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.