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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 and we include other safety warnings where generators are used.

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Auxiliary or Backup Electrical Generators & Isolation Switch Requirements

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.

Hooking up backup electrical generators

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 electrial 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 extenstion 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.

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.

Double-throw switches used as transfer switches

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).

Grounding 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 hazardf), and about unsafe storage of fuel containers for the backup generator.

Commercial & Automatic Backup Electrical Generators & Automatic Transfer Switches

Frequently Asked Questions (FAQs) about home electrical generastor hookups and 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 lpanel 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.[5] 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 conguring 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 distributer 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.

Reply:

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.

Questions & Answers regarding this article

Questions & Answers about backup electrical generators for home use and backup generators, isolation switches, electrical wiring installation, inspection, diagnosis, & repair.

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BACKUP ELECTRICAL GENERATORS

• [1] "What to look for when looking at back-up electrical generators", Bud O'Connor, The ASHI Reporter, April 1998 p. 11
• [2] National Electrical Code (U.S.A.), Article 702
• [3] "Double-throw switches used as transfer switches", Electrical Code issues and answers, Vol. 1. No. 1, January 1998, Janet Lewis, Chief Electrical Inspector, Washington State, web search 07/13/2011, original source: http://www.lni.wa.gov/tradeslicensing/electrical/files/currents/elc9801.pdf
• [4] "Basic Housing Inspection," US DHEW, S 352.75 U48, p.144, out of print, but is available in most state libraries.
• [5] 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.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Recommended books on electrical inspection, electrical wiring, electrical problem diagnosis, and electrical repair can be found in the Electrical Books section of the InspectAPedia Bookstore. (courtesy of Amazon.com)
• * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
• Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
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• Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
• "Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
• Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
• "Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
• "How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
• "Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
• "Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
• "How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
• "Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
• "Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
• "Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
• "Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
• Electrical System & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page
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