In electrical wiring, “line” refers to the incoming power and “load” refers to the outgoing power. The line side carries electricity from the source (your utility company or breaker panel) to a device, while the load side carries electricity from that device onward to the next one. This distinction matters every time you wire a switch, outlet, or protective device, because connecting wires to the wrong terminals can create serious safety hazards.
Line: The Power Supply Side
The line is always the supply side of any electrical connection. Think of it as the path electricity takes to reach a device. The very first line in your home runs from the utility company’s power lines to your main electrical panel. From there, individual line wires run from each circuit breaker to the electrical boxes throughout your house.
Line wires are always “hot,” meaning they carry an electrical current whenever the breaker is on. At any point in a circuit, the line wire is the one bringing power in. At a light switch, the line wire is the one delivering electricity from the panel to the switch. At an outlet, it’s the wire feeding power from the breaker.
Load: The Power Distribution Side
The load wire picks up where the line wire leaves off. It carries electricity from a device onward to its destination or to the next device in the chain. At a light switch, the load wire runs from the switch to the light fixture. In a series of outlets wired in sequence, the load wire feeds electricity from the first outlet to the remaining outlets downstream on the same circuit.
The key thing to understand is that “line” and “load” are relative terms. A wire that’s the load coming out of one device becomes the line feeding into the next device. The labels describe a wire’s role at a specific point in the circuit, not an inherent property of the wire itself.
Line and Load at the Electrical Panel
Your main electrical panel is the clearest place to see the line/load divide. Two main service wires come into the panel from your utility meter. Everything between the utility meter and the main breaker is considered the line side. The instant power passes through the main breaker and flows into your building’s circuits, it’s on the load side. Your utility meter or main breaker is the dividing point between the two.
This distinction becomes important for things like solar panel installations. A solar system connected between the meter and the main panel is a “line side” connection, while one wired to a breaker inside the panel is a “load side” connection. The same line/load logic applies at every scale, from the main panel down to individual switches.
Why It Matters Most on GFCI Outlets
The line/load distinction is most critical when wiring GFCI (ground-fault circuit interrupter) outlets, the safety outlets commonly required in kitchens, bathrooms, and garages. A GFCI receptacle has two sets of terminals clearly marked “line” and “load.” The line terminals connect to the wires bringing power in from the breaker panel. The load terminals connect to wires going downstream to other outlets on the circuit.
When wired correctly, the GFCI protects not just itself but every outlet downstream from it. That’s how a single GFCI outlet in a bathroom can protect additional standard outlets further along the circuit. If you connect the incoming power wires to the load terminals instead, the GFCI may appear to work normally but will offer no ground-fault protection at all. It’s one of the most common and dangerous wiring mistakes in residential electrical work.
Line and Load on Switches
On a standard single-pole light switch, the line wire connects to one terminal and the load wire connects to the other. The line brings power in, and the load sends it to the fixture. When you flip the switch off, it breaks the connection between line and load, cutting power to the light.
Three-way switches (where two switches control the same light) add complexity. One switch receives the line wire on its “common” terminal. The other switch sends the load wire to the light fixture from its common terminal. Two additional “traveler” wires connect the two switches to each other, always in a pair. Smart switches and modern controls follow the same basic logic but are especially sensitive to correct line/load orientation. A smart switch with reversed wiring may lose its Wi-Fi connection, fail to respond to commands, or appear completely dead even though power is present in the box.
How to Identify Line and Load Wires
Wire color alone won’t reliably tell you which wire is line and which is load. While industry convention uses black for the first hot phase and red for the second, the National Electrical Code doesn’t mandate specific colors for hot wires in standard residential systems. Both line and load wires are typically black in a single switch or outlet box, making them visually identical. The NEC only strictly requires that neutral wires be white or gray.
The most reliable method is testing with a non-contact voltage tester or multimeter. Start by turning off the breaker and confirming no power is flowing. Disconnect the wires from the device. Then carefully turn the breaker back on and test each wire. The wire that reads voltage is your line wire, since it’s receiving power directly from the panel. The wire that shows no voltage is the load, because it only becomes energized when connected through a device. Turn the power off again before making any connections.
If you’re replacing an existing device, take a photo of the wiring before disconnecting anything. Many devices have “line” and “load” stamped next to their terminals, so noting which wires were connected where on the old device gives you a reliable reference.
What Happens When Line and Load Are Reversed
Swapping line and load wires can create hazards that aren’t immediately obvious. The most dangerous scenario is a safety device that appears to function normally while providing no actual protection. A GFCI with reversed wiring may power on, pass its test button check, and still fail to trip during a real ground fault.
Other symptoms of reversed wiring include:
- Outlets losing downstream power: Devices further along the circuit stop working because the current path is disrupted.
- A GFCI that won’t reset: The receptacle trips immediately or refuses to stay in the “on” position.
- Lights staying on when switched off: The switch breaks the line connection but the load wire remains energized through an alternate path.
- Frequent breaker trips: Improper connections can cause overcurrent conditions that repeatedly trip the circuit breaker.
- Energized equipment during maintenance: A load wire that stays hot without a switch or protective device controlling it means connected equipment can run unexpectedly. Anyone working on that equipment may assume it’s safe to touch when it isn’t.
Heat buildup from poor connections at incorrectly wired terminals is another risk, particularly in older boxes where wire gauge and terminal size may not align perfectly. If you notice any of these symptoms after installing a switch, outlet, or GFCI, the first thing to check is whether the line and load connections are reversed.