Anytime you hear of voltages like 120/208 and 277/480, bear in mind that these are three-phase voltages and are not available to the homeowner. 120/208 is a shortened version of the full terminology of 120/208 (3-phase, 4-wire wye). In the United States, and more so in the central United States, there are but a few voltages available to the commercial and industrial industries. Among these are 240 Delta, 120/240 3-phase 4-wire delta with a "wild" leg (High-Leg Conductor Identification, NEC 110.15) and the above. Sometimes you will hear the voltages reversed, such as 208/120 volt 3-phase 4-wire wye. You will occasionally hear of voltages like 2-phase and 4-phase, but these are usually found on the East coast where electricity has been around longer.
120/208 Vs. 277/480
If the question is which is the best voltage to use, 120/208 vs. 277/480, the answer is a qualified--Yes. All buildings whether office or factory must have 120 volts to use their office equipment and in areas like small kitchens. You also need 480v to operate the roof-top air-conditioning, large motors, heating strips for molding and in gereral all factory needs. The fluorescant lighting in the offices can be 277 volts.
120/208 Volts With 277/480 Volts
A well-planned office or factory takes advantage of both voltages. 277/480 volts is brought in as the primary power and then "piped" out to the warehouse to be used for motors and machinery, and to be connected to a 120/208 volt 3-phase 4-wire wye transformer. The transformer is usually sitting next to the 277/480 volt service. This 120/208 volts is "piped" into the office for office equipment and outlets and out to the factory floor for their 120-volt needs.
Office Building or Small Factory
A good electrical engineer will design a building to take advantage of the compatibility or marriage of both 277/480 and 120/208 volts. The main service coming into the building will be 277/480 volts 3-phase 4-wire Wye. This voltage can be broken down in the building very simply with a 3-phase step down transformer. A 25 kva, 120/208 3-phase 4-wire Wye transformer can be used to furnish all the 120 volts needed in the small office and warehouse. Now we have the best of both worlds. We have high-voltage for the 277 volt fluorescent office lighting and parking lot lighting and 480 volts for the factory and rooftop air conditioning. The transformer supplies smaller voltages for all 120-volt needs. The 208 volts can be used for the office air handling equipment.
Frank Bingham started writing in 2010. A veteran of the electrical field since 1969, he worked for a small wholesale electrical supplier, moving his way up from delivery to counter manager to outside sales. Frank started his own company specializing in small industrial and commercial electrical installations. After retirement he became an instructor on the National Electrical Code, at a local trade school.