The line wire is the ungrounded conductor that delivers electrical current from the main service panel to a specific load or device. If you are wiring a standard 120V branch circuit in the US or Canada under NEC rules, the line wire is black (or red for a second hot leg). In the UK, Europe, and regions following IEC standards, the single-phase line wire is brown.
Standard Line Wire Color Codes by Region and Voltage
Wire color codes are not universal; they are strictly defined by regional electrical standards to prevent fatal cross-wiring and ensure safe maintenance. Below is the definitive reference for line (hot/live), neutral, and ground colors across the most common global systems.
| Region / Standard | System / Voltage | Line (Hot) Color | Neutral Color | Ground / Earth Color |
|---|---|---|---|---|
| US / Canada (NEC / CEC) | 120V Single-Phase | Black | White or Gray | Bare Copper or Green |
| US / Canada (NEC / CEC) | 240V Split-Phase | Black & Red | White (if needed) | Bare Copper or Green |
| US / Canada (NEC / CEC) | 208V 3-Phase (Wye) | Black, Red, Blue | White or Gray | Bare Copper or Green |
| UK / EU / AU (IEC 60446) | 230V Single-Phase | Brown | Blue | Green/Yellow Stripe |
| UK / EU / AU (IEC 60446) | 400V 3-Phase | Brown, Black, Gray | Blue | Green/Yellow Stripe |
Note: In older US installations (pre-1950s), you may encounter knob-and-tube or early cloth-sheathed wiring where the line wire was black and the neutral was white, but insulation degradation makes color identification unreliable. Always test with a meter. For comprehensive US grounding and conductor identification rules, refer to OSHA 1910.304 Wiring Design and Protection.
What the Line Wire Changes in a Real Circuit
In a real circuit, the line wire establishes the RMS voltage potential relative to the grounded neutral and earth, dictating both the current flow and the voltage drop across the conductors. It is the 'source' side of the energy transfer.
Worked Numeric Example:
Imagine a 120V, 15A receptacle circuit wired with 50 feet of 14/2 NM-B cable (Black = Line, White = Neutral, Bare = Ground). You plug in a 12A resistive space heater.
- Source Potential: The black line wire leaves the panel at 120V RMS relative to the neutral bus.
- Wire Resistance: 14 AWG copper has a resistance of approximately 2.525 ohms per 1,000 feet at 75°C. For a 50-foot run, the one-way resistance of the line wire is 0.126 ohms.
- Voltage Drop: Under the 12A load, the voltage drop across just the line wire is calculated via Ohm's Law (V = I × R): 12A × 0.126Ω = 1.51V. (The neutral wire drops another 1.51V, for a total circuit drop of 3.02V).
- Result at the Load: The space heater actually receives 116.98V (120V - 3.02V).
This example illustrates what the line wire physically changes: it is not a perfect conduit. Its physical gauge and length introduce resistance that alters the final voltage delivered to the load, which is why the NEC recommends keeping branch circuit voltage drop under 3% (3.6V on a 120V circuit).
Where You Meet the Line Wire in Practice
You will interact with line wire identification constantly when upgrading or repairing home electrical systems. Here are the three most common scenarios where identifying the line wire is critical to the job's success and safety.
1. Upgrading to Smart Switches and Dimmers
Older mechanical single-pole switches simply break the line wire to interrupt current. Modern smart switches (like Lutron Caséta or TP-Link Kasa) contain internal Wi-Fi or Zigbee radios that require constant power. To install these, you must identify the constant line wire (always hot from the panel) and connect it to the switch's 'Line' or 'Hot' terminal, while also connecting a neutral wire to complete the low-power circuit for the smart radio. If you wire the switched leg into the line terminal, the smart switch will lose power the moment you turn the light off.
2. Wiring GFCI Receptacles (Line vs. Load)
Ground Fault Circuit Interrupter (GFCI) outlets have two distinct sets of terminals: LINE and LOAD. The LINE terminals must connect to the black (line) and white (neutral) wires coming directly from the breaker panel. The LOAD terminals are used to protect downstream standard outlets. If you reverse these and feed the panel's line wire into the LOAD terminals, the GFCI will power its own receptacle, but it will not trip to protect downstream devices during a ground fault, creating a severe shock hazard.
3. Switch Loops and White Wire Re-identification
In many homes built before the 2011 NEC update, switch loops were wired using standard 2-wire cable (Black and White). In this configuration, the white wire is used as the line wire traveling up to the switch, and the black wire returns as the 'switched leg' down to the light fixture. Because a white wire is universally assumed to be a neutral, the NEC (Article 200.7(C)(2)) strictly requires that this white line wire be re-identified with black tape, black paint, or a black marker at both ends to warn future electricians that it is a hot conductor.
Line vs. Load vs. Neutral: Clearing Up the Confusion
What is the difference between Line and Load?
The Line wire brings power from the source (the breaker panel). The Load wire carries power to the next device (like a light fixture or a downstream outlet). On a standard single-pole switch, the line is the always-hot wire, and the load is the switched leg. You can identify the line wire by turning off the breaker, disconnecting the wires, turning the breaker back on, and carefully testing with a non-contact voltage tester. The wire that triggers the tester is the line; the dead wire is the load.
How do I test which wire is the line with a multimeter?
Set your multimeter to AC Voltage (V~). With the circuit energized, place the black probe on a known ground (like the bare copper ground wire or a metal junction box) and the red probe on the suspected line wire. A reading of ~114V to 126V confirms it is the line wire. If you test the suspected load wire (with the switch off), you will read 0V. If you test the neutral wire, you should also read ~0V relative to ground.
Can a line wire ever be white?
Yes, but only in specific configurations like switch loops or when pulling a 240V appliance circuit (like a baseboard heater) where no neutral is required. In both cases, the National Electrical Code mandates that the white wire be permanently re-identified with a dark color (black, red, or blue tape/marker) at every point where the insulation is visible. If you open a junction box and see a white wire with black electrical tape wrapped around it, treat it as a hot line wire.
What happens if I swap Line and Neutral on a device?
On a standard duplex receptacle, swapping line and neutral (polarity reversal) will still allow a lamp to turn on, but it creates a shock hazard. The device's internal switch will break the neutral path instead of the hot path, meaning the bulb socket remains energized at 120V even when the lamp is turned off. Always use a simple $10 receptacle tester to verify correct line/neutral orientation after installation.
