GENERATOR TRANSFER SWITCH
HOW TO CONNECT A GENERATOR TO HOME 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 center tap 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 and has its neutral wire bonded to chassis. "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 within 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.
TRANSFER SWITCH TYPES.
Generator transfer systems can be manual, automatic, or a combination of manual and automatic.
An automatic transfer switch is used with standby systems and includes a control circuit that senses mains voltage. When utility power is interrupted, it starts up the genset, disconnects your house 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 true DPDT device, it has two mechanically interlocked breakers (one is for "Genset" and the other one is for "Utility"). For the requirements of up to 30A, a cheap manual TS may cost under $350.
The wiring 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 most portable models sold in US nowadays have a GFCI (GFP) and do have their neutral bonded to the metal frame. This causes a problem: when a neutral is grounded in two places its current will split, and the GFCI on the generator trips. As the result, no outlets will function. There are several possible solutions to this problem. The safest one is to install a so-called neutral switching device (3-pole transfer switch) that controls neutral along with two power lines. The other solution is to lift the ground wire from the genset inside the transfer switch and isolate it. Finally, you may remove the neutral bond inside the genset, which disables GFCI. This option is not safe though-- if you will ever use such device without a TS, such as on a job site, you will not be protected from electric shock. Such mofication may also void the warranty. In any case, all installations should be done by a licensed electrician. And for your SAFETY, be sure to always turn OFF the main circuit breaker BEFORE starting the wiring.
All the information in this site is provided for reference purpose only and does not constitute a professional or legal advice. See the complete Disclaimer linked below. For the wiring and installation requirements refer to National Electric Code, local codes, and your product manual.