An SPDT (Single Pole, Double Throw) switch routes one common input to one of two selectable outputs. In schematic diagrams, it appears as a single contact arm pivoting between two nodes; in US residential wiring, it is universally sold and wired as a "3-way switch." Unlike a simple on/off (SPST) switch, an SPDT switch never truly breaks the circuit in a multi-location setup—it merely redirects the current path.
Below is the direct reference for identifying, interpreting, and specifying SPDT switches across electrical and electronic domains.
The Complete SPDT Symbol & Terminal Reference Table
How an SPDT switch is drawn on paper depends entirely on whether you are looking at an electronics schematic, an industrial ladder diagram, or a residential wiring plan. Use this table to decode the symbol you are looking at.
| Symbol / Context | Visual Description | Terminal Labels | Practical Meaning & Application |
|---|---|---|---|
| IEC 60617 Schematic | A single diagonal line (the pole) hinged at one end, with a dashed mechanical link pointing to two parallel nodes. | 1 (Common), 2 (Throw A), 4 (Throw B) | Standard for global electronics and industrial PLCs. The dashed line indicates simultaneous mechanical action if ganged. |
| NEMA / IEEE Schematic | A straight horizontal line (the blade) pivoting on a hinge dot, striking one of two parallel vertical lines (the jaws). | C (Common), NO (Normally Open), NC (Normally Closed) | Common in US industrial ladder logic and relay schematics. Assumes a spring-return or defined resting state. |
| US Residential Wiring (3-Way) | No standard schematic symbol on the physical device; represented in wiring diagrams as a switch with three terminal screws and a ground. | Common (Dark Screw), Traveler 1 (Brass), Traveler 2 (Brass) | Used to control a single light fixture from two physical locations. The 'Common' is always the hot source or the load. |
| Relay (Form C) | A coil symbol adjacent to a switch blade. The blade rests against one contact and moves to the other when energized. | COM (Common), NC (Normally Closed), NO (Normally Open) | Used in automotive, HVAC, and Arduino/ESP32 relay modules. 'Form C' is the industry shorthand for SPDT. |
Regional Standard Variants: NEMA vs. IEC vs. Legacy UK
The physical execution and documentation of SPDT switches shift depending on your regional wiring standards. Misapplying a regional convention to a foreign schematic is a primary cause of miswired traveler circuits.
NEMA (North America)
Governed by NEMA WD-6 standards, residential SPDT switches are strictly referred to as 3-way switches. The physical terminals are color-coded: the Common terminal is a dark (usually black or dark bronze) screw, while the two Traveler terminals are bright brass. The NEC requires the common terminal to be identified, but does not mandate specific wire colors for the travelers, though red and black are standard practice.
IEC (Europe / Global)
Under IEC 60617, SPDT switches in residential lighting are often referred to as "two-way" switches (because they are used in pairs to control a light from two places, mirroring the US "3-way" naming confusion). Terminals are typically stamped L1, L2, and COM (or 1, 2, and 3). There is no brass/dark screw color coding; identification relies entirely on stamped alphanumeric markings.
Legacy UK (BS 3939)
Older British Standard drawings used a unique SPDT symbol resembling a bow and arrow, where the "string" was the flexible contact and the "arrow" pointed to the throws. You will only encounter this in pre-1990s UK industrial panels. Treat these as standard SPDTs, but verify the mechanical action, as many legacy BS 3939 switches were heavy-duty knife switches with high let-through current risks if opened under load.
Rows People Get Wrong (And How to Fix Them)
When reading SPDT schematics or wiring physical devices, these specific misinterpretations cause the most bench and jobsite failures.
1. Confusing SPDT with DPST (Double Pole, Single Throw)
The Error: Seeing a switch with four terminals and assuming it is an SPDT with an extra ground, or wiring a DPST switch (which acts as two separate on/off switches ganged together) as if it were an SPDT.
The Fix: An SPDT has exactly three current-carrying terminals (plus ground). A DPST has four current-carrying terminals. If you need to switch both the hot and neutral of a 120V circuit simultaneously, you need a DPST, not an SPDT.
2. The "Traveler-to-Common" Miswire in 3-Way Setups
The Error: In a US 3-way residential setup, wiring the incoming hot wire to a brass traveler screw instead of the dark common screw.
The Fix: This results in a circuit where the light either stays on permanently, or only works when the other switch in the pair is in a specific position. The incoming power source (or the wire going up to the light fixture) must always land on the dark Common screw. The two brass screws are strictly for the traveler wires running between the two switches.
3. Assuming NO/NC Implies a Pushbutton
The Error: Seeing "NO" (Normally Open) and "NC" (Normally Closed) on an SPDT relay or toggle switch and assuming it is a momentary pushbutton.
The Fix: NO and NC simply describe the state of the throws relative to the common terminal when the actuator is in its resting (unactuated) position. A standard maintained toggle switch can absolutely be SPDT with NO/NC throws. Check the datasheet for "maintained" vs "momentary" action.
Safe Interpretation When Markings Are Faded or Missing
On jobsites, you frequently encounter old 3-way switches where the brass and dark screws are corroded to the same color, or PCB-mount SPDT toggles where the stamped "C", "NO", and "NC" have rubbed off. Do not guess. Use a multimeter to map the internal contacts.
- Identify the Common (C): Place one probe on a suspected terminal. Place the other probe on a second terminal. Toggle the switch. If it beeps in one position and is silent in the other, you have found a Common-to-Throw pair.
- Verify the Common: Move the second probe to the third terminal. Toggle the switch again. If it now beeps in the opposite position, the terminal holding your first probe is definitively the Common.
- Map NO and NC (for relays/toggles): With the switch in its natural resting state (unpressed, or toggle pushed down), check continuity between the Common and the remaining two terminals. The one that beeps is NC (Normally Closed). The one that is silent is NO (Normally Open).
Decision Path: Which SPDT Switch to Specify
Stop guessing which part number to order. Use this decision matrix to select the exact SPDT switch for your application.
| If your application is... | And your environment is... | Then specify this exact part / standard |
|---|---|---|
| Residential AC Lighting (3-Way) | US/Canada, 120/277V AC, 15A max, in-wall Decora style | Leviton 5603-2W (15A, 120/277V, Decora 3-Way) |
| Residential AC Lighting (2-Way) | EU/UK, 230V AC, 10A max, flush mount | Schneider Electric Merten 3-Way (IEC standard intermediate/2-way) |
| PCB / Breadboard Logic | < 24V DC, < 5A, through-hole mounting | C&K 7101J1V2BE (SPDT toggle, 5A, right-angle PC mount) |
| Industrial Motor Reversing | 480V AC, 30A+, panel mount, high vibration | Eaton E5030 (Heavy Duty 30A SPDT Toggle) |
| Arduino / ESP32 Signal Routing | 3.3V / 5V logic, < 100mA, automated switching | Omron G5V-1-DC5 (SPDT / Form C Signal Relay) |
The Default Pick: If you are building a standard US home wiring project and need an SPDT switch for a hallway or staircase, buy the Leviton 5603-2W. It is the industry baseline for 15A residential 3-way circuits, features clear dark/brass terminal differentiation to prevent traveler miswires, and is universally available at any hardware store for under $5. If you are on the bench building an ESP32 automation project that needs to physically route a low-voltage signal, default to the Omron G5V-1 series relay.
