PLC Training - Reading electrical wiring diagrams and That means it is on Rung 301, it is a switch (SW), and usually there will be a number that increments by one from left to right. So if we had a second switch it would be labeled 301SW2. The switch would be labeled 301SW1 in the field. That way when someone looks at the switch, they know exactly where to find it in the wiring diagram How to read an electrical diagram Lesson #1 - YouTube Click to view on Bing6:17Thermostat Wiring to a Furnace and AC Unit! Color Code, How it Works, Diagram! - Duration: 5:39. AC Service Tech LLC 59,781 viewsAuthor: realfixesrealfastViews: 1 CONTROL CIRCUITS:SCHEMATIC DIAGRAMS , WIRING DIAGRAMS A wiring diagram is used to represent how the circuit generally appears. To help illustrate the differences between wiring diagrams and schematics, a basic control circuit will first be explained as a schematic and then shown as a wiring diagram. READING SCHEMATIC DIAGRAMS. To read a schematic diagram, a few rules must first be learned.