Are All Rooftop Receptacles 120V?

120 Volts Versus 240 Volts

• All main electricity delivered to homes and nondomestic buildings is 240 or 220 volts; the supply is broken down to 120 or 110 volts at the fuse or circuit breaker panel. The 240 volt is typically supplied only to heavy-consumption appliances, because the voltage supplied has no impact on usage cost. The experts at Wiring Digest note that “irrespective of the voltage rating, the energy consumption of an appliance will be based on its wattage and the length of time it is operated.”

Cooling

• It's not common for commercial or industrial air conditioners to be plugged into a receptacle; such appliances are typically hard-wired to dedicated circuits. Further, it is unusual to find a domestic central or window-unit air conditioner on a roof; they are more commonly located at ground level. In the southwest, it's common practice to site swamp coolers on specially constructed rooftop supports; these are relatively simple devices, and typically use a 120-volt supply.

Landscaping and Advertising

• Landscape lighting is predominantly 12-volt, with the current supplied by a transformer. Transformers invariably plug into a 120-volt receptacle. Large installations do not use larger transformers that run on 220 volts; the advice from Residential Landscape Lighting and Design is that "you may need multiple transformers to achieve your landscape look.” It is, however, possible that advertorial material would use 220 volts; the gigantic illuminated, moving figures atop Las Vegas casinos are not running on 120 volts, but -- as with large air-conditioning installations -- nor are they plugged into receptacles. They are hard-wired to dedicated supplies.

Code and Common Sense

• Always consult with your municipality to determine local code requirements; what is mandatory for professional installers can be taken as sound advice for DIYers, also. The 2008 State of Oregon building code, section 210.63, is representative in requiring “that a 15 or 20 amp receptacle be installed within 25 feet of HVAC equipment.” The equipment -- wire, conduit, fasteners and receptacle -- should all be weather resistant, and the receptacle should be GFCI protected.