GENERATOR TRANSFER SWITCH
HOW TO CONNECT A GENERATOR TO HOUSE WIRING
The standard U.S. household electrical service is 120/240 volt single phase. It normally has two black "hot" wires and white grounded neutral. The nominal voltage between each line and neutral is 120 VAC and between two lines is 240 VAC. Note that both lines still belong to a single phase- they are obtained by grounding the centertap of the utility transformer. For such a system a generator transfer switch should be double pole double throw (DPDT) "break before make". Double throw means you can place or throw it into two positions (off position is not counted). Double pole means it transfers both line wires (poles). The neutral is normally continuous and does not have to be switched unless your genset is equipped with a GFCI. "Break before make" means it disconnects load from one source before connecting it to another one. Older 2-wire 120 volt residential systems require a single-pole double throw switch. Likewise, a three-phase generator would need a three-pole device. The TS generally should be installed within 2 feet of the main distribution panel and 30 feet of your genset inlet box.
Normally an electrical transfer switch for home generator has three positions: LINE, OFF and GEN. When switching between LINE and GEN, it always passes through the OFF position. Such break prevents arcing or short circuits during the transition. If power rating of your genset is not sufficient to power the entire house, you may install an additional distribution panel (a subpanel) and hook up the essential lines your want to backup to this panel. This sub-panel will then be wired to the transfer switch. Many commercially available TS come already with a pre-wired panel.
Generator transfer switches can be manual, automatic, or a combination of manual and automatic.
An automatic transfer switch includes a control circuit that senses mains voltage. When utility power is interrupted, it starts up the genset, disconnects the load from the utility and connects it to the genset. It then continues to monitor mains voltage, and when it is restored, switches your household wiring from the generator back to the utility. When the generator is disconnected it normally goes into a cool-down process and then automatically shuts down. Generac and other standby systems usually come with a 100A or 200A auto TS, or have it as an option. An auto TS for residential applications may cost $600 and up. Note that an auto TS can also be set to the manual mode if desired.
A manual transfer switch is usually used with portable gensets. Often, instead of a DPDT device, it has two mechanically interlocked breakers (for Genset and Utility). For requirements of up to 30A it can cost under $350.
The diagram below shows an example of connecting a portable generator to the house wiring via a DPDT switch for a 3-wire 120/240 V service. The dotted lines symbolically show the metal cases. In this diagram the neutral is not grounded inside the genset. Note that many models do have their neutral bonded to the metal frame. If such a model also has a GFCI (GFP), it may cause a problem: when a neutral is bonded to ground in two places its current will split, generator's GFCI breaker will trip, and no outlets will function. Possible solutions to this problem are either to not ground the neutral a second time at the genset, or use a so-called neutral switching device (3-pole or 4-pole transfer switch). All installations should be done by a licensed electrician. For SAFETY, be sure to always turn OFF the main circuit breaker BEFORE starting the wiring.
All the information here is provided for reference purpose only- it does not constitute a professional advice. See the Disclaimer linked below. For installation requirements refer to National Electric Code, local codes, and your product manual.