Symbols:
There are dozens of symbols used in a schematic. It is important to know what those symbols are and
how they relate to each other. Symbols are fairly universal across different industries that use wiring
schematics. For our purposes, we will show common symbols used in LiftMaster operator schematics.
First, let’s cover some common terminology in schematics.
L1, L2: Incoming power for single phase operators.
L1, L2, L3: Incoming power for three phase operators.
Normally open or NO: A switch or contact that is open; in other words the switch or contact will not
allow voltage to flow if the switch or contact is not being activated.
Normally closed or NC: A switch or contact that is closed; in other words the switch or contact will allow
voltage to flow if the switch or contact is not being activated. If the switch or contact is activated, it
stops the flow of voltage.
A straight line indicates a wire, plus the wire color:
Line thickness differentiates between field wiring (wired installed by the installer for control devices)
and factory wiring:
Field:
Factory:
This is a screw terminal: The number indicates the terminal number.
This is a wire nut:
Or:
Transformer: This is a device that takes an incoming power, say 120vac. and transforms it to a different
voltage, say 24vac.
Relays: These devices switch power from one place to another; think of it like a light switch activated
with a magnet.
Relay or contactor coil: This is where power is applied to activate the relay or contactor. The letters or
numbers inside indicate the relay or contactor.
Relay or Contactor Normally Open (NO) contact:
Relay or Contactor or Overload Normally Closed (NC) contact:
Switches:
Limit Switch (Wired NC):
Limit Switch, (Showing NO and NC):
Hand chain or Interlock Switch:
Factory Jumper Wire:
External control device wiring:
Overload:
Motors:
Brake:
Resistor:
Capacitor:
We do not use the below symbol in most of our diagrams but I will use it for demonstration: