A Single Pole Double Throw (SPDT) switch symbol represents a three-terminal device that routes a single input (the Common pole) to one of two distinct outputs (the Throws). In schematic diagrams, it is universally depicted as a single line breaking into two diverging paths. However, the exact visual representation, terminal numbering, and physical markings change drastically depending on whether you are reading an American ANSI/NEMA print, a European IEC 60617 diagram, or an automotive DIN harness. Misinterpreting these regional variants is a primary cause of dead shorts and miswired control circuits.
The SPDT Switch Symbol & Terminal Mapping Table
The table below maps the schematic symbol style to the physical terminal markings you will find on the actual hardware. Keep this reference on your bench when translating a schematic to physical wiring.
| Standard / Context | Schematic Symbol Style | Physical Terminal Labels | Polarity / Phase Convention |
|---|---|---|---|
| ANSI/NEMA (US) | Knife-switch abstraction or angled line breaking into two dots | COM, NO, NC or 1, 2, 3 | Line/Source on COM; Loads on NO/NC |
| IEC 60617 (Global) | Rectangle with internal diagonal line and external connection ticks | 11, 12, 14 (Contactors) or 1, 2, 3 (Manual) | 11/1=Common; 12/2=NC; 14/3=NO |
| JIC (US Industrial) | Standard ANSI with strict ladder logic numbering and pilot dots | L1, L2, L3 or X1, X2, X3 | Strict rung numbering; L1 always line-side |
| Automotive (DIN 72552) | Box with internal switch blade, often with relay coil shown | 30, 87, 87a | 30=Power In (Common); 87=NO; 87a=NC |
| Legacy BS 3939 (Old UK) | Similar to early IEC but with heavier line weights and distinct isolator marks | C, 1, 2 | C=Common; 1 and 2 are Throws |
How to read this table: The 'Physical Terminal Labels' column tells you what is actually stamped into the metal or plastic housing of the switch. If you are wiring a standard US-manufactured toggle switch, you will look for COM, NO (Normally Open), and NC (Normally Closed). If you are wiring a European industrial relay or contactor block acting as an SPDT, you will look for 11, 12, and 14.
Regional Schematic Variants: ANSI, IEC, and Automotive
Understanding which standard applies to your region and equipment is critical. A schematic drawn in Germany will look fundamentally different from one drawn in Ohio, even if the physical circuit performs the exact same function.
ANSI/NEMA (North America)
In the US, the NFPA 70 (NEC) governs installation, but ANSI/NEMA Y32.2 governs the symbols. The SPDT symbol typically looks like a single-pole single-throw (SPST) switch with an extra 'throw' dot added. The physical switches you buy at a US hardware store (like a Leviton or Hubbell 3-way switch, which is physically an SPDT) rarely use NO/NC markings; instead, they use a dark-colored 'Common' screw and two brass-colored 'Traveler' screws. In residential wiring, the symbol is almost never used; electricians rely on the physical terminal colors.
IEC 60617 (Europe and International)
The IEC standard abandons the 'knife switch' drawing in favor of a rectangular block. This is where terminal numbering gets highly specific. For manual control switches, IEC uses 1 (Common), 2 (Throw 1), and 3 (Throw 2). However, for electromagnetic relays and contactors, IEC 60445 dictates the 11/12/14 system: 11 is the common pole, 12 is the Normally Closed contact, and 14 is the Normally Open contact. If you see a 13/14 designation on an IEC print, that refers to the NO contact of a standard relay block.
Automotive DIN 72552
If you are wiring a 12V/24V DC system, an RV, or a marine panel, you will encounter the DIN standard. The SPDT automotive relay is the most common physical manifestation of this symbol. Terminal 30 is the heavy-duty common input (usually fed directly from the battery via a fuse). Terminal 87 is the Normally Open output (powers the load when the relay clicks). Terminal 87a is the Normally Closed output (powers the load when the relay is at rest).
Never assume US terminal conventions on imported IEC equipment. A European machine using '1, 2, 3' markings on a manual SPDT selector switch does not map to 'COM, NO, NC'. Terminal 2 and 3 are simply directional throws. Always verify with a multimeter before applying line voltage to imported control panels.
Maintained vs. Momentary Symbols
A standard SPDT symbol implies a maintained (ON-ON) position. If the switch is momentary (ON-OFF-ON or ON-(ON)), the schematic will include a small bracket or a spring symbol beneath the switch blade indicating it returns to a center or default position. Failing to notice the momentary bracket symbol will result in you buying a maintained toggle switch, causing a machine to stay running when the operator releases the control.
Rows People Get Wrong & Testing Faded Terminals
When working with SPDT switches on the bench or in the field, specific misinterpretations lead to blown fuses or malfunctioning logic. Here are the most common errors and how to resolve them.
The 'Rows People Get Wrong' Notes
- Confusing 3-Way Switches with Standard SPDT: In US residential wiring, a '3-way switch' is physically an SPDT switch. However, the schematic symbol for a 3-way lighting circuit shows two SPDT switches wired in series via traveler wires. People often miswire the second switch by treating it as a standalone SPDT rather than a series logic gate.
- Misreading IEC 11/12/14 as Line/Load: Beginners often assume '11' is Line In, '12' is Load 1, and '14' is Load 2. In reality, 11 is the Common (which can be Line OR Load depending on whether you are switching the hot leg or the neutral return—though switching the hot leg is standard practice), 12 is NC, and 14 is NO.
- Center-Off (ON-OFF-ON) vs. Standard (ON-ON): A physical switch with three positions (up, center, down) is a center-off SPDT. The schematic symbol for this includes a distinct 'X' or break in the center of the throws. If you wire a center-off switch into a circuit expecting a standard ON-ON SPDT, your circuit will unexpectedly drop power in the middle position.
Safe Interpretation When Markings are Faded or Missing
On older industrial equipment, marine panels, or heavily used bench power supplies, the stamped COM/NO/NC or 11/12/14 markings often wear off or become obscured by grease and arc flash residue. Never guess the terminal layout based on physical position (e.g., assuming the middle screw is always Common; on many toggle switches, the Common is on the side, or isolated on the opposite face).
The Field Verification Protocol:
- De-energize and Isolate: Turn off the breaker, apply LOTO (Lockout/Tagout) if applicable, and verify the circuit is dead using a non-contact voltage tester and a multimeter set to AC/DC voltage.
- Set Multimeter to Continuity: Switch your meter to the continuity/diode test mode (the setting that beeps when probes touch).
- Find the Common (COM / 11 / 30): Place one probe on Terminal A and the other on Terminal B. Toggle the switch. If continuity changes (beeps in one position, silent in the other), neither is the Common. Move the probe to Terminal C. If Terminal C shows continuity with A in position 1, and continuity with B in position 2, Terminal C is your Common.
- Identify NO and NC: With the switch in its 'resting' or 'down' (OFF) state, probe the Common and the two Throws. The Throw that shows continuity (reads < 1 ohm) with the Common in the resting state is Normally Closed (NC / 12 / 87a). The Throw that shows infinite resistance (OL) is Normally Open (NO / 14 / 87).
- Label Immediately: Use a fine-tip silver paint pen or high-temperature label tape to mark the terminals before wiring. Do not rely on memory.
By anchoring your wiring to the verified Common terminal and confirming the throw states with a meter, you eliminate the risk of shorting a power source directly to ground through a misidentified NC contact. Always defer to the specific manufacturer's datasheet for torque specifications on the terminal screws, as over-torquing a 10A micro-switch terminal can crack the internal phenolic resin and cause an internal arc fault.
