Last Updated on March 31, 2024 by Dave Farquhar
Wiring conventional outlets and switches is usually pretty easy. As long as you match up the colors and match the right colored wires to the right colored pairs of screw terminals, everything just works. But with ground-fault circuit-interrupter (GFCI) or arc-fault circuit interrupter (AFCI) outlets and occupancy detector switches, there’s an indicator for line and load. And if you mix up the line and load, nothing works. In this blog post, I’ll cover the difference between line vs load when wiring and how to find which is which in your electrical box.
In electrical wiring, the line is the upstream voltage from the source. The load is the downstream voltage to devices further along the circuit.
What are the line and load in electrical wiring?
The line in electrical wiring is the pair of wires carrying electricity from the upstream source, whether that’s a junction box somewhere or direct from your electrical panel. Where it’s coming from doesn’t matter, just that those lines are getting power.
If you only have one pair of wires coming into your outlet, that is the line. When you only have one pair of wires, there is no load.
The load is the pair of wires giving the downstream connection to the next device further along the circuit. Or in the case of a light switch, it’s the wire going to the light fixture that the switch controls. When this pair of wires isn’t connected to the outlet, it’s electrically dead. And that’s why electrical devices that care about line and load don’t work when you mix them up.
Safety precautions
To find which pair of wires is the line side and which one is the load, you have to work on the box live. Here’s the caveat working on live mains power in North America. One of my friends put it a little more rudely then this, but the family friendly version is that 110 volts is enough to make you mad, but it probably won’t kill you.
He’s not wrong, I’m still here. My track record is that I accidentally shock myself about once a decade. But I don’t make a habit of it.
In some parts of the world, mains voltage is over 200 volts. If you are in one of those countries, I don’t know how much of this will apply. I have not worked on live 220 volt power and I have no reason to.
Finding line and load wires
What you will need to do is shut off the power at the breaker so you can safely take the outlet apart, assuming you haven’t already done so. Use a circuit tracer if needed. Take the outlet out of the wall box and remove the wires from the terminals. Do what you need to do to prepare the wires for the new switch or outlet. That means shaping the wire into a loop, straightening the wires, or if you have enough slack, cutting the loop and stripping back about 9/16 of an inch of insulation to get fresh bare copper and then shaping it.
Get your digital multimeter and switch it to continuity and touch the probes together to make sure that it is working. Then switch your multimeter or voltage tester to AC voltage. In a pinch you can use a volt stick, but I find I get better results with a multimeter.
I also put on a pair of rubber gloves as a precaution. Well, except for that one time a decade I already mentioned. If I remembered the gloves every time, my track record would be better.
Connecting your device to the line and load wires
If there is anyone else in the house, close the door to the room you are working in. Let everyone in the house know you are working on mains power and you are not to be interrupted. Seriously, you are not to be interrupted. Restore power at the breaker, go back to the room, close the door to keep kids and pets out, then return to the outlet. Check the voltage on both pairs of wires. Theoretically, it’s only the black wire that matters, the black live wire or hot wire will provide 115 volts with either of the white neutral wires.
You will find 115 volts on one pair of wires and 0 volts on the other pair. Mark the pair that measures zero volts with a piece of masking tape. If you want to be nice to whoever the next person is who works on this outlet, write the word LOAD on the piece of masking tape.
Now that you found the load, it’s time to shut the power back off at the breaker. Safety first: remind everyone there is still live power exposed in that room, close the door, then go to the breaker box and shut off the breaker again.
Connecting your device
Return to the outlet, attach the line wires to the line terminals or terminal, and attach the load wires to the load terminals or terminal. Remember, black wires go to the brass terminals and white wires go to the silver terminals.
At this point, I’m always torn on whether to reassemble the outlet before restoring power and testing, or after. It is far safer to test with the outlet reassembled. But if you made a mistake, it means you have to take the outlet out of the wall again. Being a safety first kind of guy, I reassemble the outlet.
Reassemble the outlet, return to the breaker panel, restore power at the circuit breaker, then go test the outlet with an outlet tester, or in a pinch, your multimeter. But it’s really better to use an outlet tester. If the outlet is dead, push the reset button and then retest. Then press the test button, make sure it cuts off power, then press the reset button again.
Why devices care about line vs load when wiring
The reason GFCI and AFCI outlets care about wine versus load is because of how they work. Their job is to interrupt power when they sense something about the flow of power isn’t right, such as a ground fault in the case of a ground-fault circuit interrupter, or excess arcing in the case of an arc-fault circuit interrupter. In order to do that, it matters which set of terminals is line and which side is supplying the load. When the outlet trips, it will cut off power at the load side. This shuts itself off, along with any outlet downstream of it in the circuit. This also means that if you put a GFCI outlet in a room and all of them are on the same circuit, if you put the GFCI outlet in the right place, you only need one GFCI outlet in the room, and it will protect all of the outlets in the room. You can verify this by plugging a lamp into one of the downstream outlets, tripping the GFCI outlet, and switching the lamp on.
While the line versus load distinction makes things a little trickier to understand, there is also upside. With GFCI outlets costing $20 to $25 apiece, understanding line versus load can save you some serious money.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.