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Generator Transfer Switches... Your Best Bet for Safely Using Emergency Power in Your Home!

by Michael Chotiner

As power outages triggered by extreme weather events become increasingly prevalent around the U.S., more and more homeowners are exploring options for backup power. It would be nice if every home were equipped with a built-in backup generator that had the capacity to power an entire household during a utility blackout and was fitted with an automatic transfer switch to start the flow of electricity from the generator the second after the utility power fails. While some newer, pricier homes have such backup power systems, few others are so equipped.

Remodeling Magazine's 2016 Cost vs. Value Report suggests why: The average residential emergency backup power system costs about $15,000 to retrofit, and owners can expect to recover less than half that cost on the resale of their homes.

basic generator electrical wiring scheme

How much power do YOU need?

One of the first steps in planning your emergency power strategy is to decide what you need to run during an outage and how much power that will take so you can be sure your generator has ample output capacity. The most affordable option is to make do with portable generator. A 7,500-watt portable generator can deliver about 60 amps at 120 volts or 30 amps at 240 volts—enough power to keep a few lights and critical appliances like a refrigerator, well or sump pump working when utility power fails. You'd need a more expensive 15,000-watt generator to also power an electric range or central air conditioner.

But by itself, a portable generator is inconvenient to use as an emergency power source. Portable generators with gasoline engines need to be outdoors and away from open windows and doors when operating. Who wants to keep a window or door open even a crack when the weather is bad to route-in extension cords running from the generator to the fridge or a few lamps during a blackout? How would you even use a portable generator to power a furnace or boiler, central air conditioning, a sump or well pump, an electric range or any other appliances that are usually wired directly to dedicated circuits?

Enter the transfer switch...

The answer is to connect the generator to your home's main load center with a manual transfer switch. There are at least three good options:

1) Install a backfeed breaker with a generator interlock switch in your home's main service panel.

2) Install simple interlock panel with two double-pole breakers near your home's main service panel.

3) Install a transfer switch panel to control the critical circuits you need when the utility service feed is down.

Please note: Transfer Switch Wiring is NOT for Unlicensed Electricians!!

Wiring a transfer switch panel is not a job for non-licensed electricians. Handy folks with a basic understanding of home electrical systems, load calculation and safe practices will want to consider the switch panel options carefully and decide, in consultation with a pro, which will work best with the electrical system already in place for their emergency power needs.

If you really want to get involved, you can save some money by buying the necessary kits and/or components yourself and doing some of the non-electrical grunt work involved, like mounting panels and receptacles where they're needed. But unless you're a licensed electrician, don't take the cover off of your main service panel or make any wiring connections yourself. That would be dangerous and possibly illegal.

Option 1: Backfeed Breaker with Interlock Switch

One approach to connecting house wiring to a backup power source is to install a backfeed breaker in the main service panel. It is not a transfer switch, but rather a simpler, less expensive alternative.

In a backfeed breaker setup, an additional circuit breaker is installed in the main service panel and wired to accept power from the generator feed and distribute it to branch circuits connected to the panel. Because backfeeding power through a home service panel out to utility lines would pose a serious electrocution threat to technicians who may be working on them, a backfeed breaker should be installed along with an interlock switch guard, which makes it impossible for the service panel main switch and the generator breaker to be in the on-position at the same time.

backfeed breaker
Backfeed breaker on the Main Electric Panel
(Image courtesy of Schneider Electric.)

As you can see in the image above, the interlock kit is a physical barrier that prevents the main circuit breaker and the backfeed breaker from being set in the on position at the same time, making it impossible for generator power to be fed out to the grid, which would endanger technicians working on your electric lines.

For the backfeed breaker solution to be feasible, there need to be at least two unused breaker slots in your panel. You'll need a circuit breaker that's compatible with your service panel brand and the amperage of your generator. It's typical to use a 30-amp breaker for generators up to 8,000 watts and a 50-amp breaker with 8,500- to 15,000-watt generators.

Since a backfeed breaker distributes power to every circuit wired in the main panel and portable generators can't provide enough power to run everything in your home at once, you need to manage the load on the generator during an emergency. That's easy enough to do by switching off breakers that control non-essential appliances and circuits, and switching on circuits that you may need at any given time. For example, if you've been running an electric heating system for most of the day, you're probably going to have to turn off that circuit when you need to switch on the range circuit to cook supper. Overloading a generator can damage it permanently.

To connect the generator to the backfeed breaker, you'll need to install a weatherproof inlet receptacle ($50 to $80) through the house wall within 30 feet of the main service panel and run cable from the inlet to the breaker. You'll also need a 4-wire generator cable to connect the generator to the inlet receptacle.

Option 2: Simple Manual Transfer Switch Panel

simple manual transfer panel for portable generator
A single-load manual transfer switch.

The simplest, least expensive panel has one 60-amp, double-pole switch designed for use with 120/240-volt generators up to 15,000 watts. This type of switch transfers generator power to the entire service panel to which it's wired. As with the backfeed breaker setup above, it is necessary to switch off non-essential circuits in the main panel during an emergency to avoid overloading the generator.

Option 3: Manual Transfer Switch Panel with Distribution Controls

More sophisticated manual transfer switch panels for homes come in kits suitable for 30-, 60- or 100-amp service and offer up to 16 controls for individual emergency-use circuits. These kits generally include most of the components needed for complete installation, including cables that are pre-wired to the individual switches, a weatherproof inlet receptacle box and a four-wire cable for connecting it to the transfer switch panel. The most useful transfer switch panels have integral meters that help users balance the load and avoid overworking the generator.

It's important to note that the breakers within the transfer switch panel must match those in the main panel in terms of the type of protection they offer. If arc or ground fault circuit interrupters or surge protection circuit breakers are used in the main load center, they must also be used in the manual transfer switch. Get advice from your electrician, and look for a transfer switch panel with interchangeable breakers.

30 amp manual transfer switch
30-amp, 10-circuit transfer switch panel from Reliance Controls.

Manual transfer switch panels are usually mounted within a few feet of a home's main service panel. The transfer switch panel is pre-wired with a common neutral wire (white), a common ground wire (green) and a pair of hot wires (one red, one black) running from each switch. The red and black pairs are usually coded with a letter or number that corresponds to the switch in the transfer panel that controls them.

As you can see in the above graphic, each transfer panel has a maximum number of circuits it can control. All other circuits will NOT have generator power.

Your electrician will run the entire bundle of wires from the transfer panel to the main panel, usually encased in a flexible conduit. The bundle is brought into the main panel through a knockout and fastened in position with a cable clamp. The white and green wires are clamped to the neutral bus in the main panel. (Some panels have a ground busbar in addition to a neutral, in which case the green wire would be connected to that.)

Once you and the electrician have agreed on which circuits you'll need to supply with generator power during emergencies, he'll connect the individual transfer switches to those circuits in the main panel. For each circuit, he'll disconnect the existing wire from the breaker and splice it to a black wire running from the transfer switch. The red wire from the pair is clamped into the breaker terminal. For each 120-volt circuit, he'll work with one breaker and one red and black pair; for each 240-volt circuit, it will be two breakers and two red and black pairs. This setup enables the circuit to be energized by the grid during normal operation or by the generator when the transfer switch is flipped on. The transfer switch isolates the generator power so it can't be back-fed to the grid.

All told, a single-load manual transfer switch installed by a pro will take two to three hours and should cost about $300 to $500, plus the cost of the generator. Equipment and professional installation for a more sophisticated transfer switch panel will take four to six hours and run about $1,000. Not cheap, but a lot more affordable than a built-in backup system. Plus, you can take that portable generator with you when you move.

Having power in an emergency is even more important now that we rely on so many electrical devices in our daily lives. To see a history of electricity usage in the U.S., you can view this infographic on energy consumption from The Home Depot.

About the author: Michael Chotiner is a former general contractor who writes about how-to topics that range from installing a door to choosing a breaker panel. Click here to see The Home Depot's transfer switch options, including those that Michael discusses in this article.

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