Electric Generators for Temporary Use (AEN-122)

ISU Extension Pub # AEN-122
Author: Thomas H. Greiner, Extension Agricultural Engineer
Department of Agricultural and Biosystems Engineering, Iowa State University.
Revised November 1991
Content Reviewed: October 1998

Electricity has become essential to farm operation and family living. Unfortunately, few people realize that fact until the power goes off. Ventilating fans, water pumps, milking machines, mechanical feeders, refrigeration, furnace controls, and other vital modern production equipment require continuous electric service. Storm, accidents, or unavoidable breakdown of power-generating or transmitting equipment at times cause power failures. If a power outage lasts for any length of time, serious problems such as animal suffocation in windowless animal shelters, food spoilage, frozen water pipes, or loss of production will result. This ever-increasing dependence on a constant supply of electricity has caused increased interest in standby equipment for generation of electricity.

Standby power equipment can convert physical inconveniences and mental frustrations into east of mind and uninterrupted production. A farm operator or home owner must decide if he will buy the insurance of a standby generator or if he is willing to accept the risk of power failure. The
expenditures for standby generation equipment must be considered as being the same as any other kind of insurance. The cost of installation and maintenance of the system must be compared with the possible financial loss and inconveniences due to power outages. Justification will depend on the size of operation, essential electrical use, and history of power failures for the area. Standby power equipment should provide dependable electric insurance for about 20 years if it is used and maintained properly.

Standby electric power is probably not feasible for individual families who live in city apartments. It might be considered worth the expense by home owners in areas with a history of frequent or prolonged power failures to provide protection against such things as frozen water pipes and food spoilage. Farm operators, particularly those who depend on continuous electric service for such processes as mechanized feeding, milking machines, mechanical ventilation, or automatic waterers might find it a necessity to provide for emergency electric power.

Transfer Switch

The National Electrical Code requires that a standby generator be connected so as to prevent the inadvertent interconnection of the two power sources. A double-pole, double-throw switch is usually installed between the power supplier’s meter and the service entrance (100, 200, or 400 amperes). If current transformers are used for metering, a pole-top transfer switch may be used. Some power suppliers require a three-blade, double-throw switch in order to switch the neutral as well as the ungrounded conductors. Where local regulations require that the neutral not be switched, it may be necessary to use the three-blade switch and fasten all neutral lugs under one stud. Check the requirements of your local power supplier.

The standby generator should be set up in a safe, dry place. It should be within 25 feet of the transfer switch, preferably within sight. The generator should be conveniently located, especially when manually controlled or tractor driver, so no time is lost in an emergency.

The use of a double-throw switch prevents power from feeding back into the power supplier’s line and endangering the lives of linemen who may be working to restore power. It also prevents accidental reenergizing of the farm or home service system and consequent burnout of the generator when regular power service is restored. Most standby equipment guarantees are voided if the transfer switch is not used.

The double-pole, double-throw switch can be located at the service entrance, inside a building, or on a central meter pole. A single circuit or selected circuits may be connected through a transfer switch. All equipment located outdoors should be weather-proof and meet safety standards.

Illustration: Wiring of a typical transfer switch (not included)

Generator Types

Standby generating equipment can be divided into two general types: engine-driven and tractor-driven. Tractor-driven units can be stationary or portable, as a trailer mounted unit. Engine-driven units can also be stationary or portable and can be either manual start or automatic start. (Since the tractor and generator are not connected and ready to go at all times, this type of unit cannot be operated automatically.) Standby generators are available to operate either as single or three-phase. Some units are wired with four lead wires so that they can be operated either single or three-phase. If your electric service is three-phase power, consult your power supplier or generator dealer as to the type of generator and transfer switch to use. Generators must provide the same type of power at the same voltage and frequency as that supplied by power lines. This is usually 120/240 volt, single phase, 60 cycle alternating current (AC). The terms generator or alternator are used interchangeably for the same equipment.

Illustration: Portable engine generator with manual start (not included)

Illustration: Tractor-driven generator (not included)

Gasoline, LP gas (bottled gas), and diesel engines are available for powering standby generators. LP fuels are generally cleaner burning than gasoline. This tends to promote longer engine life and reduce frequency of service operations. Also, gasoline forms a varnish in the system when in storage. This may prevent starting in an emergency. Diesel units are high in initial cost except in the very large engines, but operating and maintenance costs are low.

The choice of engine is important for greatest efficiency and least care. For generators up to 15 kilowatts an air-cooled engine is recommended because of its low maintenance requirements. For generators larger than 15 kilowatts a water-cooled engine is necessary. Two to 2-1/4 hp engine capacity with the proper drive system must be available for each 1,000 watts of generator output.

There are no standard ratings for standby generators. Therefore, overload or maximum capacity, limited to short intermittent periods, must be considered in selecting the size of generator. Manufacturers’ ratings can vary from zero overload capacity to 100 percent. A large overload capacity permits you to choose a smaller unit particularly where large motors are involved.

It is necessary in a generator to provide direct current for the fields. In the past it was conventional to have a commutator and brushes with a direct current winding to provide this direct current. The DC commutator brushes are prone to spark which leads to maintenance problems. The development of the silicon diode has made it feasible to produce a self-excited generator without the use of a commutator.

Large engine generators should be located in a building, preferably one that is heated, for easier starting in winter. Inlet and outlet air ducts must be large enough to carry off the excess heat developed. Air inlets and outlets should each be at least 1/2 square foot open for each 1,000 watts of generator capacity. Provision must be made to conduct fumes of combustion outdoors safely. Exhaust pipes must be at least 6 inches away from combustible material. Mufflers are needed to keep exhaust noises at an acceptable level.

Illustration: Typical indoor installation for a large diesel-electric system (not included)

Full Load Systems

Automatically operated equipment is capable of furnishing emergency power even before the lights have gone out completely. This is accomplished through the use of an automatic transfer switch which turns on the emergency generator while the normal power is still fading. Sensitive relays are set to operate when voltage drops to 85 percent of normal or less. Less expensive equipment will automatically start the generator in 10 to 30 seconds. Automatic sequence starting (one circuit at a time) is referred to as full-load automatic service.

Automatic starting engine generators should have a sufficient capacity to carry the total emergency connected electrical load. Optional features are available for the automatic transfer systems such as under-voltage relays and time delay relays which automatically switch the load to a preset pattern. The change in output voltage of the generator from no-load to full-load should not exceed 10 percent. Full-load automatic service units are costly and should be tested weekly the same as manual start engine generators.

Part-Load Systems

Less expensive and more practical for many farms are the belt or power-take-off (PTO) driven standby generators. These units are about half as costly as engine-operated units and have the advantage of using the power of a tractor. Farm tractors are usually maintained in good operating condition; therefore, the initial cost and the starting difficulty that may be experience with an engine that is seldom run are avoided. The PTO arrangement with the tractor run at less than full throttle will probably be the choice of most dairy and livestock farmers.

Generators driven by PTO need a speed step-up drive system. Most alternating current generators will need to run at 1,800 or 3,600 rpm, and standard PTO speeds are either 540 rpm or 1,000 rpm. All PTO drives should be equipped with a free rotating shield. PTO units are often mounted on a trailer. These trailer units can then also be used for emergency lighting, field welding, or operating other tools such as drills, power saws, or augers where electric power is not available.

While belt-driven generators are less expensive, few farms have tractors with belt pulleys, and belts are more difficult to line up than PTO drives. Belt-driven units must be bolted down securely to resist the belt pull.

Never operate a generator that is not securely mounted. There is sufficient torque involved to spin the generator which could damage the unit or cause bodily injury.

Under a part-load system only the most essential equipment is operated at one time. When a power outage occurs, all electrical equipment is turned off or disconnected. After the generator is in operation, equipment is turned on with the load limited to the capacity of the generator.

Estimating the Electric Load

The size of generator needed for standby power depends on the choice of a full-load or part-load system. It must be decided in advance which electrical equipment is to be kept running in the event of a power outage. This may include the entire load, or it may be decided that certain heavy loads will not be used during the power outage. Examples might be the air conditioner, water heater, or electric range. In most cases you will ration or stagger the load by manual control. However, since a standby generator may be a “once in a lifetime” purchase, it is unwise to choose a model which will not provide reasonable comforts and convenience during a power outage. Also, some consideration should be given to electrical equipment that may be added in the future.

List the essential equipment and lights that will be operated by the standby generator in Tables I and II. Take figures from nameplate data where possible; otherwise use the figures in Tables I and II. If the nameplate lists the load for single phase in amperes only, multiply amps times volts to obtain watts. Refer to Table III to convert motor starting and running loads to watts.

Your main concern will be to provide capacity for starting motors. Motors require from three to five and occasionally nine times more electrical current to start, compared to full-load running current. If only one large motor is involved, starting capacity for this particular motor could determine the size standby generator needed. After the large motor is started, the remaining capacity can be utilized to start the smaller motors and other equipment including lights.

Calculate the wattage required to operate all the motors. See Table III. Start the largest one first. Each successive motor will add its starting load to the load already on the generator.

For example, the following equipment might be considered for a suburban home:


Equipment Size Start watts

Loading Sequence
on Generator – Watts
Starting** running**

Water pump 3/4 hp 3,345 3,345 835
Freezer 1/3 hp 1,600*  
Refrigerator 1/4 hp 1,200 4,835 1,835
Furnace 1/4 hp 1,200*  
Range l,000 W
(one top element)
Lights 750 W   3,585

* All starting at same time.
** Including previous load.

Since the water pump is the largest motor in this example, a decision must be made whether it will be operated manually or remain automatic on the pressure switch. Manual control is assumed. From Table III a 3/4 hp motor requires 3,345 watts to start and 835 watts when running. It is further assumed the freezer, refrigerator, and furnace will remain on automatic control and after some delay will all start at the same time. This will require an additional 4,000 watt capacity or a total of 4,835 watts. After these motors are running, the load drops back to 1,835 watts. A top element of the range can add l,000 watts, and 750 watts of lights make the total running load 3,585 watts. If all the listed equipment is to be started at the same time, several times more generator capacity will be required.

A 5,000 watt (5 KW) generator may have adequate capacity even if the freezer, refrigerator, and furnace motors were to start simultaneously. If the water pump is to remain on automatic operation, a larger generator (6 KW) should be considered, depending on the overload capacity.

The following equipment might be considered for a dairy:


Equipment Size Start watts

Loading Sequence on Generator-Watt
starting* running*

Gutter cleaner 5 hp 18,000 18,000 4,500
Milk cooler 3 hp 12,000 16,500 7,500
Water pump 1 hp 4,000 11,500 8,500
Ventilation fan 1/2 hp 2,300 10,800 9,075
Lights 750W   9,825

* Including previous load

In this example manual control of all motors is assumed. The largest motor is on the gutter cleaner (5 hp) and requires 18,000 watts for starting. After it is running it requires only 4,500 watts. The starting load for the 3 hp milk cooler adds 12,000 watts or a load of 16,500 watts. This reduces 7,500 watts after the milk cooler is started. The 1 hp water pump bring the load up to 11,500 watts while starting and drops back to 8,500 watts after starting. The 1/2 hp ventilating fan brings the load up to 10,800 watts while starting and drops to 9.075 after starting. Lights totalling 750 watts bring the total running load up to 9,825 watts.

An 18 KW generator capacity would be needed to start and run these motors. A 15 KW generator with a 3KW overload would be suitable. Considerable capacity remains for lights and small heaters. If the silo unloader is also 5 hp, you could make sure it did not run when operating the gutter cleaner.

Check your calculations with your power supplier or generator dealer. Most dealers are willing to demonstrate a portable generator for a 30-minute trial. This shows whether the size you selected can handle present loads.


Wiring and equipment should be installed in accordance with the National Electrical Code, local ordinances, and the requirements of the power supplier. Inspection by the local electrical inspector and the power supplier is recommended.

The tractor driven generator should be mounted on a trailer or bolted to a permanent concrete base. A location should be selected where the tractor can be easily lined up for efficient PTO operation.

The size of electric wire needed for the generator installation depends upon the amount of current and the distance the current must be carried. The expense of extra long runs of wire often makes it desirable to locate the generator within 25 feet of the switch to which it will be connected, preferably within sight.

When possible, generators should be located outdoors. Small temporary generators must never be operated indoors, even with open windows and high ventilation rates. The exhaust from the exhaust contains extremely high concentrations of toxic carbon monoxide. This colorless, odorless, tasteless, and non-irritating gas can quickly cause confusion and death. Caution must be used when generators are used outdoors to ensure the area is well-ventilated and that gases are not pulled back into an occupied structure. Carbon monoxide poisoning has occurred from generators located in entry ways, and even outside, buildings. If permanently located outdoors, provide a shelter to protect the generator from the weather. Exhaust gases must be exhausted out of the shelter by a sealed exhaust system. The shelter must be well ventilated and permit ready access to the equipment. Make certain all generator openings are covered with 1/4-inch galvanized wire mesh to prevent damage by rats and mice.

Special precautions must be taken for generators located indoors. Building codes and manufacturer’s recommendations should be consulted. Severe poisoning and fire danger exists if the units are not installed, maintained, and operated correctly. All exhaust systems must be sealed, adequate ventilation must be provided, safety controls and automatic shutdowns must be in place, carbon monoxide alarms and fire alarms must be provided, and automatic fire suppression systems installed if required. Only generator sets designed to be operated indoors may be used. Small portable generators are not designed to be operated indoors. They must never be used indoors due to the possibility of carbon monoxide poisoning and fire.

If the unit is permanently located outdoors, provide a shelter to protect it from the weather. The shelter must be well ventilated and permit ready access to the equipment.

Make sure all generator openings are covered with 1/4 -inch galvanized wire mesh to prevent damage by rats and mice.

Accessory Equipment

Alarms If a power interruption is known, steps can usually be taken to prevent loss. In the home, power failure is apparent when the home is occupied, the lights go out and the furnace goes off. Alarms are usually not needed. In unoccupied homes, alarms may be needed. In cold weather the temperature in the house can drop below freezing in only a few hours. In warm weather food in freezers and refrigerators will spoil if the interruption lasts more than two days.

On the farm, more immediate steps must be taken to prevent crop or livestock losses. An alarm to warn of failure in ventilating equipment is essential for confinement buildings, since animal stress and death can occur in less than an hour in hot weather.

There are many types of alarm systems available for warning against almost any kind of undesirable condition. These range from a simple power-off alarm to more extensive systems for sensing interruption of air flow, temperature extremes, fire, smoke, certain gases, the malfunction of a piece of equipment, or burglary.

The least expensive and most easily installed type of alarm is a commercial unit that can be plugged into an electrical outlet in the bedroom. If service to another building or a particular piece of equipment is to be monitored, a special circuit wired into the bedroom makes this possible. A relay contractor connected to the “load” wires from the control switch or to the terminals of the motor permits operating the buzzer on low voltage direct current.

Illustration: Schematic diagram of an alarm system using an electric bell and/or light operated from a battery (not included)

It is necessary to test such a unit periodically to make sure that the battery is in good enough condition to operate the bell or buzzer. This can easily be done by unplugging the unit from the outlet or using a test switch. Replace dry-cell batteries once a year or as recommended by the manufacturer.

An air horn powered by an aerosol can (compressed gas), commonly used as fog horns or distress signals for small boats, can be used with a solenoid valve that does not require a battery. The valve must be normally-open type. The solenoid coil is connected to the “load” wires or motor terminals, and as long as there is power available the valve will be closed. On interruption of power, the spring within the valve will cause it to open, allowing the compressed gas to operate the horn. Again, a test switch should be included.

Illustration: Schematic diagram of an alarm system using compressed gas (an aerosol can) to operate an air horn.
(not included)

A cooling thermostat (contacts close on temperature rise) can be used as the sensing element for a summer alarm. This system does not require a relay but simply closes a battery-operated circuit when the temperature goes above a pre-set level. It gives an alarm when the temperature rises above the set level and does not give the alarm every time the power fails. Usually the thermostat should be reset to adjust for seasonal changes: 85 to 90¡F during summer, 70 to 75¡F during winter. In the same manner a heating thermostat (contacts close on temperature drop) can be used as the sensing element for a winter failure. It gives an alarm when the temperature rises above a pre-set level. The alarm circuits can be tested by changing the thermostats’ setting.

When the location of the alarm is too far removed from the place where the electric power might fail–such as a poultry house or other animal shelter–an alarm service can sometimes be contracted through the telephone company. A lease-line is sometimes used for the warning system, or an answering service may be available.

Alarm systems are not fail-safe. Therefore, a convenient means of testing the alarm should be provided. Systems that depend on mechanical operation such as solenoid valves or air vanes must have frequent testing to be reliable. The reliability of the warning system is also largely dependent upon its source of power. A long-life battery, frequently tested, is essential to a dependable alarm service.

Illustration: A simple alarm using a cooling thermostat as the sensing element

Other Equipment A pilot lamp (neon type) connected to the power supplier’s line between the meter and the transfer switch aids in determining when normal power has failed and when it has been restored.

Time-delay fuses or circuit breakers on the generator, or between it and the transfer switch, will protect the generator against overload damage.

A voltmeter may come attached to the generator. If not, it can be bought as an accessory to assist in speed regulation. Frequency meters or tachometers also can be used to determine correct generator speed. Ammeters indicate the current being used.

An hour meter is recommended for engine-driven units to check on maintenance warm-ups and servicing schedules.

A lightning surge arrester is an inexpensive device that may be installed in the transfer switch to protect the generator as well as the circuits, appliances, and motors served by that switch. It simply acts as a safety valve, automatically draining the lightning surge from the system
before it gets into the distribution circuits.

A supply of gasoline or fuel, at least enough to start and run the generator for several minutes, should be kept readily available. This is especially important if an underground supply tank with an electric pump is used for fuel storage on the farm.


If your standby unit is automatic, normally no action will be necessary on your part. The unit should start automatically when normal power fails and stops when the power is restored.

Using an engine-driven generator with manual start or a tractor-driven unit, follow this procedure when a power outage occurs:

1. Call your power supplier and advise them of conditions.

2. Turn off or disconnect all electrical equipment.

3. Position the tractor or engine for belt or PTO drive.

4. Start the unit and bring the generator up to the proper speed (1,800 or 3,600 rpm). Check arrangements to carry off exhaust fumes and for any danger of fire. The voltmeter will indicate when the generator is ready to carry the load.

5. Put the transfer switch in the generator position.

6. Start the largest electrical motor first, adding other loads when each is up to operating speed. Be careful not to add too much too fast. If the standby generator cuts out for any reason, repeat steps 2, 4, and 6.

7. Check the voltmeter frequently. If voltage falls below 200 volts for 240 volt service or 100 volts for 120 volt service, reduce the load on the generator by turning off some of the electrical equipment. Increasing generator speed above normal operating speed will not increase the electrical power outlet.

8. When commercial power is restored, turn off or disconnect all electrical equipment. Put the transfer switch in normal power position. Stop the standby unit. Then start the largest electrical motor first, adding other loads when each is up to operating speed. This avoids a sudden load or voltage spike which could occur as power is switched. Turn on any other equipment not used on standby service.


The standby generator unit should be keep clean and in good running order at all times so that it will be ready for immediate use. An accumulation of dust and dirt on the generator can cause it to overheat when in operation.

Follow the maintenance instructions in the manufacturer’s manual. An hour of operation at set intervals will help keep the engine in good operating condition.

Table 1. Estimating Total Load and Generator Size for the Home



Usual Size

Usual Size

Your Equipment

Total lights and small appliances      



1/4 to 1/2

Window air conditioner  

1/2 to 2

Home air conditioner  

2 to 5



Electric skillet




Coffee maker


Electric iron


Electric range


Electric clothes dryer



Furnace stoker  


Furnace oil burner  


Furnace blower  

1/4 to 1/2

Electric heaters

600 and up

Electric fans


Water heater


Kitchen ventilator


Water pump  

1/2 to 2



Washing machine  

1/4 to 1/2



Sewing machine



1/4 and up

Other essential appliances      

Total Watts __________________

*Refer to Table III to convert hp to watts.

Table II. Estimating Total Load and Generator Size for the Farm



Usual Size

Usual Size

Your Equipment

Total lights      
Milking machine  

1/2 to 5

Bulk milk cooler  

1 to 12

Water pump (if separate)  

1/3 to 2

Water heater 1,000-10,0 00    
Milking parlor heat

2,000-10,0 00

Space heater

1,000-5,00 0

Ventilation fans  

1/6 to 1/2

Silo unloader  

2 to 7-1/2

Electric fence  


Feed grinding  

1 to 7-1/2

Feed mixing  

1/2 to 1

Feed conveyor  

1/2 to 5

Gutter cleaner  

3 to 5

Infrared lamp


Yard light


Shop tools  

1/6 to 1

Other essential equipment      

Total Watts_______________________

GRAND TOTAL on which to base
size of standby generator

*Refer to Table III to convert hp to watts.

Table III. Starting and Running Requirements for Commonly
Used 60-Cycle, Single-Phase Motors

App. amps Watts required* Watts required
(full load)


Approx. amps
(full load)

Watts required*

Watts required

Motor HP rating 120v. 240v.

to run
Split phase

Cap. start**


1/6 4.4  




1/4 5.8  




1/3 7.2  




1/2 9.8 4.9  



3/4 13.8 6.9  



1   8  



1-1/2   10  



2   12  



3   17  



5   28  



7-1/2   40  



10   50  



*Adapted from Cornell Extension Bulletin.
**Reduced 25 percent for repulsion induction motors.

Agricultural Engineering Notes (AEN’s) are non-reviewed publications of the Agricultural and Biosystems Engineering Department.

Source: “Standby Equipment for Electric Power Interruptions” By Joseph A. McCurdy Extension Agricultural Engineer, The Pennsylvania State University