On a physical switch toggle, the I (or 1) symbol means ON, and the O (or 0) symbol means OFF. This binary notation comes from the International Electrotechnical Commission (IEC) 60417 standard, where '1' represents a closed circuit (power flowing) and '0' represents an open circuit (power broken). When looking at the wiring terminals on the back of the switch, 'Line' (or COM/L1) is the always-hot power source, and 'Load' (or L2) is the switched leg going to the fixture.
The Master Switch Symbol & Terminal Reference Table
Before pulling any wires, match the physical markings on your switch to this reference table. This covers both the external toggle symbols and the internal terminal screw designations.
| Symbol / Marking | Meaning | Physical State | Where You See It |
|---|---|---|---|
| I (or 1) | ON / Closed | Circuit complete, current flows | External toggle face |
| O (or 0) | OFF / Open | Circuit broken, no current | External toggle face |
| COM (or Line) | Common / Power In | Always hot (120V/230V nominal) | Terminal screw (usually black or dark brass) |
| L1 (or Load) | Switched Leg Out | Hot only when switch is in 'I' position | Terminal screw (usually light brass) |
| L2 (or Traveler) | 3-Way Interconnect | Alternating hot depending on toggle | Terminal screws on 3-way/4-way switches |
| ⏚ (or Ground) | Equipment Ground | 0V, fault-current path | Green terminal screw on metal yoke |
Regional Variants: NEC (US) vs. IEC (EU/UK/AU)
Switch manufacturing and wiring color codes diverge sharply depending on your region. Assuming a US standard applies to a UK installation is a fast track to a dead short. Here is how the standards break down.
North America (NEC / NEMA WD-1)
In the US and Canada, switches are governed by NFPA 70 (NEC) Article 404 and the NEMA WD-1 General Use Snap Switches standard. Terminals are typically labeled Line, Load, and Traveler. Wire colors are strictly Black (always-hot), Red/Black (travelers or switched loads), White (neutral, rarely connected to standard single-pole switches), and Bare/Green (ground).
Europe, UK, and Australia (IEC 60445)
Regions following the IEC 60445 standard for wiring colors and identifiers use COM, L1, and L2 for terminal markings. In a standard 1-way (single-pole) switch, power enters COM and exits L1. In a 2-way (US 3-way) setup, COM is the common, and L1/L2 are the strappers (travelers). Modern wire colors are Brown (line), Blue (neutral), and Green/Yellow (earth). Note for UK readers: If you are working in a pre-2006 installation, your old color codes were Red (line) and Black (neutral/switched line), which directly conflicts with modern US black-hot conventions.
The 'Rows People Get Wrong' Notes
Even experienced DIYers misinterpret specific switch markings. Here are the most common failures and how to avoid them.
On a standard mechanical toggle, swapping Line and Load just means the switch works upside down or backfeeds the circuit. On a smart switch (like a Lutron Caseta or Kasa KS220), swapping them will fry the internal relay or cause the switch to power on but fail to control the load. Always identify the always-hot wire before connecting to the 'Line' terminal.
If you buy an IEC-standard switch (marked COM, L1, L2) for a simple single-pole light in Europe, L1 is your load. If you look at a US 3-way switch marked L1 and L2, those are travelers, not a line/load pair. Never treat L1/L2 as line/load on a North American 3-way switch.
The 'I' on the toggle face does not mean 'Input' or 'Line'. It strictly means the numerical digit '1' (binary ON). Do not wire your power source to a terminal just because it is physically located under the 'I' side of the toggle; always rely on the stamped terminal letters (COM/Line).
Safe Interpretation When Markings Are Faded or Missing
Cheap switches often have painted terminal markings that rub off after a few installations, or you might be replacing a legacy switch with no markings at all. Never guess. Use this multimeter protocol to safely identify the circuit.
- De-energize and Verify: Turn off the breaker. Use a Non-Contact Voltage Tester (NCVT) on the wires, then verify with a multimeter set to AC Voltage. Read across the black wire and ground. It must read < 1V.
- Identify the Always-Hot (Line/COM): If you are testing before turning off the breaker, the wire that reads 120V/230V to ground is your Line. Mark it immediately with black electrical tape.
- Continuity Test for Toggle Direction: With the breaker OFF and wires disconnected, set your multimeter to Continuity (the diode/beep setting). Place one probe on the COM/Line terminal and the other on the Load terminal. Flip the toggle. The terminal pair that beeps when the toggle is pushed UP (or towards the 'I' marking) is your closed circuit path.
- Watch for Phantom Voltage: If using a high-impedance digital multimeter on a disconnected load wire running parallel to hot wires in a conduit, you might read 40V-80V. This is capacitive coupling (phantom voltage). Use a low-impedance (LoZ) setting or a solenoid voltage tester to confirm true dead state.
Decision Path: Identifying Your Switch Terminals in 60 Seconds
Use this decision tree to terminate your wires correctly based on the physical markings on your specific switch.
| Your Switch Type | Markings Present | Action Required | Concrete Termination Pick |
|---|---|---|---|
| US Single-Pole | Line / Load | Connect always-hot to Line, switched leg to Load. | Terminate black (hot) wire on the black 'Line' screw; terminate red/black (load) on the brass 'Load' screw. |
| US Single-Pole (Unmarked) | No text, two brass screws | Standard mechanical switches are bidirectional. | Terminate either hot or load wire on either brass screw; orientation does not matter. |
| EU/UK 1-Way | COM / L1 | Connect permanent live to COM, switched live to L1. | Terminate brown (live) wire on the 'COM' screw; terminate switched brown/blue on the 'L1' screw. |
| EU/UK 2-Way (US 3-Way) | COM / L1 / L2 | Identify the common wire (always hot on one switch, load on the other). | Terminate the common wire on 'COM'; terminate the two strapper/traveler wires on 'L1' and 'L2' (order irrelevant). |
| Smart Switch (Any Region) | Line / Load / Neutral / Ground | Must match exactly; smart relays are directional. | Terminate always-hot to 'Line', fixture wire to 'Load', white bundle to 'Neutral', bare to 'Ground'. |
