One of our clients’ buildings, a multistory commercial office building, has a relatively new 1.5-ton split system that cools the elevator equipment room. When the building’s maintenance staff discovered the system was not running and the temperature in the equipment room was abnormally high, an A/C service tech was dispatched. The service tech was unable to find any malfunction in the equipment, but when he measured the voltage on the single-phase circuit serving the condenser and air-handler units, it was 197 with no major loads.
The building’s property manager then called the local utility. The local utility agreed to place a voltage recorder on the suspect circuit, but was quick to point out the Florida Public Service Commission (PSC) allowed it to vary voltage for non-residential service at the point of delivery by as much as 7.5 percent above or below the standard voltage. Here in the United States (and in most other countries utilizing 60 Hz as the standard frequency), utility delivery voltages are provided in multiples of 120 (i.e., 120, 240, 480, and 600). Most single-phase residential and many small commercial services are 120/240-volt three-wire systems, while many commercial buildings also have single-phase 120/208-volt service taken off a 480-volt four-wire grounded wye system.
Our client’s split system was, according to the nameplate, rated for 208-to-230-volt single-phase. Based on the PSC’s ±7.5-percent allowance, anything above approximately 192 volts is acceptable. A different basis is used for the 60-Hz voltage ratings of electric motors, the design of which is governed primarily by the National Electrical Manufacturers Association (NEMA). Motors fall into a broad class known as “utilization equipment,” the voltage standards of which are in multiples of 115 (i.e., 115, 230, 460, and 575). This is done to allow for any losses—and resultant voltage drop—between the service entrance and the point of use. A building that has 120/240-volt service will, for example, use motors rated for 115/230 volts. NEMA allows variation in the voltage of ±10 percent of rated voltage at the rated frequency (in this case, 60 Hz), but notes performance within those variations will not necessarily be the same as performance at the rated voltage.
So, what does all of this mean? For a motor nameplated for 115 volts, a ±10-percent allowance enables it to operate at 103.5 to 126.5 volts, and the utility—at least here in Florida—should be supplying power at 120 volts ±7.5 percent, or 111 to 129 volts. In that scenario, everything should work well. Our client’s equipment, however, uses a single-phase compressor motor intended for 208- to 230-volt service, and single-phase motors frequently are not built to actually be 208-volt motors. They generally are going to be either 200 volts or 230 volts, as it is not economically feasible to build motors for every possible voltage variant. In spite of the fact NEMA cautions against operation of a motor rated for 230 volts on a 208-volt system, it frequently is done, and the most significant consequence often is limited to merely derating the service factor from 1.15 to 1.0.
So, what about our client’s building? For the tech to get the system operating and prevent damage to the critical equipment located in the space, a hard-start capacitor kit was installed. The utility installed a monitor for a week or so, reporting no average readings below approximately 202 volts (it did not share with us the actual voltage readings). So, we were left with a lot of unanswered questions: What is the minimum voltage the equipment actually is receiving? What is the minimum voltage the 208-to-230-volt compressor motor really needs to start at its normal starting load? And what voltage does the motor require to properly operate after it starts?
The next time you are specifying or buying an electric motor or motor-driven equipment, you might want to do some homework and learn what voltage the local utility really will be providing, how the motor really is rated, and how that all ties together.