Virginia Cooperative Extension
Power
outages can leave a home without power for lighting, cooking,
refrigeration and pumping water. Portable generators can be
bought to provide substitute power. However, the generator must
be properly sized to start the appliances and equipment you
want to run.
(For
120-volt, plug-in appliances)
First, find the wattage of the appliance(s) you want to run by checking the nameplate. Motor-driven appliances may be listed in horsepower which must be converted to watts. Motors require four times as much power to start as they do to run. If the running wattage of a motor is 400, then the starting wattage will be 1,600. The following table gives some starting and running wattage for electrical motors:
Table
1
Watts
Required
Motor,
hp
|
| To
start
|
| To
run
|
1/6
1/4
1/3
1/2
1
5
7.5
10
|
| 1,000
1.500
2,000
2,300
4,000
18,000
28,000
36,000
|
| 215
300
400
575
1,000
4,500
7,000
9,000
|
Now
determine how many appliances you want to run at the same
time and add or total the wattage. The size of the generator
you use must be such that it will start and run the necessary
appliances. If you get a generator that is too small to run
refrigerators and freezers, they will try to start, but the
voltage will drop and their motors will overheat and burn
out. If you cannot find the wattage, an estimate can be made
from the following table:
Table
2
Typical
Equipment Wattages
Essential home equipment
|
|
Typical wattage
|
|
Refrigerator
Freezer
Furnace blower
|
|
| 400-800
600-1,000
400-600
|
Optional home equipment
|
|
Typical wattage
|
|
Electric skillet
Electric stove
Washing machine
Water pump
Water heater
Electric fan
Central air conditioner
|
|
| 1,150-1,500
3,000-4,000
400
800-2,500
1,000-5,000
75-300
2,000-5,000
|
Farm equipment
|
|
Typical wattage
|
|
Ventilator fans
Silo unloader
Feed mixing
Feed conveyor
Bulk milk cooler
Electric fence
|
|
|
300-800
2,000-7,500
800-1,500
800-5,000
1,500-12,000
7-10
|
For
example, if you want a generator to run a refrigerator and
a freezer, the wattage (table 2) of the refrigerator would
be 800 and the freezer would be 1,000. To select the correct
size generator, you decide if both refrigerator and freezer
are to start at the same time. If so, you would need (1,800
X 4) 7,200 watts. You would select the nearest larger wattage
generator. If you can be certain both appliances will not
start at the same time, you would only need 4,800 watts (to
run the refrigerator while starting the freezer).
Install
wiring and equipment to meet National Electrical Code requirements,
local regulations and the requirements of the power supplier.
Single phase standby generators are connected to the electrical
line by a double-pole, double throw transfer switch. This prevents
accidentally feeding power back into the utility lines where
it can injure neighbors or utility workers servicing the lines.
This type switch also protects the generator from damage when
power is restored.
Put
the switch in a water-tight box and properly ground it, the
central meter pole is a common location. Install the switch
between the watt hour meter and the service disconnect (main
fuse box). Note that the white (neutral) conductor is usually
not switched, but some power suppliers require it be switched
also. When the handle is up, the utility black and red conductors
are connected to the load black and red conductors, respectively.
In the down position, the load conductors are disconnected
from the utility conductors and connected to the black and
red conductors from the generators.
Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
Based
on information developed by Clemson Cooperative Extension
following Hurricane Hugo. Revised for Virginia audiences by
Virginia Cooperative Extension.
For
more information, contact your local office of Virginia Cooperative
Extension.
Publication
Number
490-303
,
August 1996
|