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MARCH 1998


With the right hardware, your generator can be wired directly to your home's circuits.

- Estimating power
- Our installation
- Mounting the new panel
- Wiring the panels
- Connecting the generator

There's nothing like a power outage to make us realize how much we rely on electricity. Electricity powers the furnace, the refrigerator, the TV, the computers, the lighting circuits, the sump pump and, in many homes, the water well, which supplies the showers and the toilets. A prolonged power outage can leave us cold, hungry, bored, unwashed, uncomfortable and largely in the dark. And if you happen to work at home, the financial losses can quickly add up.
    While an inexpensive portable generator will power the TV and a lamp or two, what's really needed is a unit that's large enough to keep a few important circuits running. The trick, though, is to connect the generator to the household wiring in a way that's safe and convenient. To do the job, we installed a manual transfer switch that routes electricity from either the municipal power supply or the generator to the vital circuits.
    A manual transfer switch is actually two switches, or breakers, positioned side by side in an auxiliary panel. One breaker is powered by the main service panel and the other by the generator. Both breakers can feed power to the chosen circuits, but not at the same time. The key is a rocking toggle clip that connects both breakers so that when one is switched on, the other automatically shuts off. While both can be off, only one can be turned on at a time, which protects the generator when municipal power is restored.
    A transfer switch is not automatic–you still have to flip the switch and plug in and start the generator–but it is affordable. We chose a Square D (Square D Co., 1601 Mercer Rd., Lexington, KY 40511) 4-circuit generator panel, model QO4-8M30DS-GP, which sells for about $100 in home centers.

Estimating power

The output of a generator is rated in watts, and this figure indicates the maximum total power draw of all the fixtures and appliances being served.
    To determine the size of the generator you'll need, start by totaling the wattage of the lights and appliances on the circuits you'd like to power. Check the labels or owner's manuals for each appliance's rating. Then, add about 20% as a reserve to handle the increased startup power requirements of most electrical devices.
    In our case, we wanted to power at least two fixtures on a general-lighting circuit (240 watts, when used sparingly), a sump pump (1500 watts), a refrigerator (600 watts) and the blower on our gas furnace (1200 watts). This gave us a practical total of 3540 watts without reserve power.
    To meet this demand, we opted for a Sears 4200-watt generator (No. 32674, about $900). This 7.8-hp unit comes with three fused AC outlets: a standard 120v, 15-amp duplex receptacle; a 120v, 30-amp 3-prong locking-type receptacle; and a 120/240v, 20-amp 4-prong locking-type receptacle.

Our installation

Because gasoline-powered generators exhaust carbon monoxide, they must be operated outdoors. Therefore, we planned to store our generator in our attached garage, and move it outside when needed. To make the electrical connection to the panel as simple as possible, we planned the generator hookup for an outside wall of the garage near the garage door. Here we mounted a box and receptacle that's wired to the new transfer switch panel. We then constructed a 6-ft. extension cord to connect the generator to the outdoor receptacle.
    Some local electrical codes may require that the cord to the generator be hardwired to the house electrical system. In this case a strain-relief device should be used at the connection.

Mounting the new panel

Begin by shutting off power to the main panel at its disconnect switch and removing the cover from the panel. Remember that while service panels are designed for safety, they should be approached cautiously. Even when switched off, terminals at the main disconnect will be hot and must be avoided.
    With the panel exposed, break a 1-in. knockout from the side of the box and mount a short threaded nipple in the hole (Photo 1). Connect the new switch panel to the opposite end of the nipple and anchor it to the wall with heavy screws (Photo 2).

1--Break a knockout from the side of the service panel and install 1-in. conduit or a threaded nipple in the opening.
2--Connect the new transfer switch panel to the other end of threaded nipple and secure the panel to the wall with screws.

    The grounding bus for the new panel comes uninstalled and there are three locations where you can secure it. Use screws to mount the grounding bus in the position that's most convenient for you (Photo 3).

3--Install the new panel's grounding bus in one of three prepared locations. Choose the one that's most convenient.
4--Disconnect the target circuits by first loosening the terminal screw on each breaker. Then, pull wires free.

Wiring the panels

Use an insulated screwdriver to remove the hot wire (black) from each breaker that controls a circuit you intend to power with the generator (Photo 4). Then, pry the breakers from the panel (Photo 5). Carefully identify the neutral (white) and grounding wires (bare) from each circuit, loosen their binding screws and pull them free (Photo 6). With all the wires disconnected, tape together each circuit's hot, neutral and ground wires (Photo 7).

5--With hot wires disconnected, carefully pry each circuit breaker from its slot on the panel's hot bus.
6--Identify the neutral and grounding wires of each circuit. Loosen the screws that secure them to the bus.

7--To prevent confusion, isolate the three wires from each circuit and tape them together in the main panel.
8--Begin wiring the new panel by pulling 10/4 cable through pipe nipple. Strip sheathing from both ends.

    Next, feed 10/4 cable through the pipe nipple (Photo 8). and strip the sheathing from both ends. If you wish, you can use individual 10-ga. insulated wires coded red, black, white and green in place of the cable. Connect the neutral and grounding wires to the main panel's neutral bus (Photo 9). If your panel has a dedicated grounding bus, bind the grounding wire to the grounding bus and the neutral wire to the neutral bus.

9--Secure the neutral (white) and ground (bare) wires to the appropriate bus bar in the main panel.
10--Connect the black and red hot wires of the 10/4 cable to the terminals of a 30-amp, 240v breaker.

    To complete the connections in the main panel, attach the incoming red and black hot wires to a 30-amp, 240v breaker (Photo 10) and press the new breaker onto the panel's hot bus (Photo 11). Remove any breakers stranded at the bottom of the panel and move them up beneath the new 240v breaker.

11--To install the breaker, hook its base over the panel's retainer clip. Press until breaker seats over the bus tabs.
12--Secure the bare ground wire from the 10/4 cable to the grounding bus in new transfer switch panel.

    The connections in the side panel are nearly identical to those in the main panel. Bind the feed line's grounding wire under a grounding bus terminal (Photo 12). Then, connect the red and black wires to one of the panel's 240v breakers and attach the white neutral wire to the neutral bus near the top of the panel. At this point, the main panel is equipped to power the side panel.
    To bring the disconnected circuits in the main panel to the new side panel, the wires must be extended. Codes vary in this regard. Some, like ours, allow twist connectors in the main panel, while others may require that you pull all disconnected circuit cables from the main panel and route them into the side panel, via junction boxes.

13--To extend circuits, connect wires from new panel to disconnected circuit wires with twist connectors.
14--In the new panel, bind the extended ground (green) and neutral (white) circuit wires to the appropriate bus bar.

    To extend the circuits within the main panel, bring insulated wires (black, white and green) through the connecting nipple, strip about 5/8 in. of insulation from each and join the new wires to the existing circuit wires with twist connectors (Photo 13). When you're done, tuck the wires neatly against the outer edges of the panel. In the side panel, attach the white neutrals to the neutral bus terminals (Photo 14) and the grounding wires to the grounding bus. Then, connect each circuit's black wire to a breaker in the side panel (Photo 15).

15--To complete the connection at the side panel, secure each black circuit wire to a 120v breaker.
16--Feed 10/4 cable through a 3/4-in. box connector at top of side panel. Tighten connector to secure the cable.

    All that remains is to install 10/4 cable that connects the new panel to the outdoor receptacle that serves as a hookup for the generator. In our case, we brought the cable from the panel up the basement wall and through the house rim joist a few feet from the garage door. Begin by mounting a 3/4-in. box connector in the top of the side panel, then run 10/4 cable into the panel through this connector. Staple the cable to the wall above the box, within 8 in., strip the cable inside the box and tighten the box connector (Photo 16). Attach the red and black hot wires to the remaining 240v breaker in the transfer switch panel and join the neutral and hot wires to their buses.
    Fill any open slots in the main panel's cover with plastic blanks available at home centers and electrical supply houses. The side panel comes with adhesive labels marked generator supply and utility supply. Stick these labels in place above the appropriate breakers of the transfer switch. Follow by labeling each circuit rerouted from the main panel on the side panel.

Connecting the generator

To install the generator hookup, first bore a 1 1/16-in. hole through the rim joist and pull the 10/4 cable through this opening. Then, thread a 3/4-in. galvanized nipple into the back of a weather-tight exterior box (Photo 17), feed the cable through this nipple and screw the box to the wall. To protect the siding, carefully caulk around the perimeter of the box. With the box installed, cut the cable to length and strip the sheathing from the last 8 in. Then install a flanged power inlet in the box. (We used a Hubbell inlet, NEMA L14-20P) This is nothing more than a male plug instead of a receptacle.

17--Thread a 3/4-in. nipple into the outdoor receptacle box. Insert the nipple through hole in the rim joist.
18---Secure extension cord wires to plugs. Attach hot wires to brass screws, white to silver screw and green to green.

    A variety of flanged inlets are available for this situation, but it makes sense to choose one exactly like the one on the generator. This not only simplifies the selection process, but it assures a compatible prong configuration and load rating. Attach the red and black hot wires to the inlet's brass terminal screws, the white neutral wire to its silver terminal and the green grounding wire to the green terminal.
    The extension cord we built has a male plug on one end and a female plug on the other. Make it with 6 ft. of rubber, water-resistant 10/4 cord with stranded wires. Strip 1 1/2 in. of sheathing from each end of the cord. Then, strip 1 1/16 in. of insulation from each wire. Slide the wires into the color-coded terminal slots on the plugs and tighten the terminal screws (Photo 18). Again, the red and black wires go to the brass-colored terminals, white goes to silver and green to green.
    With the cord made up, turn the transfer switch to generator supply and plug one end of the cord into the house-mounted flanged inlet. Plug the other end in the generator (Photo 19). For safety, always make sure to plug the cord into the house receptacle and into the generator before starting the generator. And, of course, always shut off the generator before unplugging either end of the extension cord.

19--Move the generator outside and plug the other end of cord into the appropriate receptacle. Then start generator.