The choice of system voltage according to ANSI standard C84.1

Power system design

The selection of system voltages is crucial to successful power system design. Reference lists the standard voltages for the United States and their ranges. The nominal voltages are given in Table 1 below.

As can be seen, ANSI C84.1-1989 divides system voltages into “voltage classes”.

The choice of service voltage is limited to those voltages which the serving utility provides. In most cases only one choice of electrical utility is available, and thus only one choice of service voltage. As the power requirements increase, so too does the likelihood that the utility will require a higher service voltage for a given installation.

Table 1 – Standard nominal three-phase system voltages per ANSI C84.1-1989

In some cases a choice may be given by the utility as to the service voltage desired, in which case an analysis of the various options would be required to arrive at the correct choice.

In general, the higher the service voltage the more expensive the equipment required to accommodate it will be.

Another factor to consider regarding service voltage is the voltage regulation of the utility system. Voltages defined by the utility as “distribution” should, in most cases, have adequate voltage regulation for the loads served.

Voltages defined as “subtransmission” or “transmission”, however, often require the use of voltage regulators or load-tap changing transformers at the service equipment to give adequate voltage regulation. This situation typically only occurs for service voltages above 34.5 kV, however it can occur on voltages between 20 kV and 34.5 kV. When in doubt the serving utility should be consulted.

The utilization voltage is determined by the requirements of the served loads.

For most industrial and commercial facilities this will be 480 Y/277 V, although 208 Y/120 V is also required for convenience receptacles and small machinery. Large motors may require 4160 V or higher. Distribution within a facility may be 480 Y/277 V or, for large distribution systems, medium voltage distribution may be required.

Medium voltage distribution implies a medium voltage (or higher) service voltage, and will result in higher costs of equipment, installation, and maintenance than low voltage distribution. However, this must be considered along with the fact that medium voltage distribution will generally result in smaller conductor sizes and will make control of voltage drop easier.

Principal transformer connections to supply the system voltages of Table 1 above.

The above diagrams show connections of transformer secondary windings to supply the nominal system voltages of table 1. Systems of more than 600 volts are normally three-phase and supplied by connections (3), (5) ungrounded, or (7).

Systems of 120-600 volts may be either single-phase or three phase, and all of the connections shown are used to some extent for some systems in this voltage range.

In connections (5) and (6) the ground may be connected to the midpoint of one winding as shown (if available), to one phase conductor (“corner” grounded), or omitted entirely (ungrounded).

Single-phase services and single-phase loads may be supplied from single-phase systems or from three-phase systems. They are connected phase-to-phase when supplied from three-phase, three-wire systems and either phase-to-phase or phase-to-neutral from three-phase, four-wire systems.

References:

• System Voltage Considerations – Bill Brown, P.E., Square D Engineering Services
• ANSI C84.1-2006 – American National Standard for Electric Power Systems and Equipment; Voltage Ratings (60 Hertz)