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Generator Transfer Switches: Do You Need One for a Portable?
Why plugging a generator into the house is backfeeding, how a transfer switch or interlock kit prevents it, and why an electrician and permit come first.
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Quick reference5 facts
| Never (CPSC) | Plug a generator into a wall outlet to feed house wiring (backfeeding) |
|---|---|
| The fix (Red Cross) | An electrician-installed transfer switch that meets the National Electrical Code and local codes |
| Two common setups | A manual transfer switch for chosen circuits, or a panel interlock with an inlet box |
| Paperwork | Many towns require a permit and inspection before first use |
| Cords only? | No switch is needed if appliances plug straight into the generator |
On this page (5 sections)
Yes, if a portable generator is to power circuits in your house panel, you need a transfer switch or a panel interlock, installed by a licensed electrician to the National Electrical Code and local rules. Appliances plugged into the generator with outdoor cords need no switch. Never feed the house through a wall outlet or dryer outlet.
Why backfeeding through an outlet is so dangerous
Backfeeding means pushing generator power into your home wiring by plugging the generator into an outlet. CPSC calls it extremely dangerous: it presents an electrocution risk to utility workers and to neighbors served by the same utility transformer, and it bypasses some of the home’s built-in circuit protection.
Electric utilities repeat the warning. Lake Country Power, a Minnesota cooperative, says backfeeding a portable generator can be fatal, because crews restoring power may unexpectedly meet high voltage on the lines. Poudre Valley REA, a Colorado cooperative, says backfeed is hazardous to the household where it is done and to neighboring properties.
The danger runs both ways. Generac warns that generator power can clash with incoming utility power when service is restored, which can cause fires or damage appliances and the generator itself.
No plug-into-the-wall shortcuts
How a transfer switch stops backfeed
A transfer switch makes sure the house is connected to utility power or generator power, but never both at the same time, as Generac describes it. That separation means generator power cannot flow out to the street and utility power cannot slam into the generator.
The American Red Cross says that if you want to connect a generator to house wiring, have an electrician install a power transfer switch that meets the National Electrical Code and local codes. Poudre Valley REA says the only safe way to install a generator is to disconnect it from the grid with an automatic or manual transfer switch. Our guide to portable generators for winter outages covers the cord rules for running appliances without one.
Manual transfer switch, interlock kit and inlet box compared
Most portable generator hookups use one of two setups. Both connect to the generator through an inlet box: an outdoor socket mounted on the house wall that the generator cord plugs into.
| Setup | How it works | Good to know |
|---|---|---|
| Manual transfer switch | A small subpanel next to your main panel, wired to the circuits you choose, such as the furnace, refrigerator, well pump and some lights | Only the circuits wired to it run on the generator |
| Panel interlock kit | A plate on your main panel that stops the main breaker and a generator breaker from being on at the same time | Has to be made for your panel; the electrician confirms the fit |
| Power inlet box | The outdoor socket the generator cord plugs into | Part of either setup; it is only a socket, so it does not prevent backfeed on its own |
Plug types matter. Reliance Controls’ PB31 inlet is rated 30 amps at 125 volts and takes a cord with an L5-30 connector, while its PB30 inlet is rated 30 amps at 120/240 volts and takes an L14-30 connector, the plug many larger generators use. Match the inlet, the cord and the generator’s outlet before you buy anything.
Manual transfer switches, interlock kits and inlet boxes
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Permits, inspections and your utility
Rules come from your town and your utility, and they differ. The Village of Ardsley in New York says a licensed electrician must file an electrical permit for a portable generator connection plug and manual transfer switch, and the work must be inspected and approved before the generator is used. The Borough of Ringwood in New Jersey asks for a one-line wiring diagram and the maker’s specifications for the transfer switch or interlock device with the permit.
The City of Naperville, which runs its own electric utility, says the transfer switch prevents current from flowing onto its lines during an outage, which could electrocute workers restoring power. Ask your utility whether it wants notice or any extra equipment before you hook up.
Before you call an electrician
The generator still runs outside
A transfer switch changes how power gets into the house, not where the generator sits. CPSC says to use a portable generator outside only, at least 20 feet from doors, windows and vents that could let carbon monoxide indoors. Plan the inlet box location so the cord reaches a safe spot.
Every home running a generator needs working CO alarms. Our carbon monoxide safety guide shows where to put them, and the winter power outage checklist covers food, light and water. If you are weighing a permanent unit instead, see home standby generator vs portable. If you plan to run a heater on generator power, read space heaters in a power outage first.
More outage guides are in the power outages section.
Frequently asked questions
Can I plug my generator into a dryer outlet to power the house?
No. Generac names a dryer outlet as an example of an improper connection: feeding house wiring through any outlet is backfeeding. CPSC calls it extremely dangerous because it puts utility workers and neighbors on the same transformer at risk of electrocution. A transfer switch or interlock with an inlet box, installed by an electrician, is the safe way to connect.
Do I need a transfer switch if I only use extension cords?
No. If every appliance plugs straight into the generator or into heavy-duty outdoor cords that run to it, nothing connects to house wiring, so there is no backfeed path. The Red Cross advice to use an electrician-installed transfer switch applies when you want the generator to power the house wiring.
Can I install a transfer switch myself?
The Red Cross says to have an electrician install it, and some towns, such as Ardsley in New York, require a licensed electrician to file a permit and the work to pass inspection before the generator is used. Rules vary, so call your local building department first.
Is an interlock kit as safe as a transfer switch?
Both are designed so the utility main and the generator feed cannot be on at the same time. An interlock is a plate fitted to your main panel, so it has to be one made for that panel. Towns such as Ringwood, New Jersey, ask for the maker's specifications for either device with the permit.
Sources
- Safe Generator UseAmerican Red Cross
- Portable Generator Hazards (safety alert)U.S. Consumer Product Safety Commission
- Why you should never back-feed your electric panel with a generatorGenerac
- Beware of BackfeedPoudre Valley REA
- Portable GeneratorsLake Country Power
- Electrical Generator SafetyCity of Naperville
- Is a permit required to install a portable generator?Village of Ardsley, NY
- Requirements for Portable GeneratorBorough of Ringwood, NJ
- Power Inlet Box PB31 installation instructions (30 A, 125 VAC)Reliance Controls
- Power Inlet Box PB30 installation instructions (30 A, 120/240 VAC)Reliance Controls
Filed under Power and heatPart of Winter Power Outages: Generators, CO Alarms and Food Safety