System Voltage Considerations

The selection of system voltages is crucial to successful power system design. Reference American National Standard Preferred Voltage Ratings for Electric Power Systems and Equipment (60Hz) ANSI C84.1-1989.* lists the standard voltages for the United States and their ranges. The nominal voltages from American National Standard Preferred Voltage Ratings for Electric Power Systems and Equipment (60Hz) ANSI C84.1-1989.* are given in Standard Nominal Three-phase System Voltages per ANSI C84.1-1989.

ANSI C84.1-1989 divides system voltages into “voltage classes”. Voltages 600 V and below are referred to as “low voltage”, voltages from 600 V - 69 kV are referred to as “medium voltage”, voltages from 69 kV - 230 kV are referred to as “high voltage”, and voltages 230 kV - 1,100 kV are referred to as “extra high voltage”, with 1,100 kV also referred to as “ultra-high voltage”. The emphasis of this guide is on low- and medium-voltage distribution systems.

The choice of service voltage is limited to those voltages that 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 to does the likelihood that the utility will require a higher service voltage for a given installation. 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 is required to arrive at the correct choice. In general, the higher the service voltage the more expensive the equipment required to accommodate it is. Maintenance and installation costs also increase with increasing service voltage. However, equipment such as large motors may require a service voltage of 4160 V or higher, and, further, service reliability tends to increase at higher service voltages.

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, consult the serving utility.

The utilization voltage is determined by the requirements of the served loads. For most industrial and commercial facilities this is 480Y/277 V, although 208Y/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 480Y/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 results 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 generally results in smaller conductor sizes and makes control of voltage drop easier.

Power equipment ampacity limitations impose practical limits on the available service voltage to serve a given load requirement for a single service, as shown in Equipment Design Limits to Service Voltage vs. Load Requirements, for a Single Service.

Because all conductors exhibit an impedance to the flow of electric current, the voltage is constant throughout the system, but tends to drop closer to the load. Ohm’s Law, expressed in phasor form for AC circuits, gives the basic relationship for voltage drop vs. the load current:

Where:

Thus, the larger the load current and larger the conductor impedance, the larger the voltage drop. Unbalanced loads, of course, give an unbalanced voltage drop, which leads to an unbalanced voltage at the utilization equipment.

Section 210.19(A) – branch circuits – Informational Note N° 3 recommends the voltage drop at the farthest outlet of power, heating, and lighting, or combination of such loads, to three percent of the applied voltage. Alternatively, the maximum combined voltage drop on the feeder and branch circuits to the farthest outlet should be five percent.

Section 215.2(A)(1) – feeders – Informational Note N° 2 has the same recommendations for feeders.

Those statements mean that the feeder could have a one percent voltage drop if the branch circuit had no more than four percent. Also, limiting the branch circuit voltage drop to three percent allows a two percent drop in the feeder. These or any other combinations of feeder and branch circuit voltage drops not exceeding a total of five percent are adequate.

A voltage drop of five percent or less from the utility service to the most remotely-located load is recommended by NEC article 210.19(A)(1), FPN No. 4. Because this is a note only, it is not a requirement per se but is the commonly accepted guideline.

Because conductor impedance increases with the length of the conductor, unless the power source is close to the center of the load, the voltage varies across the system, and, further, it can be more costly to maintain the maximum voltage drop across the system to within five percent of the service voltage since larger conductors must be used to offset longer conductor lengths.

Another source of concern when planning for voltage drop is the use of power-factor correction capacitors. Because these serve to reduce the reactive component of the load current, they also reduce the voltage drop per equation (4-1).

Both low and high voltage conditions, and voltage imbalance, have an adverse effect on utilization equipment (see IEEE Recommended Practice for Electric Power Distribution for Industrial PlantsEEE Standard 141-1993, December 1993.* for additional information). Voltage drop must therefore be considered during power system design to avoid future problems.