SPDT Switch Diagram

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A clear reference diagram for single-pole double-throw (SPDT) switches showing COM, NO, and NC terminal connections for switching, changeover, and two-location control circuits.

A Single-Pole Double-Throw (SPDT) switch controls one circuit input (pole) and connects it to one of two possible output positions (throws). The three terminals are:

- COM (Common): the input terminal that carries the incoming signal or supply voltage. - NO (Normally Open): the output terminal that is disconnected from COM when the switch is in its rest (un-actuated) state. It connects to COM when the switch is actuated. - NC (Normally Closed): the output terminal that is connected to COM when the switch is in its rest state. It disconnects from COM when the switch is actuated.

At any moment, the COM is connected to either NO or NC — never both simultaneously, and never open to both. This makes the SPDT an inherently changeover switch: pressing/toggling/sliding the actuator transfers the COM connection from NC to NO.

SPDT switches appear in an enormous range of applications. In a simple lighting changeover, COM connects to the supply, NC feeds one lamp (on when switch is at rest), and NO feeds a second lamp (on when switch is actuated). This creates an OR-style switching pattern without needing separate switches.

In a two-location (two-way) lighting circuit — common in staircase or corridor lighting — two SPDT switches (each called a 'two-way switch' in UK/European wiring terminology) are connected with their COM terminals to the supply and the lamp, and their NO and NC terminals cross-connected ('strapping') between the two switches. Either switch can toggle the lamp between on and off regardless of the other switch's position.

In relay and control circuits, the SPDT output is used to simultaneously energise one load and de-energise another — for example, switching between two motor directions, two signal sources, or two operating modes.

In electronic circuits, SPDT switches appear as slide switches, toggle switches, and signal routing switches (SPDT analogue switches in ICs). The same COM/NO/NC logic applies regardless of the physical form.

How to wire spdt switch diagram

Identify the three terminals on the switch Examine the switch body for terminal markings: COM (or C, or a single terminal at one end or centre), NO, and NC. On toggle switches, the COM is often the centre terminal of three. On PCB switches, refer to the datasheet pinout diagram. Use a multimeter in continuity mode to verify: with switch un-actuated, continuity should be present between COM and NC only.
Determine the required switching function Decide what the switch must do: simple on/off (use COM and NO or COM and NC only), changeover between two loads (use COM, NO, and NC), or two-way control (two-switch staircase circuit). The required function determines how all three terminals are connected.
Size the switch for the load Verify the switch voltage rating exceeds the circuit voltage. Verify the current rating exceeds the maximum load current. For inductive loads, apply a derating factor or select a switch with an inductive current rating. Check that the switch's duty cycle rating is appropriate for the switching frequency required.
Wire COM to the supply conductor For mains-voltage switching, always switch the live (L) conductor — never switch the neutral. Connect the live conductor to the COM terminal. Connect the load's live input to the NO terminal (for a normally-open circuit) or NC terminal (for a normally-closed circuit). Return the neutral directly to the load.
Connect NO and NC to load outputs (changeover application) For a changeover application, connect the first load to NO and the second load to NC. The common neutral of both loads returns directly to the supply neutral. The switch alternates the live supply between the two loads.
Wire two-way staircase circuit (if applicable) Connect the live supply to COM of switch 1. Connect the lamp's live input to COM of switch 2. Run two strapper wires between the switches: NO of switch 1 to NO of switch 2, and NC of switch 1 to NC of switch 2. Return neutral directly from the lamp to supply neutral. Test both switch positions operate the lamp.
Verify, test, and insulate Before applying power, verify all connections with a multimeter: continuity between COM and NC (switch un-actuated), continuity between COM and NO (switch actuated). After applying power, test all operating positions. Ensure all live terminals are insulated or enclosed — SPDT switches used at mains voltage must be housed in appropriate enclosures.

Specifications

Switch configuration	Single-Pole Double-Throw (SPDT); 3 terminals: COM, NO, NC
Concurrent connections	COM always connected to either NO or NC; never open to both, never to both simultaneously
Typical mains-voltage switch current rating	6 A, 10 A, 16 A, or 20 A (resistive); lower for inductive loads
Typical mains-voltage switch voltage rating	250 V AC (single-phase) or 415 V AC (three-phase, where applicable)
Low-voltage DC application range	5 V to 48 V DC (signal or logic); current per switch specification
Two-way circuit strapper wires required	2 (NO-to-NO and NC-to-NC between the two switches)
Applicable installation standard	BS 7671 (UK), NEC (USA), AS/NZS 3000, SANS 10142 (South Africa), IEC 60364
Terminal identification	COM (or C), NO (Normally Open), NC (Normally Closed)

Safety warnings

When using an SPDT switch in mains voltage circuits, always switch the live (Line/Phase) conductor. Never switch only the neutral — switching neutral alone leaves the load's internal wiring live and creates an electric shock hazard during lamp or appliance servicing.
For mains-voltage installations, all wiring must comply with the applicable wiring rules for the jurisdiction: BS 7671 (UK and many international markets), NEC (USA), AS/NZS 3000 (Australia/New Zealand), SANS 10142 (South Africa), or IEC 60364 (international). Fixed mains wiring must be performed by a licensed electrician in most jurisdictions.
Verify the switch's rated current and voltage before installation. Using a switch rated below the load current or circuit voltage can cause switch failure, contact welding, arcing, and fire.
For two-way (staircase) wiring using conductor colours and cable types, comply with the applicable wiring standard's identification requirements. Misidentified strapper wires are a common source of dangerous reverse wiring and unexpected live conductors.
SPDT switches used for low-voltage signal or logic circuits must not be interchanged with mains-rated switches. Low-voltage switches are not rated to safely interrupt mains currents and voltages.

Tools needed

Digital multimeter (continuity and resistance for terminal identification and circuit verification)
Wire stripper
Screwdrivers (flat and Phillips for terminal screws and switch plate installation)
Cable labels (for strapper wires in two-way circuits)
Non-contact voltage tester (verify circuit is dead before working on mains wiring)

Common mistakes

Switching the neutral conductor instead of the live: this leaves the load energised at the phase conductor even when the switch is open, creating a shock hazard.
Confusing NO and NC terminals: in a normally-open circuit, wiring to NC instead of NO gives a circuit that is on at rest and off when actuated — the opposite of the intended behaviour.
Using an undersized switch for inductive loads: inductive current (motors, solenoids, transformer primaries) at make and break exceeds resistive current ratings; always use the switch's inductive load current rating for inductive loads.
Incorrect strapper wire identification in two-way circuits: if the two strapper wires are transposed at one switch, the two-way circuit will have one position where both switches disconnect the lamp and no position where both positions are usable.
Leaving un-used NO or NC terminals uninsulated in mains circuits: exposed live terminals are a shock hazard; cap or insulate unused terminals.

Troubleshooting

Load does not operate in any switch position: Cause: Open circuit in COM conductor, failed switch contact, or load itself failed Fix: With supply isolated, verify continuity from supply to COM terminal. With a multimeter in continuity mode, test COM-to-NC (switch at rest) and COM-to-NO (actuated). An open reading in all positions indicates a failed switch.
Load is always on regardless of switch position: Cause: Switch contacts welded (shorted), wiring bypasses the switch, or NC and NO both connected to the same load Fix: With supply isolated, test switch contacts with a multimeter — all positions should show resistance in at least one configuration. A zero-ohm reading across all positions indicates welded contacts; replace the switch.
Two-way circuit: one switch works, other has no effect: Cause: Strapper wires crossed or one strapper wire open-circuit Fix: Isolate supply. Verify both strapper wires have continuity end-to-end. Verify strapper wires connect NO-to-NO and NC-to-NC, not NO-to-NC (which gives a circuit that only works from one switch in some positions).

Frequently asked questions

What is the difference between SPST, SPDT, DPDT, and DPST switches?

SPST (Single-Pole Single-Throw) is a simple on/off switch: one input, one output. SPDT (Single-Pole Double-Throw) has one input and two outputs (changeover). DPST (Double-Pole Single-Throw) is two SPST switches sharing one actuator — both open or both close together. DPDT (Double-Pole Double-Throw) is two SPDT switches sharing one actuator, breaking two circuits simultaneously to two alternative outputs.

How does a two-way lighting circuit use two SPDT switches?

Each switch has its COM connected to the supply or lamp, respectively. The NO terminal of switch 1 connects to the NO terminal of switch 2 (one strapper wire). The NC terminal of switch 1 connects to the NC terminal of switch 2 (second strapper wire). When both switches are in the same position, current flows through matched terminals; when switches are in opposite positions, no circuit is completed. Either switch toggles the lamp.

What does 'normally open' and 'normally closed' mean in practice?

'Normal' refers to the rest (un-actuated) state of the switch — when no force, signal, or user action is applied. NO means the circuit is open (no current flows) at rest. NC means the circuit is closed (current flows) at rest. When the switch is actuated, NO becomes closed and NC becomes open, simultaneously.

Can I use an SPDT switch as an SPST switch?

Yes. Simply leave either the NO or NC terminal unconnected (or connected to ground/common in signal circuits). Using COM and NO gives you a normally open (on when actuated) switch. Using COM and NC gives you a normally closed (on until actuated) switch. The unused terminal carries no current and causes no harm.

What rating should I look for on an SPDT switch for mains voltage use?

The switch must carry a voltage rating equal to or exceeding the mains supply voltage (e.g., 250 V AC for European installations) and a current rating equal to or exceeding the maximum load current. For inductive loads (motors, solenoids), look for a separate inductive current rating (typically lower than resistive), as switching inductive loads generates contact-damaging arcs.

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