The standard single pole switch symbol in US NEC schematics is a line breaking into a gap with an angled lever (resembling an open knife switch), while IEC standards use a simple line with a diagonal slash and a solid dot at the pivot. In physical wiring diagrams, it represents a single break in the hot (line) conductor, technically known as a Single Pole Single Throw (SPST) switch. Below is the complete reference data for identifying, drawing, and interpreting these symbols across global standards.
Single Pole Switch Symbol Reference Table
Use this spec-sheet table to identify the correct symbol for your region's schematics and understand what the graphic translates to on the jobsite. The table covers standard lighting and interlock applications.
| Symbol Name | Standard / Region | Schematic Graphic Description | Physical Wiring Meaning | Common Application |
|---|---|---|---|---|
| SPST (Normally Open) | NEMA / NEC (US/Canada) | Line with a gap and an angled lever pointing away from the gap | Standard 2-terminal wall switch; circuit is open (off) in the drawn state | Standard room lighting, exhaust fans |
| SPST (Normally Open) | IEC 60617 (EU/Global) | Straight line with a diagonal slash and a solid dot at the pivot base | Standard EU wall switch; single break in the line conductor | Global commercial and residential lighting |
| SPST (Normally Closed) | NEMA / NEC | Line with a gap, lever crossing over the gap to the opposite side | Switch is closed (on) in the resting state; opens when actuated | Safety interlocks, emergency stop circuits |
| SPST with Pilot Light | NEMA / NEC | Standard SPST symbol with a small circle or indicator attached to the lever | Switch with built-in LED or neon indicator that illuminates when load is energized | Attic lights, exterior floodlights, shop equipment |
| SPST Isolator | IEC / Old UK (BS 3939) | Diagonal slash with a perpendicular bar at the tip, indicating a physical air gap | Heavy-duty disconnect switch with visible blade separation | Industrial motor disconnects, main panel isolation |
Regional Standard Variants: NEC vs. IEC vs. Old UK
Electrical symbols are not universal. Misinterpreting a region-specific symbol can lead to wiring a circuit backward or misunderstanding a safety interlock. Here is how the major standards handle the single pole switch.
NEMA and NEC (North America)
In the US and Canada, schematics follow NEMA-style graphics, which are heavily derived from early physical knife switches. The NFPA 70 (NEC) relies on these symbols for permit drawings. The defining feature is the 'lever' that visually breaks the line. A critical rule in NEMA schematics: the switch is always drawn in its de-energized, resting state. If a light is currently on in the room, the schematic still shows the switch open if it is a momentary or normally-open relay.
IEC 60617 (Europe and Global)
The International Electrotechnical Commission uses a more minimalist approach. The single pole switch is a straight line interrupted by a diagonal slash. The solid dot at the base of the slash represents the mechanical pivot. Do not confuse this pivot dot with a wire junction node. In IEC schematics, a solid dot on a continuous line means a junction, but a dot at the base of a switch slash is strictly a mechanical hinge.
Old UK (Pre-2004 BS 3939)
Before the UK fully harmonized with IEC standards, British Standard 3939 used distinct symbols. The isolator switch, for example, featured a perpendicular bar at the tip of the lever to emphasize a physical, visible air gap—a requirement for heavy industrial maintenance. You will still see these on legacy schematics in older UK manufacturing plants and maritime installations.
Rows People Get Wrong & Safe Interpretation of Faded Markings
When reading schematics or inspecting physical hardware, assumptions lead to short circuits. Here are the most common pitfalls and how to handle degraded hardware.
The 'Rows People Get Wrong' Notes
- Confusing NO and NC States: Beginners often look at the NEMA SPST (Normally Open) symbol and assume the angled lever represents the physical 'up' position of a standard toggle switch. It does not. It represents the electrical resting state. A standard wall switch is technically Normally Open (NO) because its default, unactuated schematic state is open, even if you physically flip it 'up' to turn it on.
- The IEC Pivot Dot vs. Junction Node: As mentioned, a solid dot in IEC usually means wires are bonded. But when that dot sits exactly at the origin of a diagonal switch slash, it is a pivot. If you wire a junction there, you will bypass the switch entirely and create a dead short or an always-on load.
- Assuming Ampacity from the Symbol: The single pole switch symbol dictates logic and function, not capacity. A 15A residential toggle and a 400A industrial disconnect relay use the exact same SPST symbol. Ampacity is always noted via text annotation (e.g., '15A/120V') next to the symbol.
Safe Interpretation When Physical Markings are Faded
On older switches, the stamped '15A 120V' or 'AL/CU' lettering on the steel yoke often fades or rusts away. If you need to verify whether a switch in the wall is a standard single pole (SPST) or a 3-way switch (SPDT) before replacing it, use the terminal count and a multimeter:
- Count the Terminals: A true single pole switch has exactly two brass screws (Line and Load) and one green ground screw. A 3-way switch has three brass screws (Common + 2 Travelers) and one green ground.
- The Continuity Test: Set your multimeter to continuity or resistance (Ω). Place probes on the two brass screws. Toggle the switch. It must alternate between < 1 ohm (closed) and OL / Over Limit (open). If you measure continuity between any combination of three terminals, you are holding a 3-way switch, regardless of how it was wired in the wall.
For deeper reading on schematic interpretation, the All About Circuits reference textbook provides excellent visual breakdowns of NEMA vs IEC logic states.
Single Pole Switch Symbol FAQs
What is the difference between a single pole switch symbol and a 3-way switch symbol?
In schematics, a single pole switch is drawn as SPST (Single Pole Single Throw) with two connection points. A 3-way switch is drawn as SPDT (Single Pole Double Throw), showing a single line entering the switch and splitting into two diverging paths (the travelers). Physically, the single pole symbol translates to a device with two brass terminals, while the 3-way symbol translates to a device with three brass terminals. For comprehensive wiring diagrams showing how these symbols map to physical traveler wires, Electrical Technology offers excellent visual guides.
How do I read a single pole switch symbol on a smart home wiring diagram?
Smart switches complicate the standard SPST symbol. While the switching logic remains a single break in the hot wire, smart home schematics will add a secondary line branching off to the neutral bus (required to power the internal WiFi/Zigbee radio). Some schematics will also show a bypass resistor symbol in parallel with the load if the system is designed for older homes lacking a neutral wire. Always look for the 'N' or 'Neutral' terminal designation on the smart switch wiring diagram, which standard single pole symbols omit.
Does the single pole switch symbol indicate whether the switch is 15A or 20A?
No. The symbol only defines the switching logic (one pole, one throw). The current rating (15A vs 20A) and voltage rating (120V vs 277V) must be written as text annotations adjacent to the symbol on the blueprint. On the physical device, it is stamped into the metal yoke. Never install a 15A switch on a 20A breaker circuit, even if the schematic symbol looks identical; the NEC requires the switch rating to match or exceed the branch circuit ampacity.
Why does my European IEC schematic show a single pole switch breaking the neutral wire?
It shouldn't. Both IEC and NEC codes strictly mandate that single pole switches must break the line (hot/phase) conductor, never the neutral. If an IEC schematic shows the switch symbol on the blue (neutral) wire returning to the source, it is either a dangerous drafting error, a misinterpretation of a specific isolated equipment interlock, or a diagram for a region with non-standard historical wiring practices (like old UK back-wired fixtures). Switching the neutral leaves the light fixture energized at full mains voltage even when turned off, creating a severe shock hazard during bulb changes.
