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200-2341 7
When connecting the generator to a building
electrical distribution system, use of a transfer switch is
recommended to isolate the normal utility source from
the generator supply. By preventing backfeed of the
generator power into the utility lines during a utility power
outage, the switch provides a level of safety for electrical
line workers. An automatic transfer switch listed to
Underwriters Laboratories Standard 1008 performs this
function while also supplying a method to automatically
start and stop the generator set and transfer power to the
building electrical system from an appropriate source.
When planning for a transfer switch/generator
installation, it is necessary to know the method of
interconnection to the building system. As illustrated, it is
recommended to connect the generator to power only
circuits that are necessary during a power outage. When
following this method, the transfer switch feeds an
electrical sub-panel containing only the necessary
circuits (see figure 1). Another option is to connect the
transfer switch directly to the entire building electrical
distribution system. In this case, all circuits or loads must
be controlled to avoid overloading and possibly
damaging the generator set (see figure 2).
Switch selection is based on factors such as the size
of the building distribution system, available generator
power and required generator control system. Due to the
complex nature of electrical distribution systems, the
sizing, selection and installation of a switch is best
handled by a licensed electrician or qualified standby
generator dealer. Any switch installation must be
performed by a licensed electrician to the instructions of
the switch manufacturer and any applicable local,
regional or national codes.
AA
AA
UU
UU
TT
TT
OO
OO
MM
MM
AA
AA
TT
TT
II
II
CC
CC
TT
TT
RR
RR
AA
AA
NN
NN
SS
SS
FF
FF
EE
EE
RR
RR
SS
SS
WW
WW
II
II
TT
TT
CC
CC
HH
HH
WWAARRNNIINNGG
• Hazardous "backfeed" voltage can cause
severe injury or death. Install a transfer switch
in standby power installations to prevent
connection of standby to other sources of
power. Electrical backfeed into a utility
electrical system can cause serious injury or
death to utility personnel working on
transmission lines.
• Do not try MANUAL operation of the transfer
switch until all power supplied to the switch has
been positively turned OFF. Failure to remove
power from all sources may result in extremely
dangerous and possibly lethal electrical shock
or arching.
Figure 2
Example of Alternative Transfer
Switch Installation
(Main Panel Load)
A
B
C
D
A Utility Power
B Standby Generator
C Main Circuit Panel
D Automatic Transfer Switch
Figure 1
Example of Recommended Transfer
Switch Installation
(Essential Sub-panel Load)
A
B
C
D
E
A Utility Power
B Standby Generator
C Main Circuit Panel
D Automatic Transfer Switch
E Essential Loads Sub-panel