How Do You Read an Electrical Ladder Diagram?

To read an electrical ladder diagram, first identify the two rails that signify the supply power, and then locate the rungs that portray all of the control circuits connected to the power source. The L1 designation typically refers to the hot conductor on the 120-volt AC supply, and the L2 designation typically refers to the neutral, or grounded, conductor.

Also known as ladder logic, electrical ladder diagrams are used to illustrate how all of the switches and relays are connected. On top of the rails and rungs, other important aspects to consider when reading an electrical ladder diagram include:

  • The power source is typically left out of diagrams, but ladder diagrams can be used to display both AC and DC power systems.
  • Wires should have a coordinating label that goes along with the identification marked on the diagram, including the wire number and if it is going toward L1 or L2. Continuous wire assemblies all need to be marked with the same number for ease of maintenance and repairs.
  • The loads should ideally be drawn on the right-hand side of the rung, which is the same side as the ground connection. This allows the system to blow a fuse in the event of a faulty circuit wire, rather than forcing power to an outlet and rendering a power switch useless.