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Understanding Generator Load
03/09/10
This page explains how to determine loads so that you can size your generator
for your needs. Normally you want to power the "essential" things
and leave some room for other uses.
You must always remember that the generator cannot run at full load for long
periods. Always assume that you will need more items powered rather then less
and leave a margin of safety above and beyond the continuous load of the generator
you select.
It is less expensive to buy more power (higher wattage) in one unit then to
buy a second unit or "upgrade" by trading in your old generator for
a new one. Often installation costs for a larger unit is a fraction more initially
but if you have to upsize your connection afterwards its very expensive.
All these require common sense judgment and some experience. Ask your electrician
for advise, he can help you decide what you need.
Below is a short tutorial which will help you understand the basics. The Wattage
Guide will provide you with values for the most common items at home and on
the job. If you can get nameplate data your calculations will be more accurate,
however using the Guide will be sufficient in most cases, especially if you
leave yourself plenty of growing room. Make sure you have plenty of size by
getting generator big enough for now and the future.
Go to Calculations to use your new understanding and apply the numbers you
gathered. You should be able to determine the right size generator for you.
GENERATOR USAGE
Generators are used to perform a wide variety of chores. The wide variety of
generators meet the demands of the variety of almost all potential users. Generators
offered by GeneratorJoe provide a high quality power source that is reliable
and convenient to use.
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Generator Usage
Homeowner - Stand-by
Recreation - Boating - Camping
Commercial - Industrial - . Construction - .Rental
GENERATOR POWER
Most generators produce AC voltage, very similar to the voltage available in
your home.
The amount of power that a generator can produce is rated in watts (power).
For Example, an EM2500 generator produces a MAXIMUM 2500 watts of power. This
means the EM2500 could provide power to 25 one hundred watt light bulbs at the
same time. The generator would then be at its MAXIMUM power output.
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Generator Power
Model = Power
Power = Watts
MAXIMUM AND RATED POWER
A generator should never be operated at its MAXIMUM power output for more than
30 minutes.
RATED power is a more reliable measure of generator power. It is the power
that a generator can produce for long periods of time. Typically the RATED power
is 90% of the MAXIMUM power.
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Rated and Maximum
Model number = maximum . output in watts.
Maximum power for 1/2 hour.
Rated is usually 10% less.
LOADS
In the previous example, the light bulbs are the LOAD of the generator. The
EM2500 generator can handle a LOAD of no more than 2500 watts maximum.
The light bulb example is called a RESISTIVE type load and the POWER it requires
is pretty easy to understand. Other RESISTIVE types of LOAD are things like
toasters, convection ovens, hot plates, curling irons, coffee makers, stereos
and TV's. RESISTIVE LOADS are usually those that do not have electric motors.
Another load is the REACTIVE type and is a little more confusing. Typically,
a REACTIVE load contains an electric motor. This type of load may require up
to three times as much power (wattage) to START as it does to keep it running.
Examples of REACTIVE type loads are air conditioners, refrigerators / freezers,
furnace fans, well pumps, bench grinders and air compressors.
Loads
Resistive
Reactive
RESISTIVE LOADS
The equation shows the relationship between watts, volts and amps in a PURELY
RESISTIVE load. If you know any of the two variables, the third can be calculated.
Example: You want a generator to power a 1000 watt flood light. The light is
120V and requires 1000 watts of power. Using the equation, we can calculate
that the floodlight will draw 8.3 amps of electrical current.
For REACTIVE loads, the equation shows only a general relationship between
watts, volts and amps. That's because the power requirements for REACTIVE loads
changes with operating conditions.
Resistive Loads
Watts = Volts x Amps
REACTIVE LOADS
When determining the proper generator for REACTIVE type loads, you must consider
three modes of operation:
STARTING - The electric motor requires more power to start. The starting power
required can be THREE times the running amount.
RUNNING - The power required to run the electric motor after it has been started.
LOADED - When the electric motor begins to work (saw begins cutting wood),
its power requirement will increase. This is not applicable for most household
appliances.
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Reactive Loads
Starting
Running
Loaded
LOAD POWER REQUIREMENTS
Here are a few ways to determine power requirements for various loads that are
expected to be powered by a typical generator.
Method 1, using estimating charts, can be used to get a general idea of the
generator size.
Method 2, reading the motor data tag, is more accurate since the data tag information
is provided by the motor manufacturer. Data tag information does not always
show STARTING power requirements for REACTIVE type loads. See "CODE CHART"
at the end of this page.
*Note: Data tag information can usually be found in the owners / operating
manual as well.
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Determine Load Requirements
Estimating Chart
Read data tags
ESTIMATING CHART
Method 1 requires using the load wattage estimating chart. It gives two figures
of watts needed to power various loads; Running and starting watts.
Add up all of the watts estimated for the loads / appliances / tools that you
want to operate at the same time.
If the load is a reactive type, use the STARTING WATTS for your estimation.
Select a generator that is as large or larger than the total number estimated.
Example: If you only want to run a refrigerator (2200 starting watts) and (2)
100 watt light bulbs (200 watts) - You would need a 2500 watt generator.
Wattage Estimating Chart
What are the loads?
Add up the total wattage of loads
Use the starting watts if the load is a reactive type
Select a generator
DATA TAG
Method 2 requires a visual inspection of the data tag supplied by the electric
motor manufacturer. All electrical motors have a data tag attached to their
bodies that give volts, amps, phase, cycles, hp, and sometimes a code.
Volts (V) - The volts must be either 120 (110-120) or 120/240. 120/240 means
that the motor can be wired to operate on 120V or 240V. Typical generators are
either 120V or 120/240V.
Amps (A) - Indicates the amps required to RUN the electric motor but doesn't
consider STARTING or LOADED power requirements.
Phase (PH) - The most commonly used generators can power only single phase
motors only.
Horsepower (HP) - Rating of how much work an electric motor can perform.
Code - This isn't always supplied on the data tag. It represents the maximum
STARTING power required of the electric motor.
Cycles (Hz) - All of U.S. electric appliances run at 60 cycles per second.
Read the Motor Mfg. Tag
Volts
Amps
Phase
Hp
Code
Cycles
LOAD CODE
Example: The data tag on our electric motor shows a code of L. Our motor is
1/3 Hp. An L code is 84 amps per Hp x 1/3 (motor Hp) = 28 amps to start the
motor shown.
CODE AMPS PER HP TO START CODE AMPS PER HP TO START
A 26.0 L 83.3
B 29.5 M 93.3
C 33.3 N 104.0
D 37.4 P 116.6
E 41.6 R 133.3
F 46.6 S 149.9
G 52.4 T 166.6
H 59.0 U 186.6
J 66.6 V more than 186.6
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Load Code
Code is a letter which represents Amps per Hp to start the motor.
Multiply CODE (amps) times Hp of motor to determine starting amps.
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