How to hook up a standby generator |
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| Please notify your local electric cooperative when you purchase a standby generator so that they can be
aware of your location. The cooperatives also encourage you to have a licensed electrician install your generator. |
Connecting Your Standby Generator Safely
(Transfer Switches & Code Requirements)
The Double-Throw Switch--An Essential Element*
Adding a standby generator
to the electrical system of a home, farm or business
requires a suitable transfer switch to disconnect the electric loads
from the power supplier's utility grid. This is a requirement of the
National Electrical Code (see NEC Article 702-6) and all
electric power suppliers, for two very good reasons: (1) it prevents
the back flow of current into the utility's lines during an outage,
which could electrocute linemen working to restore power; and (2) it
prevents damage to the generator when regular electric service has
been restored, which can destroy the generator.
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Figure
1 Typical Double-Pole, Double-Throw
Transfer Switch Installation for 120/240 V, Single-Phase
Service. Grounding electrode must be bonded according to
National Electrical Code requirements and local
regulations.
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For single-phase, 120/240
volt power, the transfer switch should be a double-pole, double-throw
type. Double pole means that there are two pairs of wire lugs
available for connection of hot conductors (see Fig. 1). The third
wire (neutral wire) is continuous through the transfer enclosure, and
is typically not switched (NEC Article 230-83). The ground wire also
passes through the switch enclosure to provide a safe and continuous
ground connection.
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Figure 2
Double-Throw Switch Controlling
Individual Circuits or Subpanel
The term double-throw
means that the operator can place or "throw" the switch into
two different positions. One position feeds power from the
utility system to the load. The other position feeds power
from the standby generator to the load. Such a switch will
prevent electricity, generated by the standby unit, to flow
simultaneously to both the customer's home or farmstead, and
to the utility's system. This is essential to protect
utility personnel and to prevent generator equipment
damage.
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A three-phase generator would require a
three-pole, double-throw switch. Some electric services use current
transformer (CT) metering. This may require the use of a pole-top
transfer switch. The operation of these switches is essentially the
same, except that an extended manual switch lever is needed to allow
the user to operate the switch from ground level.
For small portable generators that serve a
single freezer, well pump, or other appliance that plugs directly
into the generator, a transfer switch is not necessary. But any time
a generator serves loads through the permanent wiring system of the
home, farm or business, a suitable transfer switch much be
used.
Typically, the transfer switch is located
between the utility meter and the loads to be served. It should be
within 25 feet or less of the generating unit for convenience and
safety. If the location of critical loads are scattered among several
outbuildings, the central meter pole may be the best location for
both the transfer switch and the generator.
If a critical load is located within a
single building, the transfer switch (and generator) can be installed
at the service entrance to that particular building.
If only one or two circuits need to be
powered during an outage, it is possible to connect only those
circuits within a building to the transfer switch as illustrated in
Fig. 2.
The size of the transfer switch is
determined by the loads to be served. If a central meter pole
location is used, the transfer switch rating must be equal to the
size of the main service (typically 100, 200 or 400 amperes). If the
standby system is designed to supply an individual building, or even
a single circuit, the transfer switch will be sized to the total
ampere rating of the connected loads.
If an engine-driven generator with
automatic start-up is used, the transfer switch is normally built
into the automatic controls of the system. In this case, the transfer
switch must be large enough to handle all electrical loads, size the
switch to match the rating of conductors which supply regular power
to the building(s) or farmstead.
*Consult a qualified
electrician for assistance, and be sure your "new" wiring is
inspected.
All standby generators should be grounded
using #6 solid copper wire and an 8-foot ground rod, which must be
properly bonded to the electrical grounding system with approved
clamp/connections to assure good electrical contact. Cover all
generator openings with wire mesh to exclude rodent entry and
potential damage. Keep these openings free of debris that might
restrict the cooling of the unit.
Other safety features to
remember:
- Keep all guards and shields in place to
protect the operator from moving parts;
- Never run a generator in a basement or
other enclosed area. Fumes that are not easily detected can be
lethal to sleeping occupants and others;
- Never shut-off the generator under
load;
- Never store fuel near the generator,
unless a proper container or fuel tank is used;
- Never re-fuel a generator when hot or
while the engine is running;
- Use extreme care under wet conditions;
making your body a path to ground can cause a fatal
shock;
- On tractor-driven units, always turn
off the tractor and PTO control to service the generator. Set the
tractor brake before starting the generator.
ELECTRIC COOPERATIVES OF ARKANSAS
1 Cooperative Way
Little Rock, AR 72209
501-570-2200
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