Utilization Voltage Selection

Very large inductive loads such as higher horsepower motors used on HVAC chillers, sewage treatment pumps and in process or other Industries can draw tremendous amounts of power. Motors also inherently have high inrush currents during full voltage starting, which can cause a significant voltage dip on the power system feeding it. As a result, many utilities have limitations on the maximum horsepower motor that can be line started directly from their system. To limit the impact of this phenomena, a variety of techniques can be used to reduce the motor’s starting inrush current. These generally involve the use of electromechanical or solid state reduced voltage starters. Variable frequency drives in both low and medium voltage are also available as shown on the System One-Line on Page 1.1-8. See Components of a Power System section for further details.

Voltage Recommendations by Motor Horsepower

Some factors affecting the selection of motor operating voltage include:

■■ Motor, motor starter and cable first cost

■■ Motor, motor starter and cable installation cost

■■ Motor and cable losses

■■ Motor availability

■■ Voltage drop

■■ Qualifications of the building operating staff; and many more

The following table is based in part on the above factors and experience. Because all the factors affecting the selection are rarely known, it is only an approximate guideline.

Table 1.1-5. Selection of Motor Horsepower Ratings as a Function of System Voltage

In higher motor hp applications, a motor’s 4.16 kV utilization voltage may be the same as the 4.16 kV service voltage. In these cases, the service equipment would need to feed power through cables or busway to a medium-voltage starter or variable frequency drive. However, in installations where there are many long cable runs that are feeding other large loads, the medium-voltage distribution may have a higher service voltage such as 13.8 kV. In this case, the service voltage would need to be stepped-down to the 4.16 kV utilization voltage through a primary unit substation transformer as illustrated by the System One-Line on Page 1.1-8. Conversely, small end loads, short runs and a high percentage of lighting and/or receptacle loads would favor lower utilization voltages such as 208 Y/120 V. If the incoming service was at 13.8 kV, as noted in the previous example, secondary unit substations, pad-mounted transformers or unitized power centers could be used to step down to the 208 Y/120 V utilization voltage required. This approach is often used to reduce or offset voltage drop issues on multi building sites such as college or hospital campuses. It is also used in large single building sites like distribution warehouses and high rise “skyscraper” buildings.

Note: The “Types of Systems” section of this Design Guide illustrates a number of power system designs that improve reliability and uptime during maintenance or service outages. Among these schemes are a variety of configurations showing medium-voltage sources feeding substation or pad-mounted transformers that step it down to the appropriate low voltage for end load utilization.

A problem can arise, however, when a low-voltage service is the only utility service option and cable distances between the incoming service and the utilization loads are great. In these instances, a practical way to offset for the voltage drop to the end utilization loads is the use of low-voltage busway in lieu of cable. Another technique to address voltage drop concerns for long cable runs is to use a step-up and step down transformer arrangement.  To accomplish this, a step-up transformer is added after the low-voltage service. The transformer primary is configured in a delta and is fed by the grounded and bonded low-voltage incoming utility service. The step-up transformer wye secondary is often at medium voltage, typically at 4.16 kV, with the transformers wye secondary grounded. A 4.16 kV delta primary step-down transformer is then located near the served load and has its wye secondary grounded in accordance with NEC Article 250.30 to create a separately derived system. This step-down transformer’s secondary voltage may be the same as the incoming service, or it may be at higher utilization voltage. Caution must be taken when selecting the step-up transformers to be used in this type of application. Step-up transformers, particularly designs that are not optimized for step-up purposes, such as a reverse-fed standard transformer, exhibit extremely high inrush during energization. Unless the step-up transformers are specifically wound for low inrush, the magnetizing current during initial energization, may exceed the 6X make capabilities of a low-voltage fused bolted pressure switch. This can result in a condition where a portion of the switch contact surface can weld before full engagement. The current passing through the smaller contact area will then eventually cause the switch to overheat and fail. Many step-up transformer applications involve a 208 Vac incoming service stepping this voltage up to the utilization voltage of 480 Vac for HVAC motor loads in a building. The design engineer must be aware of some potential pitfalls and plan ahead when involved in this type of application. Larger step-up transformers offer fewer transformer voltage taps, if any at all. They also exhibit poor voltage regulation when experiencing transient shock loads, such as motors starting. When designing power systems utilizing step up transformers to feed motor loads, a Motor Starting Analysis should be performed to ensure that the motors will start and operate as intended.

To be continued...