240-Volt Outlets I What's a 240-Volt Outlet? Newer four-prong outlets have an “L” on top,two vertical side holes,and a single half-circle hole on the bottom for the ground wire.Three-Prong vs. Four-Prong 240-Volt Outlets.Recently, 240-volt outlets have switched from three-prong to four-prong.Older three-prong outlets were designed to contain two live wires and one neutral. Each of these outlets is a 240 volt outlet designed to supply more electrical power for your larger appliances through the 240 volt wiring and 240v single phase plug.These are also quite different from the standard 120v outlets,having more wires, higher voltage,and greater hazards and safety risks. A 240V outlet is larger than a standard outlet,normally 4 1/2 inches high and 4 1/2 inches wide with either a single three prong opening in the shape of a Y or a single four prong opening that includes a ground. Step 2 Test the outlet using a multimeter.A multimeter is a electrical tool commonly used to measure current,voltage and resistance.