Guide to backup electrical generators & other emergency electrical power sources for residential & light commercial use: this article series discusses the use of emergency generators for electrical power backup at residential properties. We describe how to determine how much back up generator capacity or power in watts you'll require.
We explain how to hook up or wire an electrical generator at a building, and we describe the transfer switch or isolation switch necessary to avoid dangerous electrical shock or fire hazards. We include other safety warnings where generators are used. We describe electrical grounding and other wiring & control or installation procedures backup electrical generators.
We describe several methods for obtaining temporary or emergency electrical power from a car, RV or other 12-V or 24-V vehicle or system.
We include a list of manufacturers and sources for backup generators for residential & commercial use and a list of manufacturers of DC - AC power converters, inverters, or transverters that permit using a car or other vehicle to power small electrical devices & tools..
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Our page top photo illustrates a Briggs & Stratton portable generator capable of delivering 5000 watts with both 120V and 240V electrical output available; This unit is able to run for 10 hours continuously at 50% load. Photo courtesy of Davies Hardware, Poughkeepsie, NY. Similar models found at building suppliers range in capacity with common sizes in the 10,000 - 12,500 watt range for residential use.
At above left we illustrate an electrical transfer switch required for safe connection of a backup generator to a building's electrical system.
A common exception to the practice of using only a single power source at an electrical sub panel is the provision for backup electrical generator power that feeds an electrical sub panel (or panel) that contains critical electrical circuits for a building (such as heat, refrigeration, communications equipment, medical equipment).
In that case indeed there are two electrical power sources that in a sense "feed" an individual panel or sub panel but only one power source can be connected at a time. Typical instructions for a backup home electrical generator system include variations on the following safety warning:
Watch out: Emergency backup electrical generators produce 120V and 240V which may cause fatal electrical shock if precautions are not followed.
DO NOT under any circumstances connect your electrical generator to any circuit or receptacle that is receiving electrical power (home, office, etc) from any other sources as this is likely to result in a fire and damage all electrical systems and could also shock someone working on the system.
Our auxiliary electric generator photo at left shows a backup "home" generator that the owner had set up to keep a basement sump pump working. Some owners connect an extension cord to the generator's electrical supply receptacle and connect the other cord end to a tool or appliance - which may be safe. But using that same extension cord to "back-feed" a home electrical circuit without an isolation switch is unsafe and should not be done.
Other key safety warnings include the warning that the backup generator should not be operated indoors nor in an enclosed area - there are fire and also potentially fatal carbon monoxide hazard risks. Note that other safety precautions also apply - be sure to see the instructions provided with the generator.
Key in making that a safe installation is the use of an isolation switch that switches to provide an electrical power feed into the panel (or sub panel) from EITHER the electrical service entry from the utility company OR from the backup electrical generator.
An isolation switch, also referred to in many sources as a "transfer switch" is a double-throw switch that changes electrical connections between two alternative sources.
This is an "exclusive OR" switch that flips the panel power feed between the two sources but never allows both to be "on" or "connected" to the sub panel simultaneously. The design prevents simultaneous electrical power feed from two sources.
Our photo (left) shows a close-up view of the same isolation switch in our earlier picture. [Click any of our images to see an enlarged, detailed version]. You'll see that each individual electrical circuit is switched between "Generator", "Off", and "Line" voltage.
An isolation switch allows only one electrical power source to be connected to feed a sub panel at a given time. This avoids, for example, the hazard of shocking electrical workers who might be repairing the main electrical service.
As an example of an isolation switch set-up for an auxiliary electrical power generator hookup at a private home, our photo (above left) shows the backup generator hookup that provides limited electrical power for critical building circuits during a power outage from the electrical utility.
As pointed out by various sources including Janet Lewis, Chief Electrical Inspector for Washington State,
Listed double-throw "Enclosed Switches" that have been investigated for switching a common load from a normal supply to an optional standby system are marked "Suitable For Use In Accordance With Article 702 of the National Electrical Code" and acceptable for load transfer use on optional standby systems only. All other approved transfer switches are listed as "Transfer Switches" and marked as suitable for the intended use (such as Service Equipment, Emergency Systems, Optional Standby Systems, Automatic or Non-Automatic Transfer).
Is it possible to connect two sources of Electricity to one Sub-Panel? - Muhammad K., Jordan
By "possible to connect two sources of Electricity to one Sub-Panel" I presume you don't mean is it physically possible to connect two power sources to a single electrical panel since that would be trivial to accomplish; Rather I presume you mean is it acceptable practice or is it "safe" to connect multiple power sources into a single electrical panel or sub-panel.
The basic answer is no. In general it is very dangerous to have multiple sources of power into a single panel or sub panel because of the possibility of back feeding and shocking someone who thinks power is off from a given source. So we don't hookup simultaneous live electrical power sources to a single electrical sub panel or main panel.
We do not hook up multiple electrical power sources to a single electrical sub panel without using an isolation switch.
In our photograph of an isolation switch hook-up (above left), the main electrical panel (1) is at left. In the photo center is an isolation switch (2) that allows the homeowner to switch individual circuits from being powered either by the main electrical panel's service entry mains or by an electrical generator (located outdoors) that is connected to the isolation switch by a removable plug shown hanging on the wall (3). You can see the receiving plug receptacle at the bottom center of the isolation switch (2).
The sub panel shown at right (next to our client) was an addition to the original electrical system to support central air conditioning and is not part of this discussion.
Comment for those adding transfer switches: When I installed Generac Guardian automatic transfer switches with panels at four different locations, and inspected numerous others, they each came with a pre-wired panel of 8 or more breakers. Each branch has a color-coded wire (12AWG) threaded through a short, liquid-tight conduit. The idea is that you open a knock-out hole in the existing panel and attach the conduit and wires.
You then select which existing branch circuits you want to have powered via the transfer panel (heating/cooling, pumps, lights, etc).
All you need to do is remove the wire from the existing breaker and attach it (using a wire nut) to the wire leading to the corresponding breaker in the transfer panel. There are also three larger conductors that you run to the neutral and the two "hots" (on a 240v, single-phase feed) via a suitable two-pole breaker (e.g., 65 Amps for a 16kw generator). This breaker "feeds" the normal line voltage to the circuits in the transfer panel.
You then remove the abandoned breakers and insert a blank cover in the panel cover for each breaker slot, or rearrange the remaining breakers so that the open spaces are adjacent to each other and put the blank plates over those slots. Don't forget to re-label the branch legend on both of the panel covers, as required by code.
Some people also run another feeder (e.g., 30 amps) from the transfer panel to one or more additional subpanels with the "backup" circuits individually protected and switched there. - NH Fire Bear 2016/08/07 is a regular contributor of technical information at InspectApedia.com [ Ed.]
Thanks NH FireBear. An added comment worth noting for readers is that transfer switches for backup generators should be installed by a trained, licensed electrician, or where DIY wiring is permitted, the installation needs to be done with a building permit and with the required inspections. The risks of a mistake include, beyond the obvious dangers of shock or fire for the installer or the building, risks of back-feeding live power into the local electrical grid, risking shocking an electric-company worker who's working on restoring power after a power loss.
Shown above is a Generac automatic transfer switch intended for use with backjup generators. Generac, a home backup generator provider, supplies transfer switches such as the Generator switch discussed here.
According to ASHI home inspector Bob O'Connor, ... "If there is a ground conductor connected to the grounding electrode from the emergency power source [the generator], there must be a sign identifying the emergency and the normal ground connections posted at that location. "
O'Connor also warns that the generator must be located where exhaust fumes cannot enter the building (a carbon monoxide hazard), and about unsafe storage of fuel containers for the backup generator.
The commercial backup electrical power generator shown at left includes its own fuel storage tank (at left in the photo).
In the event of a power failure the system may start automatically and the necessary isolation switch may also be switched automatically to keep critical equipment in operation.
More information about backup and alternative electrical power systems and how they are connected is at SOLAR ENERGY SYSTEMS
and WIND ENERGY SYSTEMS,
and at WIND TURBINES & LIGHTNING
Watch out: as we describe at Step 8. Rebuild and Floodproof, locating a backup generator onto upper building floors or the rooftop may not be enough to assure that the system can continue to operate during times of area flooding. The fuel supply and isolation or transfer switches must also be located where they are protected from flood waters.
A significant portion of the cost of installing a backup generator system will involve the provision for fuel storage, transfer switches, and the establishment & connection of the system to critical electrical circuits in the building.
This discussion is now is a separate article found at CAR or TRUCK 120V GENERATORS
This discussion is now is a separate article found at WHERE TO BUY of 12-volt to 120-volt DC to AC power converters & inverters
This article topic has moved to WHERE TO BUY ELECTRICAL GENERATORS
Here are all 41 brands of electrical generator brands & product sources listed by Electric Generators Direct whose list we found was longest: .
Asco Electrical Generators, Baldor Electric Generators Briggs & Stratton Electrical Generators, Camco Electrical Generators, Century Electrical Generators, Classic Electrical Generator Accessories, Conntek Electrical Generators, Cummins Onan Electrical Generators, DeWalt Electrical Generators, DynaGen Electrical Generators, Generac Electrical Generators, GenTran Electrical Generators, Gillette Electrical Generators, Honda Electrical Generators, Honeywell Electrical Generators, IMD LLC Electrical Generators, JohnDow Industries Electrical Generators, Katolight Corporation, Protected Home Electrical Generators, Kohler Electrical Generators, PowerBoss Electrical Generators, PowerMate Electrical Generators, Praitiac Electrical Generators, Reliance Electrical Generators, Scepter Electrical Generators, Seafoam Electrical Generators, Slime Electrical Generators, StaBil Electrical Generators, Subaru Electrical Generators, TechMate Electrical Generators, TruFuel Electrical Generators, U.S. Wire & Cable Electrical Generators, Voltmaster Electrical Generators, Wagan Electrical Generators, Winco Power Systems Electrical Generators, Yamaha Electrical Generators
In a separate article found at ELECTRIC GENERATOR CAPACITY REQUIREMENTS [live link just below] we give backup generator size requirements tables for both backup electrical generators for home or commercial use and for smaller car or truck AC adapters.
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I hope you can give me some practical advice. We have a Cutler-Hammer CH series panel. We are in the process of hooking up an auxiliary generator. Our electrician tells me that our panel is full and to get a panel with at lest 24 circuits. As the house was built in 1971 that is not surprising. So I went to Lowes and picked up a 30 by 30 panel.
Now I am told that this panel cannot use the existing breakers as they are CH and it take BR. What can you suggest? Can we get a CH Cutler Hammer panel that is larger then the 20 circuit one we have now? If so where and at what prices? Thank for your help. - P.C., North Hampton NH
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem or in this case to make recommendations for what you actually need. That said, here are some things to consider:
Eaton Cutler Hammer provides a range of larger and replacement electrical panel sizes (a common synonym for "electrical panel" as you are using it is "load center") though your local supplier may not stock them.
Eaton Corp., Cutler Hammer Products Adjustable Retrofit Kits, E-CH Load Centers and load center parts can be purchased through any E-CH distributor. Cutler Hammer Contact Points: Telephone: 800-330-6479 Email: FlexCenterLincoln@eaton.com. Or see FPE REPLACEMENT PANELS This article describes Cutler Hammer's replacement options for FPE and other problematic electrical panels and load centers.
But in my OPINION, if your panel is an older one, it would be safer not to re-use the old circuit breakers during a new panel replacement, even if they fit onto the new panel bus. A whole new larger electrical panel with new circuit breakers can often be obtained for economically from your local electrical or building suppliers. Most of the upgrade cost is in the installation labor.
Before you replace your existing panel to expand the existing unit you first have a clear and correct plan for how you're going to hook up your generator to the existing house circuits. If your electrician's concern with the fully populated existing panel is that s/he has no room to add a circuit breaker to back-feed the existing panel from the generator, that would be an unsafe and improper installation - you should not be simply back-feeding your electrical panel from the generator.
Safe hookup of a generator requires an isolation switch that isolates the subset of circuits and original panel from the incoming electrical service mains when you are running the generator - it's got to be an exclusive OR - setup.
Finally, while I prefer to completely replace an older, obsolete, under-sized existing electrical panel, and while I will often upgrade the entire electrical service (incoming mains, main disconnect) at the same time (to be better able to meet contemporary electricity usage and the expanded number of circuits found in modern or upgraded homes) there are other options.
For example the electrician might install an additional sub panel, remove a pair of circuits from the existing (full) electrical panel, feed the new sub panel by a 240V
Thank you very much. You have given me rationale that changing the whole panel would be safer, even if I were not configuring for a generator setup. We do have a interlock device for the system.
Do you suggest I shop for the breakers, or just get a list of what I need and get them at the electrical parts distributor where I purchased the panel? In other words do I go to Lowes or Home depot, or Rockingham Electrical Supplies?
Thanks for your complete and quick response.
If money's tight it may be worth comparing prices between the electrical supplier and Home Depot. But watch out for third party "look alike" breakers - the panel manufacturer may object to their use, and indeed some substitute breakers made in China have performed poorly.
Can I use service the electrical service entry cable (SEC) to hook up a generator ? - Anon
Anon, if you mean can you use SEC grade or weight wire, sure, though depending on your generator size (output capacity) and distance from the building it may be overkill in wire size.
If you mean can you connect the generator to the same lugs in the electric panel as the entering SEC wires, absolutely not. You need an isolation switch.
I considered installing the standard transfer panel when I got a backup generator, but my main panel was already way too crowded. I installed a Reliance TCA1006 single load transfer switch instead.
The transfer switch is fed by a 60amp dual breaker in the main and the transfer switch has a 60amp dual breaker for supplying the sub panel I installed with 8 circuits in it. The Transfer switch also has a 30amp Dual breaker for feeding the new sub when I am using the generator. The TCA1006 is designed so when one breaker is switched "on", the other breaker is switched "off".
This prevents the generator power from feeding the commercial power lines. The only draw back is that I had to physically remove the wiring for 8 circuits from the main panel and move them to the sub panel.
I used a couple of junction boxes to accomplish this since some of the wires were too short to reach the sub. Overall, it wasn't too bad of a job. I saved about $100.00 by using the single load transfer switch and my main isn't as crowded now. - Mr. Telco 07/27/2012
Thanks for the helpful comment Mr. Telco. Indeed a common solution to wiring a backup generator is to feed a sub panel through an individual transfer switch, then wiring the critical building circuits out of the sub panel.
Quoting from Reliance Controls' product literature adds a few details about this product:
"The Panel/Link TCA offers circuit breaker combinations of 60A - 125A utility and 30A - 125A generator. The interlinked circuit breakers keep the generator isolated from the utility at all times, even with the deadfront removed.
The TCA features a heavy-duty copper bus. The TCA is suitable for service entrance and may be used as a main panel for a single load or to feed a sub-panel. Available in indoor and outdoor enclosures, the TCA has no provision for branch circuits breakers."
Contact information for Reliance Controls who sells this product can be found in our reference listing just below. 
(Jan 6, 2015) Cory A. Hicks said:
I am installing a standby generator on a meter/main panel. The panel has bus bars that connect the main disconnect to the breaker bus. The bottom of the bus has lugs that feed the house sub panel without any breakers between the disconnect and the house sub. My question is can I remove the bus bars and route 4awgcu from the disconnect to the transfer switch and then the load from the switch to the breaker bus? I know I have to remove the ground/neutral bond in the panel as it will become a "sub" panel and the ATS will become the "Main"
Thank You for your reply,
Cory, I don't have a full understanding of your electrical system, wiring, panels etc. but since all wiring runs should be protected against over current I'd expect a sub panel to be fed through a circuit breaker. One does not normally install a standby generator to power all of the circuits in a building nor a main panel nor a house sub panel that is really running all of the house circuits. To do so would require a larger standby generator than we usually find at residential properties.
Rather, key circuits that must be kept powered such as medical equipment, heat and / or cooling, refrigeration, and a few key lighting and receptacle circuits are wired through a transfer switch that swaps between the electrical utility's incoming power fed into the main panel and the generator as a power source.
it sounds as if you have a different idea and one I don't understand.
As life and safety issues as well as permit and code issues are involved I'd expect one to use a licensed electrician who is familiar with aux generator and transfer switch wiring.
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