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How to install a manual transfer switch for a backup system in 16 steps

Manual transfer switch installation

A manual transfer switch is installed next to the main service panel to override the normal electrical service with power from a backup generator during a power outage.

How to install a manual transfer switch for backup system in 16 steps
How to install a manual transfer switch for backup system in 16 steps

Manual transfer switches require an operator to change the power source, while automatic switches detect the loss of power, start the back-up generator, and switch over to the backup power feed.

Because the amount of electricity created by a backup generator is not adequate to power all of the electrical circuits in your house, you’ll need to designate a few selected circuits to get backup current.


But, select backup circuits first :)

Before you purchase a backup generator, determine which loads you will want to power from your generator in the event of a power loss. Generally you will want to power your refrigerator, freezer, and maybe a few lights. Add up the running wattage ratings of the appliances you will power up to determine how large your backup generator needs to be.

Because the startup wattage of many appliances is higher than the running wattage, avoid starting all circuits at the same time—it can cause an overload situation with your generator.

Here are some approximate running wattage guidelines (use this MS Excel Spreadsheet for calculating electrical loads):

  1. Refrigerator: 750 watts
  2. Forced air furnace: 1,100 to 1,500 watts
  3. Incandescent lights: 60 watts per bulb (CFA and LED lights use less wattage)
  4. Sump pump: 800 to 1,000 watts
  5. Garage door opener: 550 to 1,100 watts
  6. Television: 300 watts

Add the wattage values of all the loads you want to power, and multiply the sum by 1.25. This will give you the minimum wattage your generator must produce. Portable standby generators typically output 5,000 to 7,500 watts.

Most larger, stationary generators can output 10,000 to 20,000 watts (10 to 20 kilowatts).

Let’s describe 14 steps fot installing a manual transfer switch:

Step #1

Turn off the main power breaker in your electrical service panel. CAUTION: The terminals where power enters the main breakers will still be energized.

Installing transfer switch - Step #1
Installing transfer switch – Step #1

Step #2

Determine which household circuits are critical for emergency usage during a power outage. Typically this will include the refrigerator, freezer, furnace, and at least one light or small appliance circuit.

Installing transfer switch - Step #2
Installing transfer switch – Step #2


Step #3

Match your critical circuits with circuit inlet on your pre- wired transfer switch. Try to balance the load as best you can in the transfer switch: For example, if your refrigerator is on the leftmost switch circuit, connect your freezer to the circuit farthest to the right.

Double-pole (240-volt) circuits will require two 120-volt circuit connections. Also make sure that 15-amp and 20-amp circuits are not mismatched with one another.

Installing transfer switch - Step #3
Installing transfer switch – Step #3

Step #4

Select and remove a knockout at the bottom of the main service panel box. Make sure to choose a knockout that is sized to match the connector on the flexible conduit coming from the transfer switch.

Installing transfer switch - Step #4
Installing transfer switch – Step #4

Step #5

Feed the wires from the transfer switch into the knockout hole, taking care not to damage the insulation. You will note that each wire is labeled according to which circuit in the switch box it feeds.

Installing transfer switch – Step #5
Installing transfer switch – Step #5

Step #6

Secure the flexible conduit from the switch box to the main service panel using a locknut and a bushing where required.

Installing transfer switch - Step #6
Installing transfer switch – Step #6


Step #7

Attach the transfer switch box to the wall so the closer panel. Use whichever connectors make sense for your wall type.

Installing transfer switch - Step #7
Installing transfer switch – Step #7

Step #8

Remove the breaker for the first critical circuit from the main service panel box, and disconnect the hot wire lead from the lug on the breaker.

Installing transfer switch - Step #8
Installing transfer switch – Step #8

Step #9

Locate the red wire for the switch box circuit that corresponds to the circuit you’ve disconnected. Attach the red wire to the breaker you’ve just removed, and then reinstall the breaker.

Installing transfer switch - Step #9
Installing transfer switch – Step #9

Step #10

Locate the black wire from the same transfer switch circuit, and twist it together with the old feed wire, using a yellow wire connector. Tuck the wires neatly out of the way at the edges of the box.

Proceed to the next circuit, and repeat the process.

Installing transfer switch - Step #10
Installing transfer switch – Step #10

Step #11

If any of your critical circuits are 240-volt circuits, attach the red leads from the two transfer switch circuits to the double-pole breaker. The two circuits originating in the transfer switch should be next to one another, and their switches should be connected with a handle tie.

If you have no 240-volt circuits you may remove the preattached handle tie and use the circuits individually.

Installing transfer switch - Step #11
Installing transfer switch – Step #11

Step #12

Once you have made all circuit connections, attach the white neutral wire from the transfer switch to an opening in the neutral bus bar of the main service panel.

Installing transfer switch - Step #12
Installing transfer switch – Step #12

Step #13

Attach the green ground wire from the transfer switch to an open port on the grounding bar in your main service panel. This should complete the installation of the transfer switch.

Replace the cover on the service panel box, and make sure to fill in the circuit map on your switch box.

Installing transfer switch - Step #13
Installing transfer switch – Step #13

Step #14

Begin testing the transfer switch by making sure all of the be OFF at the main panel breakers.

Installing transfer switch - Step #14
Installing transfer switch – Step #14

Make sure your standby generator is operating properly and has been installed professionally.

Standby Generator
Standby Generator

Step #15

Before turning your generator on, attach the power cord from the generator to the switch box. Never attach or detach a generator cord with the generator running. Turn your standby power generator on, and let it run for a minute or two.

Installing transfer switch - Step #15
Installing transfer switch – Step #15

Step #16

Flip each circuit switch on the transfer switch box to GEN one at a time. Try to maintain balance by moving back and forth from circuits on the left and right side. Do not turn all circuits on at the same time. Observe the onboard wattage meters as you engage each circuit, and try to keep the wattage levels in balance.

When you have completed testing the switch, turn the switches back to LINE, and then shut off your generator.

Installing transfer switch - Step #16
Installing transfer switch – Step #16

Few Words on Typical Backup System

Backup generators supply power to a manual transfer switch, which disconnects the house from the main service wires and routes power from the generator through selected household circuits.

Installing a backup generator is an invaluable way to prepare your family for emergencies. The simplest backup power system is a portable gas-powered generator and an extension cord or two.

A big benefit of this approach is that you can run a refrigerator and a few worklights during a power outage with a tool that can also be transported to remote job sites or on camping trips when it’s not doing emergency backup duty. This is also the least expensive way to provide some backup power for your home.

A Typical Backup System
A Typical Backup System

You can purchase a generator at most home centers and be up and running in a matter of hours. If you take this approach, it is critically important that you make certain any loads being run by your generator are disconnected from the utility power source.

The next step up is to incorporate a manual transfer switch for your portable generator. Transfer switches are permanently hardwired to your service panel. They are mounted on either the interior or the exterior of your house between the generator and the service panel.

You provide a power feed from the generator into the switch. The switch is wired to selected essential circuits in your house, allowing you to power lights, furnace blowers, and other loads that can’t easily be run with an extension cord. But perhaps the most important job a transfer switch performs is to disconnect the utility power.

If the inactive utility power line is attached to the service panel, “backfeed” of power from your generator to the utility line can occur when the generator kicks in. This condition could be fatal to line workers who are trying to restore power. The potential for backfeed is the main reason many municipalities insist that only a licensed electrician hook up a transfer switch.

Using a transfer switch not installed by a professional may also void the warranty of the switch and the generator.

Automatic transfer switches turn on the generator and switch off the utility supply when they detect a significant drop in line voltage. They may be installed with portable generators, provided the generator is equipped with an electric starter.

Large standby generators that resemble central air conditioners are the top of the line in backup power supply systems.

Often fueled by home natural gas lines or propane tanks that offer a bottomless fuel source, standby generators are made in sizes with as much as 20 to 40 kilowatts of output – enough to supply all of the power needs of a 5,000-sq.-ft. home.

Choosing a Generator

Choosing a generator for your home’s needs requires a few calculations. The chart below gives an estimate of the size of generator typically recommended for a house of a certain size. You can get a more accurate number by adding up the power consumption (the watts) of all the circuits or devices to be powered by a generator.

It’s also important to keep in mind that, for most electrical appliances, the amount of power required at the moment you flip the ON switch is greater than the number of watts required to keep the device running.

For instance, though an air conditioner may run on 15,000 watts of power, it will require a surge of 30,000 watts at startup (the power range required to operate an appliance is usually listed somewhere on the device itself). These two numbers are called run watts and surge watts.

Generators are typically sold according to run watts (a 5,000-watt generator can sustain 5,000 watts).

They are also rated for a certain number of surge watts (a 5,000-watt generator may be able to produce a surge of 10,000 watts). If the surge watts aren’t listed, ask, or check the manual. Some generators can’t develop many more surge watts than run watts; others can produce twice as much surge as run wattage.

Size of house (in square feet)Recommended generator size (in kilowatts)
Up to 27005 -11
2071 – 370014 – 16
3701 – 470020
4701 – 700042 – 47

It’s not necessary to buy a generator large enough to match the surge potential of all your circuits (you won’t be turning everything on simultaneously), but surge watts should factor in your purchasing decision.

If you will be operating the generator at or near capacity, it is also a wise practice to stagger startups for appliances.

Reference // The complete guide to wiring by Black+Decker

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

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