How to Wire a Nema L14-20P Plug | Hunker How to Wire a Nema L14-20P Plug NEMA L14-20 wiring is virtually identical to that for the more common NEMA L14-30 plug/receptacle pair, so you can use an L14-30 wiring diagram if you need assistance. You probably won't need it, though, because both plugs follow standard 240-volt wiring. Nema L14 20p Wiring Diagram | Free Wiring Diagram Collection of nema l14 20p wiring diagram. A wiring diagram is a streamlined conventional photographic representation of an electric circuit. It reveals the components of the circuit as streamlined shapes, and the power and signal connections in between the gadgets.