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> Extension & Outreach >Electrical Publications
STANDBY POWER SYSTEMS
Agricultural Engineering Department
Michigan State University Extension
Extension Bulletin E-2709
September 1999Introduction: A reliable supply of electrical power is essential. A standby generators properly connected to essential loads can insure reliability of the electrical supply in a manner that is safe from potential hazards of fire or electrical shock.
Sizing a Standby Generator: List essential equipment that must be operated during a power outage. A refrigerator or freezer will keep food cold for a period of time if not opened. Heat the house to a comfortable temperature then shut off the furnace while other equipment is operated. Operating equipment on an as-need basis, the minimizes generator size.
Table 1 Typical load requirements for some common household appliances that may need to be operated during a power outage.
- electric fan, portable 75 to 300 watts
- electric water heater 1500 to 4500 watts
- freezer 1/4 to 1/2 hp
- furnace blower 1/4 to 1/2 hp
- furnace oil burner 1/6 hp
- fluorescent and HID lights
- wattage of lamps times 1.2
- garage door opener 1/6 to 1/2 hp
- incandescent light bulbs wattage of lamps
- radio 50 watts
- refrigerator 1/4 hp
- security system minimal
- septic system pump 1/4 to 1/2 hp
- smoke detector minimal
- sump pump 1/2 to 3/4 hp
- television 200 to 600 watts
- water pump 1/2 to 2 hp
Electric motors require about four times as much power to get started as they need to keep running. Allow 1000 Watts (1 kW) of generator capacity for each running motor horsepower (hp). Allow four times the running kW for motor starting.
Example: A standby generator supplies a 1/2 hp furnace blower and a 1/3 hp sump pump. The 1/2 hp motor will require about 500 watts for running and 2,000 watts for starting. The 1/3 hp motor will require about 333 watts for running and 1,132 watts for starting. The generator must be able to start the largest motor with the smaller motor running which in this case requires a generator with a capacity of 2,333 watts or 2.333 kW.
Sizing the Generator's Engine: Generators that are not part of an engine/generator set must be powered with an internal combustion engine with sufficient horsepower to run the load and start motors. The engine should develop at least 2-1/2 hp for each kilowatt of generator rating.
Connecting to a Standby Generator: Making the connection from the generator to the electrical system in a safe manner is essential. If connected improperly, the generator can be a shock hazard to occupants of the building or to utility personnel who may be working on the electrical distribution lines.
Generator connections must: be done in such a manner that generator power cannot get through to the utility system prevent any terminals from being energized when exposed, and have an ampere rating sufficient for the load.
The standby generator must be connected in such a way that circuits can receive power from the utility line or the generator This is accomplished by use of a double-pole transfer switch. There are three common methods of installing the double-pole switch.
Method 1: A double-pole transfer switch can be installed in the electrical service entrance to a building ahead of the service panel (Figure 1). Usually a flexible cord extends from the transfer switch to the generator. The handle of the transfer switch is either in the utility supply position or in the generator position. With this method any circuit in the service panel can be operated as long as the generator kW rating is sufficient to supply the load.
Method 2: puts the double-pole transfer switch between an adjacent separate essential circuit panel and the main service panel (Figure 2). The essential circuit panel receives power from the utility supply or from the standby generator. By using this method it is not necessary to rewire the service entrance to the building. A disadvantage is that only circuits in the essential circuit panel can receive power from the standby generator. An essential circuit panel is available with two interlocked circuit breakers that eliminate the need for a separate transfer switch (Figure 2). One of the circuit breakers connects to the main panel and the other connects to the standby generator plug.
Method 3: puts the double-pole switch on the individual circuits (Figure 3). An approved panel containing several double-pole switches is attached to the service panel. These panels are prewired and ready for connection. They also contain a plug ready for connection to the standby generator. The wire of the essential circuit is removed from the circuit breaker and connected to the double-pole switch input wire. The output wire from the double-pole switch is connected to the circuit wire that was removed from the circuit breaker.
Locating the Standby Generator: The standby generator must be located outside in a well ventilated space. Special plugs with weatherproof enclosures can be installed on the outside of the building ready for connection to a portable generator. If the connection plug is located inside the building (method 3), a flexible cord is required long enough to extend from the plug to the portable generator located outside.
The standby generator will have an output circuit protector rated in amperes. The flexible cord leading from the generator must have an ampere rating not less than the rating of the circuit protector on the generator (Table 2).
Table 2 Flexible copper conductor ampere ratings.
- AWG # 10 30 amperes
- AWG # 8 40 amperes
- AWG # 6 55 amperes
- AWG # 4 70 amperes
- AWG # 2 95 amperes
Solving Problems: Some portable standby generators are intended for use as a stand-alone system to power portable equipment and have a ground fault circuit interrupter (GFCI) protecting the output circuit. The purpose of this device is to sense when a person may be receiving a shock and automatically disconnect power to the receptacle. This same device can interfere with operation when connected to a building wiring system.
The equipment grounding wire and the neutral wire are connected together at the service panel in a building and inside the generator. The equipment grounding conductor and the neutral conductor will act as parallel conductors between the service panel and the generator. This signals a false ground fault to the generator and the GFCI will trip. In this situation the neutral conductor in the cord acts both as a current carrying conductor and a safety ground.
More Information: There are publications available that provide details on the selection, connection, and operation of standby generators. To get started please contact your local Extension office, power supplier customer service department.
Authors: Robert J. Fick, Ph.D., P.E., Truman C. Surbrook, Ph.D., P.E., Jonathan R. Althouse, Master Electrician, and Howard J. Doss, M.S., Agricultural Safety Specialist Agricultural Engineering Department Michigan State University
This publication is for educational purposes only. This publication was partially funded by the Michigan Agricultural Electric Council (MAEC) which develops and presents educational programs on electrical issues to agricultural professionals. The MAEC members include academic, regulatory and industry representatives from Michigan and surrounding states. Michigan State University is an Affirmative Action/Equal Opportunity Institution. Michigan State University Extension programs and materials are open to all without regard to race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, marital status, or family status. Issued in furtherance of MSU extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Arlen Leholm extension director, Michigan State University, East Lansing, MI 48824-1039.
