Line And Load On A Switch

Line and load are two terms that are commonly used in the electrical industry when discussing switches. Understanding the difference between these two terms is important for anyone who is working with switches or designing electrical systems. In this article, we will define line and load, discuss their differences, and address some frequently asked questions about these terms.

What is Line?

The term line refers to the power supply that is used to energize the switch. It is the supply of electrical energy that comes from your power source or utility company to your electrical panel, and then to the switch. The line is the connection that carries the electrical load from the source to your switch. In other words, it is the input side of the switch.

What is Load?

The term load refers to the device or appliance that you are controlling with the switch. It is the output side of the switch. The load is the device that is connected to the other side of the switch, which can be a light bulb, a motor, or any other electrical device. The load is what is being powered by the line.

Difference Between Line and Load

The main difference between line and load is that line is the input side of the switch, and load is the output side of the switch. When putting it another way, the line is the source of power, and the load is the device that is being powered.

There are several reasons why it is important to distinguish between line and load when wiring switches. One of the most critical is safety. If you confuse the line and load connections when wiring a switch, you could create a hazardous condition. For example, if you connect the line to the load side of the switch, power will be constantly supplied to the load even when the switch is turned off. This can lead to overheating, fires, or electrical shock. That’s why it’s very important to identify which side is the line and which side is the load when working with switches.

Another reason to distinguish between the line and load is to ensure proper functionality. If you swap the line and load connections, your device may not work correctly. For instance, if you connect a light bulb to the line side and the switch to the load side, the switch will control the power in a reverse manner. It means the light will be turned off when the button is pressed, and will turn on when released.

How to Identify Line and Load?

It is essential to note that line and load may not always be clearly labeled on your switch. In addition, wires coming into your electrical box may not show their respective connections. In other words, it can be challenging to identify which wire is the line and which is the load, especially when you are dealing with older switches, or if the wires are the same color.

Usually, the easiest way to identify line and load is to look at the wiring diagram for your switch. This diagram will show which wires are the line and which are the load. If the mapping is difficult, one common approach is to test each wire to see which one is hot or energized. Connect a tester or a voltmeter on each wire, and the wire that provides voltage or electrical activity is the line. The wire that does not provide any reading at the time of testing is the load.


Q: Can I switch line and load wires?
A: Switching line and load wires is not recommended, as it can lead to electrical hazards or malfunctions in the device being powered.

Q: Can both line and load wires be black?
A: Yes. Sometimes, both line and load wires can be the same color, making it hard to identify which is which. In this case, you may need to use a tester to determine which wire is hot and which is not.

Q: Does it matter which wire you connect to the line or load side of the switch?
A: Yes. Connecting the wrong wire to the line or load side of the switch can compromise safety, result in equipment damage, or malfunctions.


In conclusion, line and load are two crucial concepts when it comes to switches. Knowing the difference between line and load is crucial for safety and functionality purposes. Always make sure to identify which side is line and which side is load when wiring up switches. Use a wiring diagram or a tester to make sure you are connecting the right wires to the right terminal. The safety of people and equipment depends on it.