The symbol of an SPDT (Single Pole, Double Throw) switch represents a single input contact that mechanically bridges to one of two output contacts. In IEEE/ANSI schematics, it is drawn as a single pivoting blade breaking and making contact between two fixed points (break-before-make). In IEC standards, it appears as a rectangular block with a diagonal line indicating the movable wiper. Physically, an SPDT switch has three terminals: one Common (COM) and two switched paths (Normally Open/Normally Closed in DC logic, or Travelers/Strappers in AC mains wiring).

The Complete SPDT Symbol & Terminal Reference Table

Before pulling wires or soldering pins, identify which standard your schematic or physical hardware follows. The table below maps the symbol of an SPDT switch across the major global standards, detailing exactly what each terminal designation means in practice.

Standard / Domain Symbol Visual Description Terminal Designations Practical Meaning & Application
IEEE / ANSI (US Schematics) Single hinged blade between two fixed dots COM, NO, NC COM is the input. NO closes when actuated; NC opens. Used in relay logic and motor reversing.
IEC 60617 (EU Schematics) Rectangle with a diagonal line and two output strokes 11, 12, 14 (or C, NO, NC) 11 is Common, 12 is NC, 14 is NO. Standard for European industrial control panels and PLC I/O.
NEC / US Residential (Physical) Not drawn on physical devices; 1 dark screw, 2 brass screws Common, Traveler 1, Traveler 2 Used in '3-way' lighting circuits. The dark screw is always the Line or Load; brass screws carry the travelers.
BS 7671 / UK Residential (Physical) 3-pin terminal block on the rear of a wall module COM, L1, L2 Used in '2-way' lighting circuits. COM is Line/Load; L1 and L2 are the strappers connecting to the second switch.
Electronics / PCB Mount 3-pin footprint (usually 0.1' spacing) Center Pin (C), Outer Pins (A/B) Center pin is almost always Common. Used for signal routing, audio switching, and microcontroller inputs.

Regional Wiring Standards: NEC vs. IEC vs. BS 7671

The symbol of an SPDT switch on a schematic is universal, but the physical wire colors and naming conventions change drastically depending on your region. Treating a US 3-way switch like a UK 2-way switch will result in a miswired circuit and a tripped breaker.

WARNING: Any procedure involving mains voltage (>50V AC) requires you to de-energize the circuit at the breaker panel, lock/tag the breaker, and verify the wires are dead with a tested non-contact voltage tester and multimeter. Local code may require a licensed electrician for mains wiring.

United States (NEC-Style Guidance)

In the US, an SPDT switch is sold as a 3-way switch.

  • Common Terminal (Dark Brass/Black Screw): Connects to the hot Line (from the panel) or the Load (to the light fixture). Wire color: Black or Red.
  • Traveler Terminals (Light Brass Screws): Connect to the two traveler wires running between the two 3-way switches. Wire colors: Red and Black (or White re-identified with black electrical tape at both ends, per NEC 200.7(C)(1)).

United Kingdom & EU (BS 7671 / IEC)

In the UK, an SPDT switch is sold as a 2-way switch.

  • Common Terminal (COM): Connects to the permanent Live (Line) or the Switched Live (Load). Wire color: Brown.
  • Strapper Terminals (L1, L2): Connect to the strappers running to the second switch. Under current harmonized BS 7671 rules, these are Blue and Brown with brown sleeving at the terminals to indicate they are switched lives. In older UK installations, you will see Red (COM) with Yellow and Blue (Strappers).

Rows People Get Wrong: The 'Common' Terminal Trap

The most frequent error when interpreting the symbol of an SPDT switch and translating it to physical wiring is misidentifying the Common terminal. If you wire the Line hot to a Traveler/Strapper terminal instead of the Common, the circuit will exhibit a 'ghost' behavior: the light will only turn on if the other switch in the circuit is in a specific position.

Bench Tip: On physical AC wall switches, the Common terminal is almost always a different color than the travelers. On a Leviton or Eaton toggle, the Common screw is black or dark brass, while the travelers are bright brass. On a UK MK Logic module, COM is clearly stamped, but if the plastic is painted over, look for the terminal that is physically isolated from the other two on the brass busbar.

Another common mistake occurs in electronics when using PCB-mounted SPDT slide switches. Makers often assume Pin 1 is Common. Unless the datasheet explicitly states otherwise, the center pin (Pin 2 on a standard 3-pin footprint) is the Common wiper. Always verify with a multimeter before soldering.

Safe Interpretation: Testing Faded or Unmarked Switches

When salvaging industrial control switches or dealing with faded residential toggles where the 'COM' stamp is illegible, never guess. Use a digital multimeter (DMM) in continuity mode to map the internal mechanics. A healthy switch should read < 1.0 ohm across closed contacts. If you read > 10 ohms, the internal contacts are pitted from arcing and the switch must be discarded.

  1. Isolate the Switch: Remove all wires from the switch terminals. Ensure the switch is completely out of the circuit.
  2. Set DMM to Continuity: Touch the probes together to verify the meter beeps and reads near 0.0 ohms.
  3. Find the Wiper (Common): Place one probe on Terminal A and the other on Terminal B. Toggle the switch. If continuity appears and disappears, neither is the Common. Move to Terminal C.
  4. Confirm the Common: Place one probe on Terminal C. Place the other on Terminal A. Toggle the switch—it should beep in one position. Keep the probe on C, move the other to Terminal B. Toggle—it should beep in the opposite position. Terminal C is your Common.
  5. Mark the Terminal: Use a silver paint pen or a piece of heat-shrink tubing to permanently mark the Common terminal before reinstalling.

Decision Path: Selecting Your Exact SPDT Part Number

Do not use a DC electronics switch for AC mains, and do not use an AC wall switch for high-current DC battery banks (DC arcs do not self-extinguish like AC zero-crossing arcs, leading to melted contacts). Use this decision tree to select the exact part for your build.

If Your Application Is... Required Specs Concrete Part Recommendation
US Residential AC Lighting (120V) 15A, 120V AC, UL Listed, NEMA 5-15 compatible Leviton 5603 (Decora) or Eaton 323W (Toggle). Standard 3-way switch.
UK/EU Residential AC Lighting (230V) 20A, 250V AC, CE/UKCA marked, BS 7671 compliant MK Logic Plus K492 or Scolmore CM082. Standard 2-way grid module.
Low Voltage DC Electronics (PCB/Breadboard) 5A max, 120V AC / 28V DC, 0.1' pin spacing C&K 7101J16ZQE2 (Through-hole toggle) or Alps SK-12D07 (Slide switch).
High Current DC (Solar, Marine, Auto) 250A+ continuous, 12-36V DC, ignition protected Blue Sea Systems 9001 (e-Series) or Victron Energy Battery Switch. Heavy-duty DC isolator.
Industrial Panel Motor Reversing 10A, 600V AC, DIN-rail mount, IEC 60947-5-1 Schneider Electric ZB2BD25C (Selector switch) or Phoenix Contact PLC-RSP relay module.

For further reading on switch mechanics and contact bounce in logic circuits, refer to the switch types chapter in the All About Circuits DC textbook. For physical installation torque specs and dimensional diagrams of US residential switches, consult the Leviton 5603 specification sheet. When designing logic-level SPDT circuits, always account for contact bounce using either a hardware RC snubber or a software debounce delay (typically 20-50ms in Arduino/ESP32 environments).