YOUR GUIDE TO TRANSFER SWITCHES
HOW TO CONNECT A GENERATOR TO HOME WIRING
An electrical transfer switch (TS) is any device designed to power an electric load from multiple sources. In home generator applications it is utilized to connect building's wiring either to an emergency power source or to the service lines. When AC line is normal, a TS allows powering your house from the grid. When the grid fails, TS can switch the home wiring system to a generator while isolating it from mains. This guide will tell you why you need it, how it works, how to choose the right one, and how to wire it (a schematic diagram is at the bottom of this page).
It's important to know that unless you are off-grid, connecting any unauthorized power source directly into any point of electrical wiring without a changeover switch is illegal and dangerous because of so-called backfeeding. Backfeeding (also called islanding or interconnection) by definition is supplying electricity back into electric grid when it's down.Here is why it should be absolutely avoided. The voltage fed back into the lines that serve your house poses an electrocution hazard for utility workers and for your neighbors who may not know the voltage is present in the line. Aside from this, your backfeeding home generator most certainly will be overloaded by your neighbors loads in the disconnected local portion ("island") of the grid. It can even be damaged if power is restored while it is working.
REQUIREMENTS AND RATINGS
The standard U.S. household electrical service is 120/240 volt split single phase. Normally, it has red and 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 (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. The TS generally should be installed within 2 feet of the main service panel and within 30 feet of your genset inlet box.
A power transfer switche for a home generator should have three positions: LINE, OFF and GEN. When switching between LINE and GEN, they always pass through the OFF position. Such break prevents arcing or short circuits during the transition. If power rating of your genset is not sufficient to feed the entire house, you may install an additional distribution panel (a sub-panel) for the essential lines your want to backup. This sub-panel will then be wired to the transfer system. Many commercially available TS come already with a pre-wired sub-panel.
TRANSFER SWITCH TYPES
Generator transfer systems can be manual, automatic, or a combination of both. An automatic changeover is used with standby systems. It includes a control circuit that senses mains voltage. When utility power is interrupted, the control circuit starts up the genset, disconnects your house from the utility and connects it to the genset. It then continues to monitor mains status. When it's restored, it commutates your household wiring from the generator back to the utility. When the generator is disconnected, it goes into a cool-down process and then automatically shuts down. Generac and other standby systems usually include a 100A or 200A auto TS, or offer it as an option. An auto switch for residential applications may cost $600 and up. Note that you can always set it to a manual mode if desired.
A manual transfer switch is normally used with portable gensets. Usually, instead of a true DPDT device, it consists of two double-pole breakers with mechanically interlocked handles (one is for "Genset" and the other one is for "Line"). For the load requirements of up to 30A, a cheap manual TS with a sub-panel may cost under $300. A simple interlock panel with two breakers runs under $100.
The wiring diagram below shows an example of connecting a 4-prong 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 neutral is not grounded inside the genset. For simplicity an inlet is not shown.
Here is a little known detail. Many portable models sold in US nowadays have a GFCI (GFP) and their neutral is bonded to the metal frame. This causes a problem when you are trying to use them with a TS: when 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 isolation switch). It will control neutral along with two power lines. Another solution is to lift inside the transfer equipment the ground wire from the genset (just isolate its loose end). Your genset will still be grounded via its neutral bus. Finally, one may remove the neutral bond inside the genset, which disables GFCI. This option is not safe though-- if you will ever use such a device in the field without a TS (for example on a job site), you will not be protected from electric shock. Such modification may also void the manufacturer's warranty. In any case, all installations should be done by a licensed electrician. You can find one and/or get an estimate by using the search tool above. And for everyone's SAFETY, make sure the main circuit breaker is turned OFF BEFORE any wiring work started.
All the information in this site is provided for reference purpose only and does not constitute a professional or legal advice. See complete Disclaimer linked below. For the wiring and installation requirements refer to National Electric Code® 2014, local codes, and your product manual.
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