97 / 156
Electrical Theory & Applications for HVACR
Chapter 4: Understanding Wiring Diagrams
Page 85
READING A WIRING SCHEMATIC
Reading a wiring schemaƟc can be made easier by following a few simple rules:
SchemaƟcs are read like books, from top to boƩom and leŌ to right.
There must be a complete circuit in order for current to flow through a component.
Electrical contacts and switches are shown in their normal “off” posiƟon, unless otherwise stated
on the diagram.
When a relay is energized, all of its contacts change posiƟon. Normally open contacts close and
normally closed contacts open. All contact symbols with the same coil number or leƩer are
controlled by that coil, regardless of locaƟon in the circuit.
Switches or contacts used to provide the stop funcƟon are normally closed and generally wired in
series.
Switches or contacts used to provide start funcƟon are normally open.
Fig. 4‐9: Wiring schemaƟc