An electric transfer switch is any appliance intended to alternatively direct power from multiple sources to an electric load. The device is used in residential generator applications to connect a home’s wiring either to a generator or the national grid. When the AC service line is functional, a transfer switch (TS) directs power from the grid to your home. When there is a power outage, TS channels the generator output to wire your home powering system. This TS guide explains its importance, how it works, guides you on how to select the correct one, and the procedure of wiring, it in your home.
In general, you should read: How to Connect a Portable Generator to a House
Unless you are completely off the grid, directly connecting your emergency power source to your electrical wiring when there is no transfer switch is strongly discouraged. The reason is, doing so is both illegal and hazardous due to the danger of back feeding (also referred to as interconnection/islanding). What is backfeeding? It means supplying electricity right into the national grid when there is a power outage. Also, avoid doing so completely because the voltage you feed back into the grid can potentially electrocute utility workers or your neighbors who could be working on the line unaware that electric energy exists in the line. Additionally, back feeding makes your neighbors’ loads overload your generator in the separate local section of the grid. Should the service power be restored while your system is back feeding your, generator will be damaged severely.
TS power needs and ratings
The official United States household electrical line is rated 120/240 V, split single phase. Red and black cables denote “Live” wires while white indicates grounded neutral. The minimum voltage between the neutral and each of the two lines is 120 VAC (alternating voltage) while that between the lines is 240 VAC. The lines both lines are single phased (the utility transformer’s center tap is grounded, and the remaining two leads extended to form the two lines). The generator TS of such a system is typically Double Pole Double Throw (DPDT) besides being “Break Before Make”.
So, What is the meaning of DPDT and "Break before make"?
It means that you can put (throw) the generator transfer switch into two positions (excluding the “Off” position). And the double pole implies TS transfers both grid wires (poles). Since the grounded line is usually continuous, it is not necessary to switch it on except when your generator comes with GCFI (Ground Fault Circuit Interrupter) and its neutral wire is connected to the chassis. “Break before Make,” implies TS commutates load from a given source, say national grid, before connecting the load to another source, say your gen set.
How to position your TS system?
Older two-cable 120 V home systems need a one-pole double throw switch. You should install the Transfer Switch a max of 2 feet from the mains service line and a max distance of 30 feet from your generator inlet box.
Functions of the three TS positions
Power TS for a home generator has three positions, namely LINE, POSITION & GEN. When switching from LINE to GEN, the TS always go through OFF position. This arrangement prevents short circuits (arching) while switching from one source to another. If your generator’s output is not enough to support all the devices in your home, you should install an extra distribution panel (called a sub-panel) dedicated to the crucial appliances you wish to backup. Subsequently, wire the sub-panel to the TS system. Note, though, that commercially available transfer switches come with an already pre-wired sub-panel.
Types of TS systems
Generator TS systems can be automatic, manual or both.
Auto TS systems are utilized with standby generator systems. It comes with a control circuit that monitors service line voltage. When the service line is interrupted, the circuit automatically switches on the generator and transfers the supply to your house from the utility to the standby generator. It then continues monitoring the grid status. Immediately it is restored, the control circuit transfers back your household wiring from the gen set to the national grid. As soon as the generator is released, it cools down and finally shuts down automatically. An automatic switch for home applications costs a minimum of 600 Dollars. It is possible to set your auto system to manual mode should there be a need to do so.
Manual TS systems, usually used with portable generators, differ significantly with automatic TS systems. Instead of a proper DPDT appliance, the system contains two double-pole breakers fitted with mechanically interconnected handles (for “Genset” and “Line,” respectively). If your electric load requirement does not exceed 30 A, an inexpensive manual TS, complete with a sub-panel, costing less than 300 Dollars will suffice. A simple interlock panel containing two breakers costs less than 100 dollars.
Transfer Switch Schematic diagram
Below is a Generator wiring diagram showing a typical way of connecting a four-prong portable gen set to a home system through a DPDT TS for a 3-cable 120/240V grid. The dotted lines indicate, figuratively, the metallic casing. Note that neutral is not grounded within the portable generator. An inlet has deliberately been omitted to make the diagram simple.
A crucial detail you probably didn’t know
Many portable generators in the USA nowadays come with a GFCI (GFP) while the neutral is connected to the metallic frame. This scenario causes issues when you try using them with a transfer switch; grounding the neutral in two different places splits its current, making the GFCI on the gen set to trip. Consequently, neither of the two outlets will operate. One safe solution is to install a neutral switching appliance (a 3-pole separating switch). The device will control the neutral together with two power cables. Alternatively, remove the neutral link inside your generator so as to disable the GFCI. This second option is not safe; there is a danger of electric shock should you use the appliance in a remote location where there is no transfer switch. The alterations may also nullify the manufacturer’s warranty.