The symbol for SPDT (Single Pole, Double Throw) switch depends entirely on the drafting standard you are reading. In US ANSI/IEEE schematics, it appears as a single hinged line (the pole) throwing between two parallel contact points. In international IEC 60617 diagrams, it is drawn as a single contact node with a diagonal actuator line branching to two distinct outputs. In physical US home wiring governed by the NEC, an SPDT switch is universally sold, wired, and referred to as a "3-way switch."
The Complete SPDT Switch Symbol Reference Table
Before tracing any circuit, identify which standard the schematic or wiring diagram follows. Use this reference table to map the symbol to the physical terminals on your bench or in the junction box.
| Standard / Region | Symbol Visual Description | Terminal Designations | Typical Application |
|---|---|---|---|
| ANSI/IEEE 315 (US Schematics) | Hinged line (pole) pivoting to one of two parallel lines | Common (C), Throw 1, Throw 2 (or NO/NC) | US electronic schematics, DC control logic |
| IEC 60617 (International) | Single input node, diagonal actuator line pointing to two output nodes | Input (1), Outputs (2, 4) | EU/International schematics, PLC programming |
| NEC / US Residential | Physical device mapping (no universal schematic symbol) | Common (Dark Screw), Travelers (Brass Screws), Ground (Green) | US 3-way stairway, hallway, and multi-point lighting |
| NEMA (Industrial Control) | Enclosed switch outline with internal SPDT knife-blade symbol | Line (L), Load 1 (A), Load 2 (B) | Motor reversing, industrial control panels, limit switches |
Regional Variants: ANSI vs. IEC vs. Legacy UK
The divergence in switch symbols stems from different engineering philosophies. The NFPA 70 (NEC) and ANSI standards heavily reflect the mechanical heritage of early knife switches. When you see the ANSI SPDT symbol, you are looking at a literal side-profile drawing of a physical blade pivoting on a hinge to make contact with one of two metal jaws. This makes it highly intuitive for technicians troubleshooting physical relay contacts.
Conversely, the IEC 60617 standard prioritizes electrical node connectivity over mechanical representation. The IEC symbol for an SPDT switch looks more like a logic gate or a flowchart node. It strips away the "hinge" imagery and simply shows current entering node 1 and exiting either node 2 or node 4. This cleaner aesthetic is preferred in dense PLC ladder logic and modern European PCB design.
In the UK, legacy wiring diagrams (pre-2006 harmonization) sometimes used unique cross-hatching or specific traveler notations for two-way lighting circuits. Today, the UK strictly follows IEC 60617 for schematics and BS 7671 for physical wiring colors. If you are opening an older UK consumer unit or junction box, expect to see red (old live), black (old neutral), and green (old earth). In modern UK two-way SPDT switch drops using 3-core and earth cable, brown is the common, while blue and black (sleeved with brown) act as the travelers.
Rows People Get Wrong & Faded Marking Protocols
Mistake 1: Confusing SPDT with DPST
The most common schematic error is misreading a DPST (Double Pole, Single Throw) symbol as an SPDT. A DPST symbol features two distinct hinged poles that move together (often linked by a dashed mechanical line) but only have one throw each. An SPDT has only one pole, but it throws to two different contacts. If your symbol has two input lines and two output lines, it is DPST, not SPDT.
Mistake 2: Misidentifying the Common Terminal on a US 3-Way Switch
When wiring a physical SPDT switch in a US home, the "Common" terminal is the odd-colored screw—usually black, dark copper, or distinctly marked "COM". The two "Traveler" terminals are the matching brass-colored screws. A frequent DIY error is landing the line hot on a brass traveler screw instead of the dark common screw, which results in the circuit only working when the other switch in the 3-way pair is in a specific position.
Protocol for Faded or Missing Terminal Markings
If you are salvaging a relay or an industrial SPDT toggle switch and the "C", "NO", and "NC" stampings are worn off or obscured by grime, use a multimeter to map the pins. According to standard electrical switching principles, you can deduce the pinout safely:
- Set your multimeter to continuity mode (the diode/sound wave icon).
- With the switch actuator in Position A, probe all combinations of the three suspect pins.
- The pin that shows < 1 ohm (continuity) to one pin, and infinite resistance (OL) to the other, is your Common.
- Flip the actuator to Position B. The Common pin will now show < 1 ohm to the other pin. The pin it was previously connected to is now OL.
- Label the Common. The remaining two pins are your Throw 1 and Throw 2 (or NO/NC, depending on the actuator's resting state).
SPDT Switch Wiring and Symbol FAQs
What is the difference between an SPDT switch symbol and a 3-way switch?
Electrically, they are the exact same device: a single input that can be routed to one of two outputs. The difference is purely semantic and regional. "SPDT" is the technical schematic terminology used globally in electronics and industrial controls. "3-way switch" is the North American retail and NEC terminology for the physical wall switch used in residential lighting. The "3" in 3-way refers to the three physical screw terminals on the device (excluding the ground screw), not the number of electrical poles.
How do I read an SPDT symbol on an industrial ladder diagram?
In NEMA-style ladder logic, an SPDT limit switch or selector switch is drawn vertically between two horizontal rungs. The common terminal (the pole) connects to the incoming rail or a previous contact. The two throws branch off to two separate parallel rungs, usually controlling two mutually exclusive loads (like a motor's "Up" and "Down" contactor coils). Look for the mechanical interlock symbol (a dashed line or physical linkage graphic) which indicates that throwing the switch to one rung physically breaks the connection to the other.
Can I wire an SPDT switch to act as a standard SPST (Single Pole Single Throw)?
Yes, this is a common bench and jobsite workaround if you only have SPDT switches on hand. To use an SPDT as an SPST, simply connect your line to the Common terminal and your load to one of the Throw terminals. Leave the second Throw terminal completely empty and insulated. The switch will now function as a basic on/off SPST. Never jumper the two throw terminals together in an attempt to "increase contact area"; this can cause asymmetric arcing and premature contact failure.
What does a dotted mechanical line between two SPDT symbols mean?
When you see a dashed or dotted line connecting the actuator arms of two separate SPDT (or SPST) symbols on a schematic, it indicates a gang or mechanical linkage. This means both switches are physically mounted on the same shaft or actuator. When you flip the physical toggle or push the button, both switch poles change state simultaneously. This is frequently seen in DPDT (Double Pole Double Throw) configurations, which are simply two SPDT switches ganged together.
