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How to Wire an Automatic Transfer Switch to a Sub-Panel

You can connect either a manual or automatic transfer switch to a sub-panel in your home electrical system so that the emergency generator only provides power to the circuits routed through that panel. Transfer switches are required by the National Electric Code (NEC) for generators that are directly connected to home wiring. The power comes in to your home from the power company and then is fed into the main electrical panel in your house, from there the circuits that will be active when the emergency generator is running are diverted to a transfer switch which is connected to the generator and the subpanel.

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Very few homeowners can afford or consider it necessary to supply power to all of the homes electrical devices, fixtures and appliances in the event of a power failure. Because the generator chosen can only supply power to a limited number of items, it is necessary to decide which circuits will be powered by the standby generator in the event of a utility power outage.

As mentioned in another article, there are two types of transfer switches: Manual and Automatic Transfer Switch. In each case, manual or automatic transfer switch the circuitry is the same.

The meter is fed from the utility via an overhead or underground conduit as shown in the following figure. The electrical service main disconnect and electrical distribution panel are shown as 2 separate items, however newer circuit breaker distribution panels have the main disconnect located at the top of the electrical distribution panel (large circuit breaker).

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Adding a standby generator into your electrical service system, as shown in the next figure:

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Steps of wiring an automatic transfer switch to a sub-panel

1 Switch off the main power to the house and disconnect the generator from the transfer switch.

2 Feed one end of the 3-wire plus ground cable into the output area of your transfer switch. Depending on the amperage your sub-panel will draw you will need between 6 and 10 gauge wire. For 30 amps use 10 gauge wire, 40 amps use 8 gauge wire and for 50 amps and above use 6 gauge wire.

3 Cut the outer sheath off the cable and separate the individual wires. Strip the last half-inch of insulation from all the constituent wires.

4 Twist the green wire from the cable to the green wire coming into the transfer switch. This is the ground and it should bypass

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the switch. Secure the splice with a wire nut. Repeat for the white, common wire.

5 Insert the bare metal of the red wire into the top terminal on the switch and tighten the screw down hard. Repeat for the black wire on the bottom terminal of the switch.

6 Feed the opposite end of the cable into the sub-panel input hole. Cut the cable so that you have enough wire to lead connect to the main terminals at the top of the panel and to the bus bars for the neutral and ground.

7 Remove the outer sheath of the cable and strip off the last halfinch of insulation from each wire. Connect the green ground wire to the ground bus bar and the white wire to the neutral bus bar. Tighten the screws firmly. Note that in a sub-panel installation the ground and neutral bus bars should be isolated.

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8 Insert the red wire into either the top or the right main terminal. Insert the black wire into the remaining main terminal. Tighten the terminal screws firmly.

Tips & Warnings 1-Many automatic transfer switches have built-in sub-panels. 2-Use an automatic transfer switch that is compatible with your generator.

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How to wire an automatic transfer switch to a sub panel  
How to wire an automatic transfer switch to a sub panel  

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