The selection of the ac supply system voltage typically begins at the service entrance of the facility. In most commercial environments in the U. S., the utility supplies three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V.
In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility.
In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system.
Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over 208 Y/120 V systems:
a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.
b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.
c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.
Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.
Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.
It is not recommended practice to step-up the voltage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment. Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.
Due to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.
In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises. In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment.