Auxiliary electrical generator ready for (unsafe) use © D Friedman at Backup Electrical Generators
Backup or standby electrical generator types, sources, transfer switches, wiring guides, + car power inverters, DC to AC Converters

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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.

Also discussed here: How to Connect or Hook up a backup electrical generator. Isolation Switches, Transfer Switches, Double-Throw Switches to Prevent Backfeeds, Fires, Electrical Shock. Double-throw switches used as transfer switches for electrical generators. Electrical Grounding Requirements for Back-up Electrical Generators.

High Capacity Commercial & Automatic Backup Electrical Generators & Automatic Transfer Switches. Use a car or truck as an emergency electrical generator for charging cell phones & operating tools. Sources of 12-volt to 120-volt DC to AC power converters & inverters. Sources of Emergency & Temporary-Use Electrical Generators & Typical Backup Generator System Costs.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Guide to Installing & Using Emergency & Temporary Electrician Power Generators, Switches, & AC-DC Power Converters

Backup electrical generator hookup © D Friedman at Article Series Contents

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.[12] 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.

How to Connect or Hook up a backup electrical generator

Auxiliary electrical generator ready for (unsafe) use © D Friedman at 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.

Electric Generator Isolation Switches, Transfer Switches, Double-Throw Switches to Prevent Backfeeds, Fires, Electrical Shock

Isolation switch for backup electrical generator © D Friedman at Auxiliary or Backup Electrical Generators & Isolation Switch Requirements

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.

Use of Double-throw switches used as transfer switches for electrical generators

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).

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 or "transfer switch" to avoid simultaneous (and unsafe) electrical power feeds to an electrical panel or sub panel - use backup electrical generator hookups as an example

Backup generastor, 12,000 - 12,500 Watt range, PowerMate brand (C) 2013 Daniel FriedmanBy "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: on installing pre-wired automatic transfer switches for home backup generator system use

Generac transfer swtich for backup generators - at Generac is at NHFireBear said:

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 [ 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.

Electrical Grounding Requirements for Back-up Electrical Generators

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.

Clearance Distances for Backup Generators

On 2018-02-17 by Jim asked: Is there a code for clearance of an automatic whole house generator ...

Is there a code for clearance of an automatic whole house generator from an a/c condenser? Maybe at least the exhaust end?

This Q&A were posted originally at CLEARANCE DISTANCE, HVAC and are currently also found at CLEARANCE DISTANCE, HVAC FAQs.

On 2018-02-18 by (mod) - code for clearance distance of a backup generator from an a/c condenser?

Fair question, Jim, not one I find in HVAC installation guides since the electrical generator is a pretty-specific device. Codes and guidelines can't anticipate everything that might be installed around a building, so usually more general constraints are given.

Clearance distances for electrical generators used at private homes are specified for the following reasons:

Proper air flow around the generator for proper generator operation

Fire safety - clearance from combustibles

Carbon monoxide fatal poisoning safety - clearances from buidling openings like windows and doors

Typical residential electrical generator minimum clearance distances


The installation of a backup electric generator must comply strictly with NFPA 37, NFPA 54, NFPA 58 and NFPA 70 standards. We give excerpts from NFPA 37 below.

NFPA 37, Section 4.1.4, Engines Located Outdoors. Clearance Distances

Engines, and their weatherproof housings if provided, that are installed outdoors shall be located at least 5 ft. from openings in walls and at least 5 ft. from structures having combustible walls. A minimum separation shall not be required where the following conditions exist:

1. The adjacent wall of the structure has a fire resistance rating of at least 1 hour.

2. The weatherproof enclosure is constructed of noncombustible materials and it has been demonstrated that a fire within the enclosure will not ignite combustible materials outside the enclosure.

You may find closer distances allowed by the installation manual for specific generators including Generac who in some documents supports clearance to combustible walls of as little as 18" - again no mention of A/C compressors.

The following quote is from a Generac brochure and is pretty standard, notwithstanding smaller distances I've just cited.

Allow at least five feet of clearance on all sides of the generator when operating. - source: Generac, "Portable Generators" [brochure, PDF], retrieved 2018/02/17, original source: Generac Power Systems, Inc.S45 W29290 Hwy. 59, Waukesha, WI 531891-888-GENERAC (1-888-436-3722)0180710SBY Rev. B 08/2014

Gas Supply for LPG or Natural Gas Backup Generators

This topic is now found at GAS SUPPLY for ELECTRIC GENERATORS

Examples of High Capacity Commercial & Automatic Backup Electrical Generators & Automatic Transfer Switches

Commercial backup electrical power generator © D Friedman at

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



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 system will involve the provision for fuel storage, transfer switches, and the establishment & connection of the system to critical electrical circuits in the building.

Use a car, RV, truck or its battery as an emergency electrical generator for charging cell phones & operating tools

This discussion is now is a separate article found at CAR or TRUCK 120V GENERATORS

Sources of 12-volt to 120-volt DC to AC power converters & inverters (power transverters)

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

Steps to Turn a Backup Generator On or Off


Del said:

I just bought a house with generator and transfer switch. I am wondering if someone can outline steps to switching from grid to generator and back.

Specifically I am looking for order of steps. (Ex. Start generator, connect generator, flip transfer switch) to ensure no surges and safety to the power grid. I am curious if flipping the switch while grid power is on can cause damage. - 2018/02/01


Watch out: The installation and operation manual for your portable or permanently-installed backup generator will have explicit instructions that you should follow, since the manufacturer may have steps in mind that are not in my general answer below.

Be sure to take a look as there are life-safety concerns involved in any electrical system operation.

In general that if you have a correctly installed transfer switch, it's an either-or switch.

That is your electric panel is connected either to the incoming electrical service from the utility company OR a portion of its circuits are connected to the generator supply and the rest of the panel is considered offline.

Summarizing: the purpose of the transfer switch is twofold, one is to switch the power source for critical circuits out of the main electric panel service entry source and on to the generator as the electricity source for just those critical circuits.

The second purpose that transfer switches serve is preventing you from back feeding power from your generator into the electric panel and out through the entry mains where you could shock or electrocute a line worker who's working outside to restore power.

General Procedure to Turn ON a Backup Electrical Generator using a Manually-Operated Transfer Switch

  1. At the main electrical panel: Optional: turn off the main switch in the main electrical panel. This manually isolates your whole home from the electrical grid, doing a favor for the utility company by reducing the power surge that the grid experiences when everyone's power is turned back on at once.
  2. Connect the generator: For a generator that is not permanently connected to the transfer switch, connect the generator to the transfer switch power-in receptacle. Usually this is a special electrical receptacle on or close to the transfer switch itself. The generator's output power cord is connected to that receptacle.
  3. At the transfer switch box: Turn off the circuit breakers in the transfer switch box
  4. Start the generator. If your portable generator was in storage you'll usually need to move it outdoors to a sheltered location close-enough to connect its power cord to the transfer box before you start the engine.
  5. Run the generator to let it get up to speed and to normal operating temperature. Inspect the generator to make sure it's operating safely, for example that it is not sending exhaust fumes into the building (risking fatal carbon monoxide poisoning), then
  6. Flip the transfer switch to the Generator position
  7. Turn on the sub breakers in the transfer box. Doing this 1 by 1 minimizes load surge on the generator

General Procedure to Turn the Backup Emergency Electrical Generator OFF

  1. At the transfer switch box: turn the transfer switch back to Utility Power (or an equivalent label that means you are disconnecting from the generator and connecting those circuits back to the utility company's system)
  2. At the main electrical panel: turn on the main circuit breaker if you previously turned it off. Turn on any other circuit breakers that you'd turned off.
  3. At the generator: turn off the backup generator motor
  4. Disconnect the backup generator power cord from the transfer box
  5. Store it: Return the backup generator to its storage location
  6. Re-Fuel: If your generator uses a re-fillable fuel canister, you probably want to assure that the fuel tank or canister (e.g. LP gas) has been re-filled promptly to be ready for the next use.

Procedure for Using a Backup Electrical Generator with an Automatic Transfer Switch

Automatic transfer switches, also referred to as "universal" transfer switches, will automatically switch the pre-selected electrical circuits over to the backup generator in the event of a loss of electrical power from the utility company.

If you have such a system installed you should not have to take any steps to provide backup electrical power when the utility's power system is down.

However all backup electrical generator systems should be tested periodically to assure that the system is in good operating condtion and available for use when needed.

Backup Electrical Generator Installation & Operation Manuals

Watch out: all of these devices and their manuals contain instructions that, if not strictly complied with, will result in serious personal injury, including death, in addition to property damage.

Manufacturers of Emergency & Temporary-Use Electrical Generators & Typical Backup Generator System Costs

Backup generator, commercial grade (C) Daniel FriedmanThis 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: [10].

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

How Much Backup Generator Capacity do We Need?

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.


Continue reading at ELECTRIC GENERATOR CAPACITY REQUIREMENTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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