central position in which both sets of contacts are open — neither pole is connected to either throw. This allows the switch to be placed in an off state between the two active positions, useful for motor control where a positive off position is required between forward and reverse.

What does it mean for a DPDT switch to be break-before-make versus make-before-break?

Break-before-make (BBM): the common disconnects from the current throw before connecting to the new throw. There is a momentary open-circuit state. This prevents simultaneous connection of both throws, which is required in supply changeover applications where connecting two supplies together would cause a short circuit. Make-before-break (MBM): the common briefly connects both throws simultaneously. This maintains signal continuity during switching, useful in analogue signal routing where a momentary interruption would cause noise.