Low-Voltage Utilization

With most low-voltage services, the service voltage is the same as the utilization voltage. However, when the engineer is faced with a decision between 208Y/120 V and 480Y/277 V secondary distribution for commercial and institutional buildings, the choice depends on several factors. The most important of these are the size and types of loads (motors, fluorescent lighting, incandescent lighting, receptacles) and length of feeders. In general, power system designs with HVAC equipment with a significant quantity of motors, predominantly fluorescent lighting loads, and long feeders, will tend to make 480Y/277 V more economical. 

Industrial installations with large motor loads are almost always 480 V resistance grounded, wye systems (see further discussion on this topic in the Grounding/ Ground Fault Protection section of this Design Guide). 

Practical Factors

Because most low-voltage distribution equipment available is rated for up to 600 V, and conductors are insulated for 600 V, the installation of 480 V systems uses the same techniques and is essentially no more difficult, costly or hazardous than for 208 V systems. The major difference is that an arc of 120 V to ground tends to be self-extinguishing, while an arc of 277 V to ground tends to be self-sustaining and likely to cause severe damage. 

For this reason, Article 230.95 of the National Electrical Code requires ground fault protection of equipment on grounded wye services of more than 150 V to ground, but not exceeding 600 V phase to-phase (for practical purpose, 480Y/277 V services), for any service disconnecting means rated 1000 A or more. 

Article 215.10 of the NEC extends this equipment ground fault requirement to feeder conductors and clarifies the need for equipment ground fault protection for 1000 A and above, feeder circuit protective devices on the 480/277 Vac secondary of transformers. Article 210.13 has been added to the 2014 NEC, essentially recognizing the same need for equipment ground fault protection on 1000 A branch circuits being fed from the 480/277 Vac secondary of transformers. 

The National Electrical Code permits voltage up to 300 V to ground on circuits supplying permanently installed electric discharge lamp fixtures, provided the luminaires do not have an integral manual switch and are mounted at least 8 ft (2.4 m) above the floor. This permits a three-phase, four-wire, solidly grounded 480Y/277 V system to supply directly all of the fluorescent and high-intensity discharge (HID) lighting in a building at 277 V, as well as motors at 480 V.

Technical Factors

The principal advantage of the use of higher secondary voltages in buildings is that for a given load, less current means smaller conductors and lower voltage drop. Also, a given conductor size can supply a large load at the same voltage drop in volts, but a lower percentage voltage drop because of the higher supply voltage. Fewer or smaller circuits can be used to transmit the power from the service entrance point to the final distribution points. Smaller conductors can be used in many branch circuits supplying power loads, and a reduction in the number of lighting branch circuits is usually possible. 

It is easier to keep voltage drops within acceptable limits on 480 V circuits than on 208 V circuits. When 120 V loads are supplied from a 480 V system through step-down transformers, voltage drop in the 480 V supply conductors can be compensated for by the tap adjustments on the transformer, resulting in full 120 V output. Because these transformers are usually located close to the 120 V loads, secondary voltage drop should not be a problem. If it is, taps may be used to compensate by raising the voltage at the transformer. 

The interrupting ratings of circuit breakers and fuses at 480 V have increased considerably in recent years, and protective devices are now available for any required fault duty at 480 V. In addition, many of these protective devices are current limiting, and can be used to protect downstream equipment against these high fault currents.

Economic Factors

Utilization equipment suitable for principal loads in most buildings is available for either 480 V or 208 V systems. Three-phase motors and their controls can be obtained for either voltage, and for a given horsepower are less costly at 480 V. LED lighting as well as earlier technologies including fluorescent, HID and high pressure sodium can all be applied in either 480 V or 208 V systems. However, in almost all cases, the installed equipment will have a lower total cost at the higher voltage.

Figure 1.1-23. Typical Power Distribution and Riser Diagram for a Commercial Office Building