Standby Electric Generators for Emergency Farm Use

Factsheet - ISSN 1198-712X   -   Copyright Queen's Printer for Ontario
Agdex#: 771
Publication Date: 05/99
Order#: 99-005
Last Reviewed: 05/99
History: Original Factsheet
Written by: Hugh Fraser - Engineer, Horticultural Crop Structures & Equipment Specialist/OMAFRA; John Johnson - Engineer, Structural Design Specialist/OMAFRA

PDF Version - 127 KB

Table of Contents

  1. Introduction
  2. Generator Types
  3. Planning the Type of Generator System
  4. Sizing a Full Electrical Load, or Part Electrical Load Generator System
  5. Connecting the Generator to an Electrical System
  6. Operation
  7. Maintenance
  8. Safety
  9. Other Considerations


The winter of 1998 will long be remembered as having the Ice Storm of the Century in Eastern Ontario, Southern Quebec, and Northern New York State. An area equivalent to the size of the Province of New Brunswick received at least 100 mm (4 inches) of freezing rain and ice over a period of 5 days, wreaking havoc with trees, buildings and power lines.

Several thousand farmers were without electric power for between one to four weeks, and they found out how dependent they have become on electric power in their operations. Those who foresaw the need to own generators were better off than their neighbours who were without one. Thousands of generators from all over North America were purchased, borrowed and rented as farmers became desperate to find ways to run their electrical equipment.

This Factsheet is intended to help farmers in the selection, installation and operation of a commercial size generator on their farms. Small, portable generators, designed for lights or small appliances are not discussed. This Factsheet is not intended to serve as a comprehensive set of guidelines for the installation and use of standby generators on farms. Specific details for generators are available from manufacturers, distributors and electrical contractors.

Generator Types

There are two types of commercial size generators used on farms, driven either by:

  • their own diesel, gasoline or natural gas engines (that is, self-contained) and are usually permanently mounted near the transformer pole, or
  • a tractor PTO (power takeoff) that is usually portable, but can be permanently mounted near the transformer pole

Either type is available to produce single or three-phase power. Figure 1 shows a 100 kW self-contained generator used at a poultry farm. It is mounted on a concrete floor base inside a small building. Figure 2 shows some portable PTO driven generators mounted on trailers used during the Ice Storm.

A generator driven by its own engine, housed in a small building, and installed near the transformer pole, is ready with a flick of a switch, or automatically, if the power goes off. (Picture courtesyof Dobbie Poultry Farms, Ventnor, Ontario)

Figure 1. A generator driven by its own engine, housed in a small building, and installed near the transformer pole, is ready with a flick of a switch, or automatically, if the power goes off. (Picture courtesy of Dobbie Poultry Farms, Ventnor, Ontario)

The advantages of owning and operating a generator that is driven by its own engine are:

  • it is quieter during operation
  • it uses fuel more efficiently
  • it can be switched on quickly, either manually or automatically, when the power goes off
  • it can operate for long, continuous periods

Generators driven by a tractor PTO can be mounted on trailers for portability, but are usually limited in output capacity from about 10 kW to 100 kW.

Figure 2. Generators driven by a tractor PTO can be mounted on trailers for portability, but are usually limited in output capacity from about 10 kW to 100 kW.

The advantages of owning and operating a generator that is driven by a tractor PTO are:

  • it costs less; 1998 prices of about $200/kW of output power, versus $400/kW for a generator with its own engine
  • it is easier to maintain, since it doesn't have its own engine to keep tuned up
  • it can be moved from location to location.

Planning the Type of Generator System

Generators are rated by the amount of electrical power they generate. Power is rated by two different terms; kilovolt-amperes, (kVA), and Watts (W) or kilowatts (kW); 1 kW = 1000 W.

The first term, kVA, is the apparent power drawn by a motor:

kVA = [Volts (V) x Amps (A)] ÷ 1000

The second term, kW, is the amount of real power drawn by a motor, or a measure of power that actually does work (mechanical work plus heat losses):

kW = kVA x Power Factor

The power factor is a measurement of the difference in phase between voltage and current in an electric circuit. It ranges from about 0.65 for single-phase motors under 1 hp in size, to about 0.95 for single-phase motors over 5 hp.

So, for a ½ hp motor, if the line voltage was 240 Volts, and the nameplate current was 5 Amps, the real power in kW would be:

(240 V x 5 A) ÷ (1000) x 0.65 = 0.78 k

Two ratings for generators are often listed, a continuous rating during operation and a peak rating allowing for some short-term increased loading, such as when a motor starts. Generators must provide power of the same voltage and frequency as that delivered to the site. This is usually 120/240 volts, single-phase, 60 Hz alternating current (AC).

The majority of farms use a single-phase power distribution network operating at 120 or 240 V. This type of system requires the use of single-phase motors from ¼ hp to about 10 hp. Motor sizes above this require a three-phase distribution system. This Factsheet deals with single-phase motors only.

It is important to realize that electric motors require a much higher amount of power in watts to start them, than to run them. The startup watts can range from two to twelve times the operating watts in extreme cases, depending on:

  • the type of motor (Table 1), and
  • the load on the motor (a silo unloader buried in silage, or a stable cleaner buried in manure requires more startup watts than if each is free and clear of materials)

To be prudent, use four times the operating watts to determine the startup watts.

Motors for agricultural use are generally referred to as farm duty motors. This term is used by manufacturers to describe a motor with the following features:

  • a corrosion resistant nameplate
  • a manual reset thermal overload protection
  • a gasketed conduit box, and
  • a shaft flinger to help prevent water ingress at the shaft
There are five types of single-phase motors generally used for farm applications, although there are other designs as well (see Table 1).
  • Capacitor start (CS) motors provide good starting performance, operating efficiency, and are generally used for farm motors under 2 hp in size.
  • Capacitor start/capacitor run (CS/CR) motors provide similar starting performance as CS motors, but with lower operating currents, and are generally used for farm motors greater than 2 hp in size.
  • Permanent split capacitor (PSC) motors give excellent efficiency and power factor, allow for variable speed operation with relatively simple controls, and are generally used for ventilation fans and blowers.
  • Split phase (SP) motors are inexpensive, of lower efficiency than capacitor start designs and used for small intermittent loads
  • High torque CS/CR motors are used in sizes greater than 5 hp for hard to start loads, such as silage unloaders.
Table 1. Typical motor type depending on the farm application (see previous section for abbreviation definitions)
 Farm Application  Typical Motor Type
 Manure pumps
 Jet pumps
 Material handling
 Machine tools
 SP or CS
 Submersible pumps
 SP or CS
 Sump pumps
 CS or CS/CR > 3 hp
 Vacuum pumps
 CS or CS/CR > 3 hp
 Silo unloaders
 High torque CS/CR
 Ventilation fans

To tell which kind of motor you have, use Table 1, or consult your electrical contractor.

Heaters and lights are resistance loads, so the startup watts is the same as the operating watts. Table 2 gives guidelines on startup and operating watts for several single-phase motor sizes, based on information compiled from actual farm motor testing data, where available.

It is important to note that the operating and startup loads for the motor sizes listed in Table 2 are lower than those in Table 45 contained in the Electrical Safety Code, because Table 45 is meant as a conservative guide only where motor nameplate information is unavailable, and do not necessarily represent those found in agricultural applications. All 120 V loads should be balanced. That is, there should be the same amperage on each line. Consult your electrical contractor for more information.

Full Electrical Load Systems

Generators should be sized for a full electrical load if they start automatically when the power goes off. They should have enough capacity to handle all the electrical loads at once, or else the electrical equipment and/or the generator could be damaged. These systems use permanently mounted, self-contained generators driven by their own engines.

Part Electrical Load Systems

Generators are sized as part electrical load systems if they are used only for critical electrical loads such as milking equipment, fans, or ventilation equipment. So, the resulting generator capacity in a part electrical load system is much smaller than in a full electrical load system. This means the generator could require more attention during operation, since there is a higher likelihood of power problems. Someone must detect the power failure, start the generator, and carefully add electrical loads, one at a time. These systems are usually designed for tractor PTO driven generators.

Table 2. Motor output in horsepower (hp) vs. startup and operating watts for single-phase motors at 240 V. For startup, four times operating levels are assumed, although it can be two to twelve times as much. See electrical contractor for details.*
 Motor Output (hp)  Startup Watts (W) (assuming 4 times operating)  Typical Operating Watts (W)
 2,300 W
 575 W
 3,200 W
 800 W
 4,300 W
 1,075 W
 7,400 W
 1,850 W
 12,300 W
 3,075 W
 18,200 W
 4,550 W
 7 ½
 27,000 W
 6,750 W
 36,000 W
 9,000 W

*Data is typical for capacitor start, capacitor start/capacitor run, and permanent split capacitor motors used for agricultural use (compiled by Enertech Solutions Inc. for the Ice Storm Recovery Assistance Program 1999).

Sizing a Full Electrical Load, or Part Electrical Load Generator System

Suppose there are ten pieces of equipment in a barn, eight with motors (Table 3). House equipment is not included. All motors are 240 volts. Assume that the startup watts for all motors is four times the operating watts as in Table 2.

Full Electrical Load System Example

  1. There are 4 steps to sizing:

    1. List operating and startup electrical loads, in watts, of all motors that must operate at the same time. Check motor nameplates and Table 2.  Table 3 lists the equipment and their operating and startup watts. Note that the startup and operating watts for the electric heater and lights are the same, since they are resistance loads.

    Table 3. Ten pieces of equipment in case study
     Equipment and motor size (hp) Startup watts (W)
    (4 times operating)
     Operating watts (W)
     Fan #1; ½ hp 2,300 575
     Fan #2; ½ hp 2,300 575
     Water pump; ½ hp 2,300 575
     Electric heater 4,800 4,800
     Lights 2,000 2,000
     Bulk milk cooler; 2 hp 7,400 1,850
     Milking machine; 1 hp 4,300 1,075
     Silo unloader; 5 hp 18,200 4,550
     Stable cleaner; 3 hp 12,300 3,075
     Freezer; ½ hp 2,300 575
     Totals 58,200 19,650

  2. Check which motors would not run at the same time, then only include largest ones in the total starting watts. The silo unloader requires 18,200 W on startup, but is never used when the stable cleaner runs, which requires 12,300 W on startup. So, include the larger silo unloader. From Table 3, subtract 12,300 W from the 58,200 W on startup, resulting in 45,900 W on startup.

  3. Add 20% for future expansion, then round up to the nearest 5 kW.

    45,900 W x 120% = 55,080 W peak (60 kW)

  4. Determine the tractor size for a PTO driven generator, based on 2 brake horsepower per 1 kW of electrical output by the generator.

    2 hp/kW x 60 kW = 120 hp tractor size

Part Electrical Load System Example

There are 4 steps to sizing:

  1. List operating and startup electrical loads, in watts, of only critical motors that must operate at the same time, in order of highest starting watts. Check Table 2 and motor nameplates. The operation sequence starts by adding largest motors first. The silo unloader and Fan #1 are not essential, and resistance loads (electric heater and lights) should be added last. see Table 4.

    Table 4. Eight motors and appliances used in part electrical load case study in order of starting watt amount.
     Equipment and motor size (hp)  Startup watts (W)
    (4 times operating)
     Operating watts (W)
     Stable cleaner; 3 hp
    12,300  3,075 
     Bulk milk cooler; 2 hp
    7,400  1,850 
     Milking machine; 1 hp
    4,300   1,075 
     Fan #2; ½ hp
    2,300   575 
     Water pump; ½ hp
    2,300   575 
     Freezer; ½ hp
    2,300  575 
     Electric heater
    4,800  4,800 
    2,000  2,000 

  2. Determine the peak required watts as each of the eight loads is added. Add the startup watts of the next load to the watts needed for those operating. Table 5 shows the load at each step, with the peak at Step 8 when the lights are added. This peak level is 14,525 W. Even though the greatest required watt level is in the final step in this case study, it isn't always in the last step.
  3. 3. Add 20% for future expansion, then round up to the nearest 5 kW.
    14,525 W x 120% = 17,430 W peak (20kW)
  4. Determine the tractor size for a PTO driven generator, based on 2 brake horsepower per 1 kW of electrical output by the generator.
    2 hp/kW x 20 kW = 40 hp tractor size
(Note that the resulting generator capacity in the part electrical load system is only one third of the one in the full electrical load system)
For further information on sizing, see your generator supplier and electrical contractor.
The advantages of a full electrical load system:
  • all equipment can be started at the same time, unlike for a part electrical load system
  • there are less management decisions to make when under the stress of a power outage (see operation later)
  • it can be set up to start automatically, so that power disruption doesn't occur
  • the power is off for only a short period, which could be critical for ventilation fans
  • there is more flexibility to add loads
  • the greater capacity of the generator means there should be less of a likelihood of power problems during operation than in a generator sized for a part electrical load

The advantages of a part electrical load system:

  • results in a smaller capacity, less expensive generator
  • if using a tractor PTO driven generator, it is portable for use anywhere, provided it is grounded and connected properly

Table 5. Peak electrical loads using sequence of motors shown in Table 4. 'S' indicates the startup load, while 'R' indicates the operating load after the motor is running.
 Step 1
 Step 2
Step 3
Step 4
 Step 5
Step 6
Step 7
 Step 8
Stable cleaner; 3 hp  12,300 S  3,075 R  3,075 R  3,075 R  3,075 R  3,075 R  3,075 R  3,075 R 
Bulk milk cooler; 2 hp    7,400 S  1,850 R  1,850 R  1,850 R  1,850 R  1,850 R  1,850 R 
 Milking machine; 1 hp     4,300 S  1,075 R  1,075 R  1,075 R  1,075 R  1,075 R 
 Fan #2; ½ hp       2,300 S  575 R  575 R  575 R   575 R 
Water pump; ½ hp          2,300 S  575 R  575 R  575 R 
 Freezer; ½ hp           2,300 S  575 R  575 R 
 Electric heater             4,800 S  4,800 R 
 Lights               2,000 S 
Peak watts/sequence 12,300  10,475  9,225   8,300  8,875  9,450  12,525  14,525 

Connecting the Generator to an Electrical System

Generators can pose serious safety hazards to people and livestock, so please follow all safety instructions provided by the manufacturer and required by the Ontario Electrical Safety Code. Mistakes can happen when people are tired, in a hurry, or under stress. Mistakes can be fatal to those using the generator, and to those working on the power lines.

Transfer Devices (Pole-Top Switches, or Automatic Transfer Switches)

The Ontario Electrical Safety Code specifies that 'under no circumstances shall a generator be connected to any portion of a wiring system except where it is connected through a transfer device that makes it impossible for the generator to feed back into the normal power supply.' A transfer device is required between the utility system and the generator for safety, to prevent:

  • power from feeding back into the power supply line and endangering the lives of hydroelectric workers trying to restore power, and
  • accidental re-energizing of the farm or home service system and consequent burnout of the generator when regular service is restored; generator guarantees are voided if a transfer switch is not used.

The transfer device should be sized according to the current rating of each service connected to it. It must be sized for 100% of the largest service, and 75% of the balance of services. The transfer device should be installed only by a qualified electrical contractor following local electrical codes. Depending on many factors, installing a transfer device could cost in the range of $2,000 to $3,000 or more.

Stabilizing the Generator

If the generator is permanently mounted near the transformer pole, it should be anchored on a 150 mm (6") concrete pad and sheltered from the weather. Make sure the axle of a tractor PTO driven generator is strong and long enough to prevent flipping of the generator under load.

Wiring Considerations

Equipment damage can result from voltage drops. Therefore, to prevent excessive voltage drop between the generator and the equipment, the power cable should be of adequate gauge for the length used. See your supplier and electrical contractor for the proper equipment. Always make sure that motors are under no load when starting, if possible. Always use qualified personnel to initially connect and test generators.


There are several things to consider when operating a tractor PTO driven generator:

  • clearly identify all circuits on your electrical panels
  • make sure you run it at the correct RPM (540 or 1000 RPM); never assume the tractor RPM is correct
  • running a generator faster will not produce more power, but it may damage it
  • make sure the PTO shaft is designed for the horsepower required
  • follow the information on the plate on the side of the generator
  • disconnect all loads before starting, including hot water heaters, heating cables, heated water troughs; reconnect them slowly after essential motors are running
  • turn on the motors one at a time
  • generators may handle short term heavy loads, but may not for long periods
  • monitor voltage and keep 240 V (± 10%) at the generator
  • for continuous operation, try to run the system at 80% capacity or less, if possible
  • use an amp-meter to check total loading and compare it to the generator's specifications
  • if the circuit breaker on the generator trips, reduce the loading and make sure your 120 V loads are balanced; if more loads come from one side of the fuse box, they should be shut off to balance the load; remember the generator does not average loads, but trips on the highest loading from one side
  • keep fuel on hand for at least 72 hours; during the Ice Storm, farmers averaged 160 litres of fuel daily, with reports of as high as 400 litres
  • plan ahead for which tractor will drive the generator, and keep it fueled up at all times; remember that the fuel pump may not operate, nor the block heater


  • Shut off the tractor and generator when lubricating, checking connections, or fueling
  • Make sure the gearbox and grease joints have proper lubrication; wait until the generator has cooled down before checking
  • Monitor the tractor for coolant leaks, oil leaks, or fan belt breakage which could destroy a tractor in a short period
  • Murphy Switches are automatic shutdown systems to prevent damage in case of: high coolant temperature; low coolant level; low oil pressure; low oil level; $200 invested could save a $50,000 tractor


  • Provide 500 cm2 (0.5 ft2) of inlet and outlet air opening for each 1 kW of generator capacity, if housed in a building, to allow the tremendous amount of heat from the generator to escape
  • Generators get hot, so be careful of burns
  • Direct combustion fumes outdoors using piping; during the Ice Storm, several people were treated for carbon monoxide poisoning
  • Keep exhaust pipes at least 15 cm (6") away from combustible materials
  • Only operate tractor PTO driven generators if they are securely mounted; there is sufficient torque to spin the whole generator
  • Make sure that PTO shields are in place and that children and others are kept away from this equipment because of the chance of burns, shocks or entanglement
  • Do not remove radiator caps when engines are running, as the coolant will be very hot
  • Keep an eye on the tractor's gauges and use an automatic shutdown system, just in case
  • Electricity is a hazardous commodity, so use caution when working with it

Other Considerations

  • At least 2 brake horsepower of tractor capacity are needed per 1 kW of generator output; during the Ice Storm, farmers often used oversized tractors for generators up to 35 kW in output, but undersized tractors for generators over 35 kW
  • in output
  • A three-phase rated system will produce about 60% of the power on single-phase
  • For tractor PTO driven generators, pick a convenient spot to park and mark it well to help you get the tractor aligned correctly, but quickly
  • Store the PTO shaft with the PTO driven generator
  • Cover all openings into the generator with 5 mm (1/4") galvanized wire mesh to prevent damage by mice or rats
  • Operate the generator under at least 50% load at least four or more times annually to ensure it works when needed, to keep components dry with the heat produced, and to eliminate flat spots on the bearings
  • Buy a good multimeter to check voltages, amperages, frequency of current, power availability, and learn how to safely use it
  • On generators with their own engines, replace or use the fuel supply every couple of months to prevent moisture condensation in the tank or fuel deterioration; the fuel tank should be kept full
  • An alarm system should be purchased to tell you when your electrical power is off

The information in this Factsheet was condensed from several sources. It is not intended to be a substitute for professional advice from a manufacturer or supplier of generators. Always consult your electrical contractor. Remember, all installations of electrical equipment are subject to the inspection requirements contained in the Ontario Electrical Safety Code. Special thanks for information in this Factsheet go to:

  • Ontario and B.C. Hydro
  • Thomas Greiner, Extension Agricultural Engineer, Iowa State University
  • Mid-West Plan Service, Iowa
  • Results of The Ice Storm Survey, by Steve Clarke, P.Eng., OMAFRA
  • Ontario Hydro Reports; TSDD-92-033, Single-Phase Motor Test Program and E92-2-H, Single-Phase Fractional Horsepower Motor Test Program

For more information:
Toll Free: 1-877-424-1300