It's important to understand generator sizing in order to account for the load it will need to handle. Electric motors are particularly difficult for a generator because starting an electric motor requires 2 to 3* times its nameplate amperage or wattage.
A current surge of short duration can be supplied by a generator, but a current demand of longer duration, such as a heavily loaded motor starting a high inertia system can overload a generator, possibly damaging both the generator and motor.
For this reason, when determining the power your generator is to provide, it is important to calculate electric motor requirements at 3* times the running watts to compensate for the surge needed to start the motor. When sizing by KVA use the following calculation to determine watts: KW= KVA x Power Factor. With this in mind, the following guidelines can be helpful in selecting the right size generator for your application.
How do you determine what size generator you need?
Total the wattages of all small appliances, tools, and light bulbs to be operated at the same time. Most appliances have labels showing wattage (if volts and amps are given: volts x amps = wattage (VA) times power factor).
For example you may wish to operate the following equipment:
|Eight 100w light bulbs||800|
The next item to look at for generator sizing is the volt-amperes (wattage) requirement of electric motor to be operated (e.g., furnace blower motor 1/3 HP), remembering that the starting requires 2 to 3* times the nameplate or running (rated) watts. Thus, if running watts of the motor is 600, multiply the number by 3 to determine approximate max. VA needed.
Total watts and VA in Steps 1 and 2 to get total requirements:
|Running = 1800W|
|Starting = 600 x 3 = 1800 Max. VA|
To allow for anticipated future needs or use of extra equipment, add 25% to total in Step 3 when considering generator sizing.
See performance data charts for each generator listing for the unit that meets your total load criteria.
In summary: For example, a generator that provides at least 4500 max. VA and 4000 watts meets the load requirement for the example above. A 3000-watt generator should not be selected because it does not have enough max. VA to start the electric motor load.
Note: The starting (max.) VA can also be determined by referring to the motor code listing that indicates starting KVA per horsepower (see chart below).
(*) Some motors can require as much as 5 to 6 times their full load amps at start. Check the motor code listing to be sure.
|Code||KVA Per Horsepower||Code||KVA Per Horsepower||Code||KVA Per Horsepower|
|A||0 to 3.15||H||6.3 to 7.1||R||14.0 to 16.0|
|B||3.15 to 3.55||J||7.1 to 8.0||S||16.0 to 18.0|
|C||3.55 to 4.0||K||8.0 to 9.0||T||18.0 to 20.0|
|D||4.0 to 4.5||L||9.0 to 10.0||U||20.0 to 22.4|
|E||4.5 to 5.0||M||10.0 to 11.2||V||22.4 & up|
|F||5.0 to 5.6||N||11.2 to 12.5|
|G||5.6 to 6.3||P||12.5 to 14.0|
|Equipment||Running Watts||Maximum VA (Surge Watts)|
|Light bulb (100W)||100||100|
|1/3 HP Furnace w/Blower||600||1800|
|1/3 HP Sump Pump||700||2100|
|6" Circular Saw||800||2400|
|Water heater (storage-type)||5000||5000|
Note: This chart is provided to suggest typical values. Actual wattages may be significantly higher or lower depending on age, size make and condition of appliance.
Recommended Extension Cords for Use with Generators
|Ampere Rating||50 ft.||100 ft.||150 ft|
It is important to use extension cords of adequate current carrying capacity when utilizing a generator to operate portable electric tools. Undersized cords result in excessive voltage drops and additional generator loading. This also causes excessive heating of the portable tool because voltage drop reduces tool capacity and increases amp draw.
The product statements contained herein are intended for informational purposes only. Such product statements do not constitute a product recommendation or representation as to the appropriateness for a specific application or use. W. W. Grainger, Inc. does not guarantee the result of product operation or assume any liability for personal injury or property damage resulting from the use of such products.