14 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. A low power factor, as in our example, can be improved by certain corrective measures, such as changing various systems components and installing capacitors. 1.4 Utilization Voltages There are fi ve basic voltages derived from utility-supplied distribution. The commercial building application depends on load require- ments and economic considerations. • Single-phase, three-wire system, although most commonly used in residences, is often applicable to small commercial buildings and apartment complexes. See Figure 1-19. The 240-volt portion can be used for the larger power loads, while the 120-volt portion serves the lighting and receptacles. • Three-phase, three-wire system is a common secondary system used in commercial buildings. This is a delta- connected secondary with 240 volts, 480 volts, or 600 volts between phase conductors. The system is used when motor loads make up the majority of the overall load. Step-down transformers can be used to provide 120-volt service for lighting and receptacles. See Figure 1-20. • Three-phase, four-wire wye is the most common three-phase system, particularly the type providing 120/208-volt service. This system can serve a variety of power and lighting load conditions: four-wire, 120/208-volt loads three-wire, 120/208-volt Formulas: Apparent Power = V × I Power Factor = True Power ________ Apparent Power Solution: Apparent Power = 240 V × 10 A = 2400 VA Power Factor = 1900 W ____ 2400 VA = 0.79 loads three-wire, 208-volt loads two-wire, 208-volt loads and two-wire, 120-volt loads. Figure 1-21 illustrates this highly adaptable and fl exible system. • Three-phase, 480/277-volt, four-wire system is another extremely fl exible variation of the four-wire wye. It is a very economic commercial system and has advantages over the 120/208-volt system. This system provides for three types of loads: 277-volt, single-phase 480-volt, three- phase and 120/208-volt or 120/240-volt loads with the addition of a step-down transformer. The 480/277-volt, four-wire wye confi guration is shown in Figure 1-22. • Three-phase, four-wire, delta system is basically a modifi cation of the three-phase, three-wire delta. In this arrangement, the grounded neutral is added as a center tap to one of the secondary transformer windings, as shown in Figure 1-23. CAUTION In a four-wire delta system, the phase that is not connected to the tapped winding is called the high leg. The voltage diff erence between the high leg and the neutral is greater than the diff erence between the other phases and the neutral. The high leg conductor must be identifi ed with orange marking to prevent an electrician from mistaking it for one of the other phase conductors. 120 V 120 V 240 V Ground Neutral Goodheart-Willcox Publisher Figure 1-19. Single-phase, three-wire system. By connecting a “hot” wire to the grounded wire, 120-volt service is obtained. Connecting the two hot wires supplies 240 volts.