Line vs Load – Practical Implications, How to Differentiate, GFCI


In electrical terms, line refers to the wires that supply electricity to a device. Such a device could be a switch or panel that receives power into a building from a utility company. On the other hand, load refers to the wires taking power away from a device. Thus, the line vs load concept is very important in electrical wiring.

Line and load connections on an electrical panel

In this article, you will learn the practical implications of line vs. load, how to differentiate between them, and GFCI applications.

Implications of Line vs Load

Understanding that the terms ‘line’ vs ‘load’ with respect to a single device is important. When power comes into a building, the line connections bring it from the utility supply to the panel. Then load connections take the power to devices in the circuit. With respect to these devices, the load wires from the panel are the line wires of the devices. While the load wire of a device is the line wire of another device downstream in the circuit. When looking at a circuit, this may bring a bit of confusion, thus, there are a few indicators to differentiate between live and load wiring.

In addition, each device in a circuit is to have indicators of its line and load side to ensure proper connections. However, reversing these connections may not be of consequence in some devices. An example is a single-pole, single-throw switch, which works perfectly even when there is a reversal in connection. But, in multiple throw switches, the terminal connections are directional, so it will not function properly if reversed. In any case, reversing polarity creates a potential shock hazard, short circuit, or fire outbreak. This is because a device will remain energized even if the switch is off when there has been a polarity reversal.

Line vs Load Wires

To avoid undesirable consequences of reversing polarity in electrical connections, there are several conventions in the industry to differentiate between line and load wires. Some of the parameters to differentiate between the wires are as follows:

  • Position of the wires: By convention, the connection of the line wires to an electrical panel or switch enters from the bottom. The load wires come in from the top. In addition, these line vs load connection points have markings to indicate which type of wires a point is for.
  • Color coding: In electrical connections, color codes serve to identify various types of wires. Moreover, these codes vary according to country. For example, in some countries, black means line/upstream wires, while red means load wires. In addition, in some climes, the screws at each terminal follow a particular color code. Therefore, it is necessary to master the color-coding applicable to a region.
Black line wire connected to the bottom of a terminal
Courtesy: ORRO
Red load wire connected to the top of a terminal
Courtesy: ORRO
  • Size of wires: Because devices typically reduce the voltage or current, the amount of power a line wire carries is more than that of a load wire. As a result, line side wires are typically larger than load side wires. This especially holds true if the power difference is significant. Although in devices such as overloads or protection, there is no change to the voltage or current, rendering this method ineffective.
  • Measuring power features: As electrical power downstream of a device is less than upstream, measuring the voltage or current at these ends can help differentiate between line and load wires. Moreover, devices such as a volt stick/pen and digital multimeter provide non-intrusive ways of measuring these parameters. The neon screwdriver can measure these parameters when in contact with the bare wire or terminal screw.

GFCI Applications

Earlier sections of this article highlight the dangers of reversing the line vs load connection. If there is suspicion of reverse polarity, turn off the power to the room or outlet immediately. Then, use a standard outlet tester with a voltage chart to determine if the wiring of the outlet is correct. If the wiring is improperly connected, a simple switch of the wires resolves the problem. Unfortunately, this is a reactive approach and still exposes equipment and humans to the dangers of reverse polarity.

Outlet tester for checking the polarity of outlets
Courtesy: WAYPOINT

This is where the ground-fault circuit-interrupter (GFCI) outlets add value. Unlike a fuse, which serves to protect devices, GFCI integrates into the outlet and prevents the risk of shock. Moreover, it constantly monitors the current flow and interrupts it immediately there is a surge. Hence, it provides protection against any current abnormalities.

It is advisable to have a GFCI connection to both line and load terminals to provide protection for that outlet and others downstream the circuit. In addition, a GFCI outlet can also suffer reverse polarity. Thus, getting the line vs load connection at this outlet right is pivotal to the safety of all outlets downstream of the circuit.