Understanding the Fuse Box
Understanding how power from a generator feeds through a fuse box requires some understanding of electrical wiring and fuse box design. Electricity comes into the fuse box on three wires.
Two of these wires each carry 120 volts, while the third wire is a ground. The power wires are connected to one leg of the main breaker, while the ground wire is connected to the ground buss bar in the fuse box.
The main breaker is connected to the two live buss bars in the center of the fuse box. When 120 volt power, such as for house lighting, is required, a fuse holder or circuit breaker is connected to one of these live busses.
Several lights or receptacles may be connected to one fuse or breaker with the house wiring; 220 volt breakers connect to both live busses with each leg fused to protect the wiring. Any power fed back into any circuit from a generator will go back through the fuse or breaker on that circuit and feed power to the buss bar and subsequently to all fuses or breakers connected to that buss.
Connecting a Small Generator
Connecting a small generator to your home's wiring can easily be done with a cable and a manual transfer switch. The cable is plugged in to the generator receptacle, and the female connector plugged onto a transfer switch.
Using an extension cord to directly power a circuit is illegal because the current from the generator will feed back to the outside power lines where a worker trying to restore power could be electrocuted. The main breaker should also be opened for an additional safety factor.
Permanent Generator Connection
Larger generators with the ability to power most or all of your home's electrical needs should be permanently connected to your house wiring. This requires the use of a transfer switch.
These switches allow the selection of either generator or line power and isolate the source not selected. There are two types of transfer switches: manual and automatic.
As the names imply, manual switches require the homeowner to take an action to switch from line power to generator power, while automatic switches make the transfer without aid. This is the type of switch one would use with a generator that has an automatic starting system.
Transfer switches must be installed by a qualified electrician.