How to Make a Transfer Switch

Transfer switches are the switches you use to change from using commercially supplied power to using a private generator in situations like during a hurricane. There are two kinds of transfer switches. Manual switches are turned on when the commercial powers goes out and then turned off when the power comes back. Automatic transfer switches start the generator as soon as the commercial power fails. Putting the system back on-line may be either automatic or manual.

Transfer switches change the power source.

Step 1

Choose a manual transfer switch, which is the most simple solution.  Connect the commercial line to one throw of the switch, and connect the generator output to the other throw of the switch.

Connect the pole of the switch to the house circuit. 

Step 2

Throw the switch when the power goes out, and throw it back when the power comes back on.  When thrown one way, the commercial utility line comes into the house.

When thrown the other way, the generator supplies to the house. 

Step 3

Use a latching relay to automatically switch to generator backup when the commercial utility line goes out.  The latching relay stays on once it is turned on, and releases when the power is interrupted.

It will need to be reset when the power comes back on. 

Step 4

Connect the commercial line to one of the relay contacts that is normally open.  Connect three wires, which are a wire leading to the relay coil, a wire leading to the house circuits, and a wire leading to the pushbutton, to the pole of this same contact.

The other side of the pushbutton goes to the same contact that is connected to the utility line.  The normally closed path through the other set of relay contacts goes to the circuit that starts the generator.

Step 5

Restart the system when the power comes back on by pushing the pushbutton, which causes the relay to latch.  This will open the circuit that activates the generator.

Closing down the generator is a complicated process that is only automatic in the most expensive systems. 

Things You Will Need

  • Large SPDT switch (for manual transfer switches)
  • DPDT Latching relay (for automatic transfer switches)
  • Normally open pushbutton (for automatic transfer switches)


  • The disadvantage of the manual system is that the generator does not automatically start when the power goes out. If there is no critical machines, like life support machines in a hospital, then a manual transfer switch is all you need. An automatic system will start the generator even if no body is at home at the time the power goes out. In a system where the generator outputs to a bank of batteries and the batteries are connected to the house, the automatic transfer switch can ensure that the house sees only a few seconds of power outage.


  • Closing down the generator once the commercial power returns can be a complicated process. For example, you might want to keep the generator running to recharge the bank of batteries. The expensive automatic systems will do everything for you and warn you about situations that may arise, such as when the generator is low on fuel.

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