Line vs load: what’s the real difference in wiring?
People often ask about line vs load because the labels look simple, yet wiring mistakes here can cause real problems.
The direct answer is this: line is where power comes from, and load is where power goes to. If these are reversed, a device may still turn on, but built-in safety features often won’t work as intended.
In basic electrical wiring, the line wire carries electricity from the power source, usually the breaker or main supply. The load wire carries electricity away from the device to whatever it controls, such as a light, outlet, or appliance. The device sits between the two points.
This difference matters because many modern devices depend on correct power direction. GFCI outlets, smart switches, dimmers, and timers need line-side power to run internal protection or electronics. When line and load are swapped, protection can quietly fail.
In simple terms, line feeds the device and load continues the circuit beyond it. You can think of line as incoming power and load as outgoing power, but the device must be wired correctly to manage that flow.
A quick way to remember:
- Line: power coming from the breaker
- Load: power going to lights or outlets
- Line: stays energized when the breaker is on
- Load: only energized after passing through the device
Where confusion usually happens is inside switch boxes. Wire colors are not always reliable indicators, and in some setups power enters at the fixture instead of the switch. That’s why testing wires matters more than assuming.
In real use, reversing line and load can cause subtle issues. A GFCI may reset but fail to protect downstream outlets. A smart switch may flicker, disconnect, or behave unpredictably under load.
Keep in mind, turning off a switch does not always cut power to the line wire. The line side often remains live unless the breaker is off, which is why proper identification is important.
For now, that’s the situation.