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Guide to backup electrical generators & other emergency electrical power sources for residential & light commercial use: this article discusses the use of emergency generators for electrical power backup at residential properties. We explain the need for an isolation switch to avoid dangerous electrical shock or fire hazards & we include other safety warnings where generators are used. We describe 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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Guide to Installing & Using Emergency & Temporary Electrician Power Generators, Switches, & AC-DC Power Converters
Auxiliary or Backup Electrical Generators & Isolation Switch Requirements
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 left we illustrate an electrical transfer switch required for safe connection of a backup generator to a building's electrical system.
Question: Can I Hook Up Two Power Sources to One Electrical Panel?
Is it possible to connect two sources of Electricity to one Sub-Panel? - Muhammad K., Jordan
Reply: Use an isolation switch to avoid simultaneous (and unsafe) electrical power feeds to an electrical panel or sub panel - use backup electrical generator hookups as an example
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 subpanel because of the possibility of backfeeding 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.
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:
Electric Generator Isolation Switches, Transfer Switches, Double-Throw Switches to Prevent Backfeeds, Fires, Electrical Shock
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,
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.
Examples of High Capacity Commercial & Automatic Backup Electrical Generators & Automatic Transfer Switches
True Installation Cost for Backup Electrical Generators Must Include Fuel Storage, Transfer Switches, Electrical Circuits, & Flood or Disaster Protection
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 sytsem will involve the provision for fuel storage, transfer switches, and the establishment & connection of the system to critical electrical circuits in the building.
How to use a car, RV, truck or its battery as an emergency electrical generator for charging cell phones & operating tools
The methods we describe below permit use of a vehicle or just a vehicle 12-volt battery as an emergency power source sufficient to provide emergency cell phone recharging, computer use, lighting, even light power tool operation.
Reasonable uses & wattage requirements for car, boat, truck, RV and similar 12-VDC to 120V AC charging devices and transverters include
How to use ordinary cars, trucks, vans, etc. as light-use backup emergency electrical power sources - recharging a cellphone or operating a tablet or laptop computer
There are several approaches to using the 12-V electrical system in a vehicle for light-duty electrical use, such as re-charging a cellphone, flashlight, or computer, ranging in difficulty from trivial (just plug it in) to suitable for a handy person who has a knife and some small gauge electrical wires at hand.
Connect a 12-volt USB adapter to the vehicle's utility outlet or cigarette lighter socket
Standard cell phone chargers such as the unit provided by Samsung for our example here are intended to connect to a 120-V electrical receptacle or a 240V AC receptacle (most adapters handle both voltages) and to provide an output voltage of 5-Volts DC.
Because the current requirements to re-charge a cellphone is very small, you can do a lot of cellphone re-charging from a car battery without much worry about running down the battery even if the vehicle engine is not running.
At left we illustrate the easiest way to re-charge a cellphone for emergency use when there is no electrical power in nearby buildings. We used a Griffin 12-V car adapter to plug into the 12-V outlet in this Jeep Sahara, then connected our Samsung cell phone's USB adapter cable between the car adapter and the cell phone's charging connector at the phone base.
On Jeeps and some other vehicles there may be two 21-V outlets, one switched by the vehicle's ignition and a second (shown here) that is "always on" provided that the vehicle has a functional battery. The advantage of using the "always-on" circuit is that you don't have to turn on the vehicle's ignition (which in turn operates lights and other systems, reducing vehicle battery life if the engine cannot be run).
If the vehicle does not already have a built-in USB connection, if you've planned ahead you will have already purchased a 12-V adapter that plugs into the cigarette lighter or 12-V utility outlet on your car, truck, boat or other vehicle. Connect the USB-power cord for your cellphone to the adapter or to the USB connector in the vehicle. Depending on individual vehicle's wiring, you may need to also turn on the vehicle's ignition switch. That source provides low-voltage electrical power sufficient to recharge a cellphone and some other appliances that connect via a USB cable such as a tablet computer or iPad.
DC to AC Power Adapter Volts, Amps, Watts Data for Common Small Electrical Devices
How to Connect a 12-volt to 120-volt DC to AC power converter / inverter (a power transverter) to the vehicle's utility outlet
A 12-V power inverter (also called a power transverter, power adapter, or DC to AC power converter) is a small device designed to be connected to a vehicle's 12-V utility outlet (cigarette lighter outlet). The power inverter or "transverter" converts 12-V DC current to 120-V AC current and includes one or more 120-V electrical outlets to which you can connect a 120-V device. However unless you've purchased and installed a high-capacity transverter, most likely the unit can not drive higher-wattage electrical devices, tools, or other appliances that draw high wattage (or high current - amperage).
We have successfully used a small 12-V power transverter/adapter to drive and re-charge a laptop computer and to operate the recharging station for battery-operated electrical drills and saws - very useful at a jobsite or in a disaster area where conventional electrical power sources are not available.
Typically a 150-watt rated DC to AC power converter/inverter connects directly to the 12-volt utility outlet or cigarette lighter socket in a vehicle, while more powerful models such as a 325-watt rated DC to AC power converter/inverter connects to the vehicle battery directly using its own cables and battery terminal clips. Still larger DC to AC power converter/inverters connect to a vehicle battery by direct wiring or by using automobile jumper cables.
Watch out: if you over-tax a power converter it will probably shut off, either using an automatic internal overload safety switch or by blowing a fuse. And while power convert/transverter prices vary over a large range, so do their capacities and features, including not only the actual wattage delivered, but the consistency of their power output. A converter that overheats and shuts off or that drops its voltage level too low after 1/2 hour of laptop computer use may make you wish you'd purchased a different model. Wagan provides this interesting technical detail about the sustained power output of typical DC to AC power Inverters.
How to use (& limits of) vehicles with built-in 120-V electrical power outlets as backup electrical generators
In an emergency you can use the 12-volt electrical system, battery, or generator/alternator found in just about any car, truck, boat or other vehicle as a source of low-capacity electrical power suitable for charging a cellphone or even operating light 120 V electrical tools such as a drill, saw, or lighting.
Some vehicles including most recreational vehicles (RVs) and motor homes, and some cars or light trucks (such as our Toyota FJ Cruiser) already include one or more 120-V electrical power outlets. In this case plugging in a small (low amperage draw) tool, light, even appliance is simple. Turn on the 120-V outlet using a rocker switch on the car's dashboard, and plug in the appliance. Some vehicles such as the FJ Cruiser provide two different power levels depending on whether or not the vehicle's engine is operating.
Obviously, running a car or truck motor continuously just to operate its electrical system is not practical for more than a brief use, and in some circumstances operating the vehicle engine may be impossible or even dangerous.
Watch out: do not operate a vehicle in dangerous conditions nor in an enclosed space - doing so risks death by carbon monoxide poisoning just as we warn above when we discuss where to locate a gas or diesel fuel operated backup electrical generator.
Watch out: using the vehicle's built-in 120-V power outlet for protracted intervals will run down the vehicle battery making it impossible to re-start the engine.
How to Salvage a 12-Volt battery from a Car or other Vehicle to use as a temporary power source
A third method can be used to provide 12-V electrical power to operate a USB adapter plug or a 12-V DC to 120-V AC electrical power transverter to make use of the battery from a car or truck even if the vehicle itself is not operable. For example if a vehicle has been flooded so that its engine cannot be operated, you may be able to salvage the vehicle battery.
Remove the 12-V battery from the car, truck, van, boat etc.
Connect the input positive and negative terminals (using any makeshift means) from the USB adapter or the 12V to 120V transverter power cord to the corresponding positive and negative terminals on the battery.
Watch out: connect negative to negative (black to black or - to - ) and positive to positive (red to red or + to +) when hooking up an adapter directly to a car battery. If you reverse polarity you will damage the devices or adapter you are attaching, probably blowing a fuse or burning up the device.
Can You Use a Vehicle 12-Volt System for Emergency Survival Heat?
While a vehicle parked safely outdoors in open air and that has an operable engine can be run for a while (fuel runs out) to heat the engine and thus the vehicle heater, some readers have asked about using the vehicle's electric circuit as an alternative heating source, perhaps thinking that they can run the vehicle as a stationary generator and operate an electric heater in a nearby shelter. This approach is not likely to work and is likely to be dangerous.
Watch out: do not try to operate a conventional 120V electric heater (nor other high-current or high wattage devices) from a typical car cigarette lighter DC to AC adapter: The high wattage used by the electric heater and high current draw (amps) will most likely blow any fuses in the vehicle's wiring system if it does not melt the wiring itself, and there is a real risk that you will overheat the vehicle wiring inviting a short circuit or fire. The maximum electricity usage from a typical car cigarette lighter circuit is about 240 Watts at 12-volts DC, protected usually by a 20Amp 12-V DC fuse. Note that that's 20 Amps at 12-volts, not 20-Amps at 120-volts. Very roughly, because there are other factors, a 12-V 20A electrical circuit in a vehicle has just about one tenth of the wattage capacity of a 120-V 10A circuit in a building.
There are 12-volt operated vehicle seating pads intended to give limited warmth to the driver or occupant (just as some vehicles have electric seats) though that approach means people have to sit in the vehicle and of course run the engine.
Watch out: as we warned earlier, do not operate a car, RV nor other vehicle in dangerous conditions (vehicle was flooded, wiring is unsafe, etc), nor in an enclosed space - doing so risks death by carbon monoxide poisoning just as we warn above when we discuss where to locate a gas or diesel fuel operated backup electrical generator.
Where to Buy / Manufacturers of Emergency & Temporary-Use Electrical Generators & Typical Backup Generator System Costs
A properly and safely wired & installed backup or standby electrical generator with sufficient wattage output and fuel storage to power a typical residential home for several days in an emergency costs "well over $10,000"  though with some care for safety one can power a few critical electrical circuits in a home (heat, refrigerator, a lighting circuit) using a less costly contractor-grade portable electrical generator as well.
The commecial-grade backup generator by Katolight Corporation shown at left is installed at an adult care facility in New York.
Factors driving up the cost of an installed backup electrical generator for the home include
According to a recent article in the New York Times, Generac, the largest manufacturer of electrical generators in the U.S. was working overtime to fill demand for backup electrical generators in the Northeastern U.S. following Hurricane Sandy in 2012. The same article noted that in 2012, about 3 percent of "stand-alone" homes in the U.S. have a standby generator installed.  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
While these backup generator companies can be contacted directly, consumers usually shop at local suppliers or shop online from suppliers
How Much Backup Generator Capacity do We Need?
The minimum wattage capacity your standby or backup electrical generator needs to provide depends on your building, its occupants, their special needs, etc. Here we list the more critical electrical circuits and devices you should consider powering. Use this list, with additions or deletions to arrive at the total minimum wattage that your backup or standby generator system will need to provide.
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Frequently Asked Questions (FAQs) about home electrical generators, generator hookups, transfer switches & electrical wiring
Question: Our electrical panel does not have room to connect to the backup electrical generator system
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
Reply: Electric panel expansion options for backup generator hookups
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
Follow-up: shopping for circuit breakers?
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.
If money's tight it may be worth comparing prices between the electrical supplier and Home Depot. But watch out for third party "lookalike" breakers - the panel manufacturer may object to their use, and indeed some substitute breakers made in China have performed poorly.
Question: service entry cable to hook up a generator
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.
Question: Reader comments on using the Reliance TCA 1006 single load transfer switch to install a generator at an overcrowded electrical panel
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.
Contact information for Reliance Controls who sells this product can be found in our reference listing just below. 
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