Double Throw Switch

Adding a standby generator to the electrical system of a home, farm, or business requires a suitable transfer switch to disconnect the electric loads from the power supplier's utility grid. This is a requirement of the National Electrical Code and all electric power suppliers, for two very good reasons: (1) it prevents the back flow of current into the utility's lines during an outage, which could electrocute linemen working to restore power; and (2) it prevents damage to the generator when regular electric service has been restored, which can destroy the generator.

For single-phase, 120/240 volt power, the transfer switch should be a double-pole, double throw type. Double pole means that there are two pairs of wire lugs available for connection of hot conductors. The third wire (neutral wire) is continuous through the transfer enclosure, and is typically not switched. The ground wire also passes through the switch enclosure to provide a safe and continuous ground connection.

The term double-throw means that the operator can place or "throw" the switch into two different positions. One position feeds power from the utility system to the load. The other position feeds power from the standby generator to the load. Such a switch will prevent electricity, generated by the standby unit, to flow simultaneously to both the customer's home or farmstead, and to the utility's system. This is essential to protect utility personnel and to prevent generator equipment damage.

For small portable generators that serve a single freezer, well pump, or other appliance that plugs directly into the generator, a transfer switch is not necessary. But any time a generator serves loads through the permanent wiring system of the home, farm or business, a suitable transfer switch must be used.

Typically, the transfer switch is located between the utility meter and the loads to be served. It should be within 25 feet or less of the generating unit for convenience and safety. If the location of critical loads are scattered among several outbuildings, the central meter pole may be the best location for the transfer switch and the generator.