The single pole double throw switch schematic symbol represents a three-terminal electromechanical device that routes a single input (the pole) to one of two distinct outputs (the throws). In US residential wiring, this is the standard 3-way switch used for stairwell lighting; in industrial control and electronics, it is the foundational component for motor reversing, transfer switches, and logic gating.
The Complete SPDT Schematic Symbol & Terminal Reference
Because electrical schematics vary globally, reading an SPDT symbol correctly requires knowing which standard the drafter used. The table below maps the schematic representations to their physical terminal markings across the three dominant standards.
| Standard / Context | Schematic Symbol Description | Schematic Terminal Names | Physical Markings (US Residential) | Physical Markings (Industrial / Electronics) |
|---|---|---|---|---|
| NEMA / US NEC (Residential 3-Way) | A single line (pole) breaking into a V-shape pointing to two parallel lines (throws). | Pole, Throw 1, Throw 2 | Common (dark screw), Traveler 1 & 2 (brass screws) | N/A (Residential specific) |
| IEC 60617 (International / EU) | A diagonal line (moving contact) pivoting from a fixed dot toward two other fixed dots. | Common (1), NO (2), NC (3) | N/A | C, NO, NC or P, A, B |
| JIC (Industrial Control) | Similar to NEMA but often includes a mechanical linkage line if ganged with other poles. | Common, Normally Open, Normally Closed | N/A | C, NO, NC (often numbered 11, 12, 14) |
| Old UK / BS (Pre-Harmonization) | Knife-switch style representation with a single blade pivoting between two contacts. | Common, Way 1, Way 2 | COM, L1, L2 (used in 2-way lighting) | C, 1, 2 |
Regional Standards and the "Rows People Get Wrong"
When interpreting the standard electrical schematic symbols, misidentifying the SPDT configuration is one of the most common causes of wiring faults. The standard that applies to you depends on your region and application: US electricians follow NFPA 70 (NEC) and NEMA symbols, while EU and international engineers use IEC 60617.
Rows and Symbols People Get Wrong
- Assuming NO/NC applies to residential 3-way switches: In the IEC and JIC rows, the throws are labeled Normally Open (NO) and Normally Closed (NC) because industrial switches are typically spring-return (momentary). A US residential 3-way switch is a maintained-contact switch. It has no "normal" state; it simply toggles between Traveler 1 and Traveler 2. Wiring a 3-way switch expecting a spring-return will result in a permanently dead circuit.
- Confusing the Pole (Common) with the Throws: On a physical US 3-way switch, the Common terminal is the odd one out—it uses a dark-colored (usually black or dark bronze) screw, while the two Travelers use brass-colored screws. On a schematic, the Pole is the single line entering the switch, while the Throws are the two lines exiting. Reversing these guarantees the switch will not function as intended.
- Misreading the IEC pivot point: In the IEC 60617 symbol, the point where the diagonal line pivots is the Common terminal. The two dots at the other end are the throws. Beginners often mistake the pivot for a ground symbol or a mechanical hinge rather than the primary current path input.
Safe Interpretation When Markings Are Faded or Missing
In older homes or salvaged industrial panels, the physical terminal markings (COM, NO, NC) or the distinct screw colors on a 3-way switch may be painted over, corroded, or completely faded. You can safely map the physical switch to the single pole double throw switch schematic symbol using a digital multimeter.
- De-energize and Verify: Turn off the breaker. Test the wires with a multimeter set to AC Voltage to confirm 0V across all terminal combinations.
- Set to Continuity: Switch your multimeter to the continuity setting (the diode/soundwave icon). It should beep when probes touch, indicating a resistance of less than 1 ohm.
- Identify the Common (Pole): Place one probe on Terminal A and the other on Terminal B. Toggle the switch. If it beeps in one position but not the other, neither is the Common. Move the probe to Terminal C. The terminal that shows continuity to both of the other terminals (depending on the toggle position) is the Common (Pole).
- Identify the Throws: Once the Common is found, the remaining two terminals are your Throws (Travelers). In a maintained SPDT switch, they are interchangeable. In a momentary industrial SPDT switch, you must note which throw engages when the actuator is pressed (NO) and which engages when released (NC).
Frequently Asked Questions
What is the difference between an SPDT and a DPDT schematic symbol?
A Single Pole Double Throw (SPDT) symbol shows one input line splitting into two output lines. A Double Pole Double Throw (DPDT) symbol shows two completely isolated SPDT switches drawn side-by-side, usually connected by a dashed mechanical linkage line. The DPDT switch has six physical terminals (two Commons, two NOs, two NCs) and is used to simultaneously switch two separate circuits, such as reversing the polarity of a DC motor by swapping both the positive and negative leads at once.
How do I wire an SPDT switch symbol in a 3-way lighting circuit?
In a US NEC-compliant 3-way lighting circuit, you use two SPDT switches. The line voltage (hot) connects to the Common (dark screw) of the first switch. The two Travelers (brass screws) of the first switch connect via a 3-wire cable (typically red and black THHN or NM-B wires) to the two Travelers of the second switch. Finally, the Common of the second switch connects to the load (the light fixture). The neutral wire bypasses both switches and runs directly to the light fixture.
Why does my SPDT switch symbol show a center-off position?
If the schematic symbol shows a gap between the pole and the throws, or a third fixed dot in the center, it represents an SPDT Center-Off (On-Off-On) switch. In this configuration, the switch is still a single pole with two throws, but the mechanical actuator has a stable middle position where the pole connects to neither throw, breaking the circuit entirely. These are common in HVAC fan speed controls and reversible motor toggles where a hard stop is required before changing direction.
