In electrical wiring, “line” refers to the wire bringing power in from the source, and “load” refers to the wire carrying power out to the next device. Think of it as a one-way street: electricity flows from your breaker panel through the line wire into a device, then continues through the load wire to whatever comes next in the circuit. These terms are sometimes called “incoming” and “outgoing,” or “upstream” and “downstream.”
How Line and Load Work in a Circuit
Every electrical circuit is a chain. Power leaves your breaker panel, travels through wires, passes through outlets or switches, and eventually reaches the device you’re powering. At each stop along that chain, the wire delivering power into the device is the line, and the wire sending power onward is the load.
Here’s the key concept that trips people up: line and load are relative to each device, not fixed labels for specific wires. The load wire leaving your first outlet becomes the line wire for the second outlet downstream. A single wire can be “load” from one device’s perspective and “line” from another’s. An electrician on the Mike Holt forum put it clearly: follow the line side and you’ll eventually trace your way back to the power generator; follow the load side and you’ll end up at whatever’s consuming the electricity.
This chain continues through every level of your electrical system:
- The load side of your utility transformer feeds the line side of your electric meter.
- The load side of your meter feeds the line side of your main breaker.
- The load side of your main breaker feeds the line side of each branch circuit breaker.
- The load side of a branch breaker feeds the line side of the first outlet or switch on that circuit.
Why It Matters Most on GFCI Outlets
For standard outlets, mixing up line and load isn’t usually a concern because both sets of terminals function the same way. GFCI outlets are different. They have two clearly marked pairs of screw terminals, one labeled “LINE” and one labeled “LOAD,” and connecting wires to the wrong pair can leave you unprotected.
The line terminals connect the incoming power from your breaker panel. This is what powers the GFCI outlet itself and gives it the ability to monitor for ground faults. The load terminals are for wiring additional regular outlets downstream that you want the GFCI to protect. If you wire everything to the line terminals only, the GFCI protects itself but nothing else on the circuit. If you connect both sets of terminals correctly, a single GFCI can extend its protection to every downstream outlet, which is why building codes often require only one GFCI per circuit in kitchens, bathrooms, and other wet areas.
If you accidentally swap the connections, putting the incoming power on the load terminals and the downstream wires on the line terminals, the GFCI may appear to work but its fault protection won’t function properly. During a ground fault, the device may not trip as designed, leaving you exposed to shock.
What Happens When Line and Load Are Reversed
On switches and other devices where line and load aren’t interchangeable, reversing the wires creates problems that range from annoying to dangerous. A light switch wired backward may leave the fixture partially energized even when the switch is off. The light might flicker, refuse to turn off completely, or keep internal components live when you think everything is safely de-energized. Parts of the circuit that should be inactive when switched off can remain hot.
The most serious risk is electric shock. When wiring is reversed, a device that appears to be off can still have energized internal components or a live metal casing. Touching those surfaces can deliver a dangerous shock. Over time, components under stress from incorrect wiring can overheat, degrade insulation, and increase fire risk. If an outlet, switch, or connected appliance feels warm during normal use, that’s a warning sign worth investigating.
Safety devices like circuit breakers and GFCI outlets may also fail to trip correctly during a fault when wired in reverse, because current is flowing through an unintended path. This defeats the entire purpose of having those protections in place.
How to Tell Which Wire Is Which
If you’re working on an existing circuit and aren’t sure which wires are line and which are load, the most reliable method is a non-contact voltage tester. With the power on (and being careful not to touch any bare conductors), hold the tester near each wire. The wire that reads as live is the line wire, since it’s being fed directly from the breaker panel. The load wire won’t be energized unless the device it passes through is completing the circuit.
On devices themselves, manufacturers often stamp “LINE” and “LOAD” directly on the housing near the corresponding terminals. GFCI outlets almost always have these markings, and many smart switches, timers, and dimmers do as well. When markings aren’t present, the line terminals are typically the ones closest to the power source entry point on the device, though this varies by manufacturer.
You can also identify line and load by circuit position. If two sets of wires enter an electrical box, the set coming from the direction of the breaker panel is the line, and the set continuing to the next outlet or switch is the load. In a single-gang box with only one set of wires, there is no load connection because that box is the last stop on the circuit.
“Load” as Electrical Demand
You’ll also encounter the word “load” used in a completely different sense. Beyond describing a specific wire, “load” is a general term for the electrical demand that devices place on a circuit. When someone says a circuit is “under heavy load,” they mean the total power draw from all connected devices is high. A 20-amp kitchen circuit running a toaster, microwave, and coffee maker simultaneously is under a heavier load than the same circuit running just a single appliance. This meaning is separate from the line-and-load wiring concept, but you’ll see both uses in electrical guides and on product labels.