Industrial and commercial wiring can be delivered in 120/208-volt and 277/480-volt systems. Heating and ventilation systems are run at 480 volts while industrial scale lighting runs off 277 volts. The 277/480-volt system powers the industrial and commercial equipment at 480 volts while the branches of 277 volt wiring keep the lights on.
Electrical discharge lighting is designed to run on 277-volt wiring. A three-phase, four-wire distribution system running at 277 volts is used for high voltage lighting like fluorescent lights. In "Commercial Electrical Wiring" the authors state, "Most larger commercial buildings utilize a 480/277-volt Y-connected service entrance."
A 277/480V, three-phase system can provide single phase or three-phase power at 277 volts or 480 volts. Electrical distribution systems with 277-volt wiring use step down transformers to provide either 240-volt or 120-volt power as required. Residential AC circuits use power at 120 amps and 277 volts. Wiring of a 277/480-volt system can supply both industrial and residential users. Using 277-volt wiring permits more lights to be powered by a single branch circuit. This reduces the amount of high voltage wiring that must be installed to connect banks of fluorescent lights or appliances.
National Electrical Code
The National Fire Protection Association (NFPA) publication 70E, also called the National Electrical Code (NEC), defines electrical safety standards in the United States. Alternating current (AC) 277/480-volt systems must have a 277-volt rating based on the National Electric Code (NEC). Article 210 of the National Electrical Code addresses 277- and 480-volt branch circuits. Section 215.10 of the NEC mandates ground-fault protection of equipment (GFPE) on branch of a 277/480-volt circuit. Switches are used to control the flow of power to lighting. The NEC prohibits voltage between circuits from exceeding 300 volts. On 277/480V systems, the voltage is checked between any powered leg of the Y and the circuit's ground. Yellow, brown and orange wires typically identify the ungrounded wires in a three-phase, four-wire, 277-volt system.
Wiring carrying 277 volts can create an arc flash hazard. The NEC states that alternating current wiring carrying more than 50 volts must be grounded unless one of several exemptions is met. According to "Handbook of OSHA Construction Safety and Health" by Charles Reese and James Eidson, grounding is not required "… if the system is nominally rated 480Y/277 volt, 3-phase, 4-wire in which the neutral is used as a circuit conductor." The grounded conductor wire is identified by its gray color.