240-Volt Outlets I What's a 240-Volt Outlet? - Mr. Electric Compared to standard 120-volt receptacles, 240-volt outlets are larger, with rounded tops, and three or four holes depending on the age of the outlet. Older three-prong outlets have a hole like a backward “L” on top and two diagonal holes on the sides. 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. 240-volt receptacles come in styles that can be mounted directly onto a surface or recessed into a wall. For added safety, you might consider an industrial model, which has a feature that locks the plug in the slot. This is particularly useful for large or very heavy power tools. GFCI Receptacles. 240-Volt window or through-wall room air conditioner. 240-Volt equipment such as small welders. Below are two additional configurations and types of 240 VAC electrical receptacles. The white three-bladed 240-V receptacle below is a 20-Amp Leviton product that is a non-grounding type electrical receptacle.