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TRANSFER SWITCH BASICS
What is a transfer
switch as used with a standby generator and your home? Well, the simple answer
is that it isolates the generator from the home’s electric utility service and
vice versa. Without a transfer switch, the only way to tie a generator into
your home’s electrical system is to perform the very dangerous and likely
illegal act of “backfeeding”.
Back feeding is done by connecting the output of
your generator to your home’s electrical system, typically by plugging into a
receptacle. This is dangerous for several reasons. The first reason that this
is unacceptable is that, with a large enough generator, can likely overload the
circuit that you are back feeding through resulting in damage, fire or injury.
Another reason that one must be mindful of the dangers of back feeding is the
simple fact that you can energize not only your entire house, but your
neighborhood and even primary service lines. It is important to note that when you
back feed, the same transformer that reduces line voltages to your household voltages
can work in reverse. That is, your 120 volts put out by your generator can be stepped
up to line voltage back feeding through a transformer. You could imagine how dangerous
this can be for utility workers repairing downed lines, damaged equipment, etc.
The basic function
of a transfer switch in a standby generator system is to isolate two separate
feed sources for a load(s) from each other, preventing any chance of one power
system feeding the other and vice versa. The following graphic illustrates a basic
schematic of a single transfer switch. The illustrated switch is a single pole,
double throw (SPDT) type. A common example of a SPDT switch is a three-way
switch found in the home. As drawn, the load/appliance is connected to the
electric service of the building and/or residence. The generator supply in this
example is isolated from the load and the electric service.
When the transfer
switch is actuated/switched, the connection between the electric service/line
power will “break” and then the switch will “make” the connection between the
load and the generator. Note the terms “make” and “break” for these are
important. It is possible to have two different types of SPDT switches: make
before break and break before make. A make before break switch will actually
have a brief period of time during switching where both the line power and the generator
will be connected. The break before make switch, the common type, theoretically
should never allow the connection of the two power sources. Obviously, this is
the type of switch that we need. The three-way switch mentioned earlier is typically
this type of SPDT switch.
There are several
companies that manufacture and sell preassembled transfer switches, basically,
off-the-shelf solutions. Two of these companies are GenTran and Reliance Controls.
For what these
switches are, sets of SPDT switches and some push button breakers (for the
basic manual options), We have yet to meet anyone that thought these were a bargain.
Based on our needs and what is available on the market, we opted to design and
install a much more economical, custom matched to our needs and just as safe
for property and life transfer switch system.
TRANSFER SWITCH DESIGN
& INSTALLATION
The above figure illustrates
the transfer switch design. Some important safety oriented characteristics to
note regarding this design:
1.
Note that all boxes,
grounded receptacles and grounded switches are bonded to the generator’s ground
and in turn bonded to the ground bus at the electric service panel for your
house.
2.
The neutral from the
generator is bonded to the neutral bus of the service panel which, in turn, is
bonded to the ground bus of the service panel. Note:
portable generator like Honda Eu2000i does not have a neutral bonded to
ground at its feed points.
3.
In this particular
example, you do NOT want to bond the neutral
and ground at the generator, Honda Eu2000i, if you are bonding the neutral and
ground buses at the service panel. Some generators come from the factory with a
neutral to ground bond, but that is not so in our case. In our example, if we
were to bond the neutral and ground at our generator and have the neutral
grounded at the panel, we now have two different paths for return current to
flow to our generator. Return current on the ground will create voltage on the
ground and that is NOT good. For now, you
theoretically have an “energized” ground and this is both a life safety and
property concern.
4.
All circuits being
dual served are 15 amp in size in the main service panel. While the generator
has 20 amp circuit protection, this is too much for your home’s 15 amp branch
circuit wiring. So, we would be able to overload a 15 amp branch circuit with
19.9 amps and never trip the generator circuit protection out. Also, keep in
mind that 15.1 amps will not instantaneously trip a 15 amp breaker. As a matter
of fact, 15 amps on a 15 amp protected circuit could take minutes or longer
before tripping. It is also important to note that a 20 amp breaker will not
trip as fast with higher currents as what a 15 amp breaker would.
All of these
concerns/issues could be major fire safety hazards. To prevent any issues, each
transfer switch’s generator side line is protected by a 15 amp breaker (All of
the 15 amp circuits utilize 14/2 W/ Ground wiring).
5.
The feed cable (12/2
W/ Ground) from the L5-20R receptacle (where the generator is hooked up
outside) and the receptacle itself are sized to accommodate the 20 amp
protected circuit provided by the generator as is the utility receptacle.
6.
The transfer
switches are 20 Amp models with Heavy Duty Contacts to prevent welded or
sticking contacts in order to meet the break before make requirement as
mentioned above.
The
basic “flow” of the system is as follows:
Generator >
Service Cable > Twist-Lock Receptacle > Split to Utility Outlet &
Generator 2 Breaker, 4 Circuit Load Center > Transfer Switch > Load (e.g.
Appliance)
The
materials list is as follows:
Outdoor Receptacle
· L5-20I Flanged
Insert Receptacle
· Outdoor, Deep
Receptacle Box
· Hubbell Flanged
Insert Outdoor Receptacle Box Cover
· Vinyl Siding J-Block
Generator Load Center
· Cutler-Hammer 2
Breaker, 4 Circuit Load Center (Cutler-Hammer to match existing service panel)
· Cutler-Hammer Twin
15/15 Amp BR Breakers
Utility Outlet
·
Single Metal Box
·
20 Amp 120V Duplex
Receptacle
·
Duplex Receptacle
Cover
Transfer Switches
·
20 Amp Heavy Duty
Three-Way SPDT Switches
·
Double Metal Boxes
·
Double Switch Covers
Miscellaneous: Wiring, Conduit, Etc.
·
12/2 W/ Ground
·
14/2 W/ Ground
·
3/4” EMT Conduit
·
3/4” Conduit Clamps
·
3/4” Conduit
Setscrew Connectors
·
Wire Nuts &
Electrical Tape
At a minimum, your
design, layout and installation must comply with the currentrevision of the National Electrical Code (NEC).
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