In electrical wiring, “line” refers to the wire bringing power in from your electrical panel, and “load” refers to the wire carrying power out to a device or the next outlet in the circuit. Think of line as the supply and load as the delivery. This distinction matters every time you wire a switch, outlet, or GFCI receptacle, because connecting wires to the wrong terminals can prevent devices from working or eliminate important safety protections.
How Line and Load Wires Work
A line wire is always “hot,” meaning it carries electrical current from your breaker panel to a device regardless of whether that device is switched on or off. It’s the input. A load wire is the output: it carries power from that device onward to whatever comes next, whether that’s a light fixture, an appliance, or another outlet downstream in the same circuit.
In a light switch circuit, for example, the line wire brings power from the panel to the switch. The load wire then runs from the switch up to the light fixture. When you flip the switch off, you’re breaking the connection between line and load, which cuts power to the fixture. The line wire stays energized the entire time.
If you have a series of outlets wired in sequence, the same idea applies. Power enters the first outlet on the line wire, and a load wire feeds electricity from that outlet to the remaining outlets on the circuit. Each outlet’s “load” becomes the next outlet’s “line.”
Why It Matters Most: GFCI Outlets
The place where getting line and load right is most critical is a GFCI (ground-fault circuit interrupter) outlet. GFCI receptacles have two sets of terminals clearly labeled “LINE” and “LOAD,” and mixing them up causes a specific, testable failure: the GFCI will not reset.
The LINE terminals connect to the incoming power from your breaker panel. A GFCI will function and protect itself with only these terminals wired. The LOAD terminals are optional. They send power out to other standard outlets further along the circuit, and when connected correctly, every downstream outlet receives GFCI protection from that single device. This is how builders protect an entire bathroom or kitchen circuit with one GFCI receptacle near the panel end of the run.
If you reverse the connections, putting the incoming power on the LOAD terminals instead of LINE, modern GFCI receptacles (built to standards updated in 2006) are designed to lock out. They won’t reset and won’t supply power to the receptacle face. This is a safety feature, not a defect. If your newly installed GFCI won’t reset, swapped line and load connections are the first thing to check.
Line and Load on Light Switches
On a standard single-pole light switch, both brass terminals are interchangeable. It doesn’t technically matter which one gets the line wire and which gets the load wire. The switch simply opens and closes the circuit between them. That said, electricians typically follow a consistent pattern, connecting line to the top terminal and load to the bottom, because it makes troubleshooting easier later.
The neutral (white) wires in a switch box usually bypass the switch entirely. They’re connected together with a wire nut and pass straight through. The switch only interrupts the hot conductor.
Line and Load at the Panel Level
The same line/load concept scales up to your entire electrical system. The main service wires coming from the utility meter into your main breaker are the “line side” of your home’s power. Everything on the other side of that main breaker, flowing out into your home’s circuits, is the “load side.” Your utility meter or main breaker is the dividing point between the two. Individual circuit breakers inside the panel then distribute power to each branch circuit, where the line/load relationship repeats at every device along the way.
How to Tell Line From Load
When you open an electrical box and find two sets of same-colored wires, figuring out which is line and which is load takes a few steps.
- Check for labels. On GFCI outlets and some smart devices, the terminals are stamped “LINE” and “LOAD.” Some electricians also label wires with tape during installation.
- Trace the wire path. The line wire usually comes from the direction of the panel, often entering the bottom of the box. The load wire typically exits toward the ceiling (for a light fixture) or continues to the next box in the run.
- Use a voltage tester. With the power turned back on carefully, test both wires using a non-contact voltage tester or multimeter. The wire that reads voltage is the line wire. The wire that shows no voltage (assuming any connected switch or device is off) is the load.
When both wires are black, a voltage tester is the most reliable method. The wire carrying voltage from the breaker panel is always the line. The other, which continues the circuit to the next device, is the load.
Why Smart Switches Require Correct Wiring
Smart switches and electronic dimmers are more demanding than traditional switches because they contain circuitry that needs a constant trickle of power. Most require a neutral wire in addition to correctly identified line and load connections. Unlike a basic single-pole switch where the two terminals are interchangeable, a smart switch typically has separate, labeled pigtail wires or terminals for line (black), load (red or a second black), neutral (white), and ground (green). Connecting line to the load terminal, or vice versa, can prevent the switch from powering on, damage its electronics, or cause it to behave erratically.
If you’re upgrading from a basic switch to a smart one, identifying your line and load wires before you disconnect anything saves significant frustration. Mark them with tape, note which wire stays hot when the old switch is in the off position (that’s your line), and follow the smart switch manufacturer’s wiring diagram exactly.
Wire Colors Vary by Region
In the United States (following the National Electrical Code), single-phase line wires are typically black, with red used as a second hot conductor. In countries following IEC standards, the single-phase line wire is brown, and three-phase systems use brown, black, and grey instead of the American black, red, and blue. Neutral is white in the U.S. and blue under IEC standards. These differences matter if you’re working in an older home with non-standard wiring or referencing guides from another country. When in doubt, test with a meter rather than relying on color alone.