All services shall be alternating current - 60 Hertz and will be delivered in accordance to American National Standard for Electric Power Systems and Equipment voltage ratings 60 Hertz (ANSI C84.1 or current edition). Normal secondary delivery voltages are 120/208 or 120/240 volts single phase, and 120/208 or 277/480 volts three phase.
In the case of large loads, power may be delivered at other voltages approved by the District. In general, motor loads up to and including five horsepower may be served at 240 volts single phase. Motors of five horsepower to fifty horsepower may be served at 208 or 240 volts three phase. Motor loads aggregating 25 horsepower and more may be served at 480 volts three phase. The District may require customers to install reduced voltage starting equipment in cases where across-the-line starting would result in excessive voltage disturbances to the District's system.
When transformers are connected in a Delta configuration, required to supply 120/240 or 240/480 volt three phase services, there exists a potential that the resulting electrical system can enter into ferroresonance. Some of the symptoms of this condition are three phase motor reversal, over voltage up to five times normal, unusual secondary voltages, transformer and equipment failure due to high voltage stress or high current flows. It is most common to experience ferroresonance when underground high voltage cable is part of the electrical system. Ferroresonance can also occur when there are long distances from the service to the supply station.
Ferroresonance cannot easily be predicted or prevented. Customers with 120/240 or 240/480 volt three phase services should install over-voltage protection at the service entrance, and on all equipment. Often this requires two or more layers of over-voltage protection including at the main service panel, and at the individual pieces of equipment. Protection should be added that measures incoming voltage and disconnects all legs of the service if any one supply leg is lost (single phasing protection). Because ferroresonance is often associated with a loss of one high-voltage electrical phase, this protection must be carefully considered and applied. Protection must also be applied on all types of loads.
