In an electrical circuit, "line" and "load" are the terms that describe the two points of connection for a device within a circuit. Line wire refers to the hot wire that comes directly from the power source, such as the electrical panel, to other devices, including switches and outlets.

Load wire carries the power forward from switches and outlets to electrical appliances and fixtures. Line wires deliver the power into the box, while load wires are the ones that pass electricity along. Using these terms makes the most sense in the context of a single box, as the load wire for one outlet becomes the line wire for the next outlet. 

Line and load wires are located at different sides of an electrical box. They are both hot wires color-coded either in black or red color. Line wire is more likely to be black, while load wire is more likely to be red. However, it is not guaranteed. Both line and load wires can be red or black.

When is it necessary to determine line wire from load wire?

There are several instances when it is necessary to determine line wire from load wire. These include installing, replacing, and troubleshooting devices like GFCI outlets, light switches, and dimmers. While working with these devices, you must know wiring line vs load to not mix up the cables and wire the devices correctly so that they function properly.

Here are some specifics of the different instances when the knowledge of wiring load vs. line is a must:

• GFCI outlets: GFCI is designed to shut the power in the event of the ground fault, with separate terminals for line vs. load wiring. It is important not to mix them up and connect the wires correctly. 

With GFCI outlets, mixing line and load wire is actually dangerous because GFCI outlets have internal breakers, and the downstream load wire will keep working, which may lead to accidental electrocution. With most other circuits, line and load wires will simply not work if mixed up.

• Light switches and dimmers will not work if the load and line wires are mixed up.
• Troubleshooting any electrical issue: When troubleshooting any electrical issue with a device not working, knowing where the line and load wires are located helps determine whether the power is reaching the device.

How to determine line wire from load wire? Multimeter vs. voltage tester vs. voltage detector pen

You will need to use extra devices to determine a line wire from a load wire. The devices you could use for this purpose include a multimeter, voltage tester, or voltage detector pen (or non-contact voltage tester).

The wire that will have the voltage after switching off outlets and switches is the line wire because it is connected to the power source.

With a multimeter and voltage tester, one probe must touch a hot wire, and the other probe touches the grounded point.

A voltage detector pen or non-contact voltage tester is a safer device for those with no prior experience in handling live wires. To determine whether the wire is live, bring the device close to the wire. A voltage detector pen is a less accurate device compared to the multimeter or voltage tester, as it only gets you a yes/no answer to whether the voltage is present and can sometimes give false positives due to capacitive coupling (where a non-hot wire picks up a weak signal just from being near a hot wire).

Another way to determine the wiring load vs. line is to turn off the power and manually examine the wires.

Once you have safely determined which wire is line and which is load, mark the wires with the power switched off to ensure there are no mix-ups in the future.

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