What Is a Generator Transfer Switch? Manual, Automatic, Interlock and Inlet Box

A generator transfer switch connects chosen house circuits to either the utility or your generator, and never to both at once. That rule keeps your generator from pushing power back onto lines where utility crews work. On my farm I run cords from a generator to single loads myself, but I leave every house connection to a licensed electrician. Below I cover manual and automatic types, interlock kits, inlet boxes and how to match your generator’s outlet.

What Is a Generator Transfer Switch? The Short Answer

The device sits between your main electrical panel and your power sources. According to Schneider Electric, it disconnects the load from one source and connects it to another. In a house, the circuits it feeds run from the utility or from the generator, one at a time.

A licensed electrician installs the switch next to the panel and adds an outdoor inlet box, where the generator’s cord plugs in. The generator stays outside, and the house wiring does the rest. Placing the generator and running the cord safely is covered in my guide to connecting a generator safely, so here I stay on the device.

How a Generator Transfer Switch Works

The utility feed and the generator feed both land on it, and a lever or a relay decides which of them reaches your circuits.

Why One Source at a Time Matters

OSHA says never to attach a generator directly to the electrical system of a structure unless a qualified electrician has properly installed it with a transfer switch. Without one, the generator can energize wiring systems for great distances, and OSHA names electrocution as the risk to utility workers and others. The switch is the part that makes that impossible.

It also protects your own gear. When utility power returns while the generator still runs, a switch keeps the two sources apart, so the wiring and the generator are not hit by two supplies at once.

A wall outlet and a cord with two male plugs create the same backfeed with no switch at all. Generac calls backfeeding dangerous, often illegal, and something that should never be done. I never do it, and my guide on plugging into a wall outlet explains the danger in plain terms.

What the Switch Feeds

The Generac manual models I describe feed a short list of circuits the electrician wires to them. Other transfer equipment can feed a whole panel or subpanel, within the generator’s capacity. Generac’s manual models offer 8 to 10 circuits on the 30 amp versions and 16 circuits on the 50 amp versions, and Generac says the design lets you stagger loads so the generator is not overloaded. You do the staggering.

My list starts with the fridge, the freezer, the water pump and the workshop lights. Hardwired loads, such as a furnace blower, can sit on the list too, which a cord alone cannot do. My guide on generator power for a refrigerator gives the watts and the safe setup for that first item.

The generator’s rating still sets the limit. Here is a worked example with round numbers:

1. The generator’s running rating is 5,000 watts.
2. The circuits on the list add up to 3,800 running watts.
3. 5,000 minus 3,800 leaves 1,200 watts, and the largest motor still needs extra power for a moment at start-up. My page on starting watts and running watts explains that surge.
4. If the list added up to 6,200 watts, the extra circuits would have to come off the list.

Manual vs Automatic Transfer Switches

The two types differ in who does the switching. A manual switch waits for you. An automatic switch watches the utility and acts on its own.

A Manual Transfer Switch for Generator Use at Home

Electricians usually pair a portable unit with a manual switch. According to Schneider Electric, a manual switch stays connected to one source until someone operates it. Generac adds that its manual models need a power inlet box, do not sense the utility and cannot be used with an automatic home standby generator.

With a manual switch and a portable unit, you notice the outage, start the engine outdoors and then flip the lever. I am home when the power drops, so for my use that is fine. If you are away for days, a manual switch cannot help while you are gone.

An Automatic Transfer Switch for Generator Backup

An automatic transfer switch for generator backup works without anyone present. According to Schneider Electric, it monitors voltage and frequency on the utility, sends a start signal to the engine and transfers only after the generator’s power is acceptable. It switches back once the utility is stable, with delays built in so a short flicker does not trigger it.

This is the switch inside a standby system, which is a permanent installation under permit. I do not cover those here, because they are a job for licensed trades.

Interlock vs Transfer Switch for Generator Hookups

An interlock kit is the alternative that electricians most often offer for a portable generator. It is a metal bracket fixed to the main panel. It covers the main breaker, so the main cannot be switched on while the generator’s breaker is on.

The kit still needs an outdoor inlet box, and an electrician must install both. Because the bracket opens the whole panel to the generator, nothing but you stops a loaded circuit from being switched on too early. A manual circuit switch feeds only the circuits on its list, which keeps the load list short. With either one, you keep the running load within the generator’s rating and allow for motor start-up. To size the unit for your own list, see my page on the generator size you need.

Codes differ by area. Ask the electrician and the local inspector whether an interlock is accepted where you live and whether the kit must be a listed one. The National Electrical Code calls for transfer equipment that stops the utility and a generator from being connected together, and the inspector has the last word on how your area applies it.

Inlet Boxes and Matching the Generator’s Outlet

The inlet box is a purpose-built outdoor power inlet with male contacts, fixed to the wall. It is not a household outlet. One end of a heavy cord plugs into the generator and the other into the inlet. Because the cord ends at the wall, no window has to stay open for it.

Match the Inlet to the Outlet

The inlet, the generator’s outlet and the cord must agree on voltage and amps.

The cord can carry different connectors at its two ends, so the electrician checks both. Generac pairs its 30 amp models with an L14-30 inlet for generators that have an L14-30R outlet. Its 50 amp models use a matching inlet for generators with a 14-50R outlet. An adapter never adds capacity.

The table shows 120 volts times the amps on each leg, so usable power is the lowest rating among the generator, its outlet and breaker, the cord, the inlet and the switch. Read the generator’s outlet panel before you buy a switch. Many small inverter generators have 120 volt outlets only, and a 240 volt well pump cannot run from those, with or without a switch. Generac says only some portable models work with its manual switches.

Before I choose a switch, I put the generator’s outlet panel next to the pump’s nameplate and check that the voltage and amps agree. For the full answer to can a generator run a well pump, see my guide on matching the pump to the unit.

Cord and Placement

Use the cord the generator’s manual calls for, with the gauge and length from its chart, outdoor rated and undamaged. The generator then stands far from the house, as the next section explains.

Who Installs It and What to Ask

Generac says installation requires electrical knowledge and compliance with local codes. I leave out the wiring steps on purpose: they are for a licensed electrician, and a wrong step in the panel can kill.

Bring these questions to the electrician:

• Circuits: ask which circuits the switch will feed and whether your generator’s running rating covers them.
• Inlet: ask which inlet matches your generator’s outlet and where on the wall it will sit, away from windows.
• Rules: ask whether your area needs a permit or an inspection.
• Device: ask whether an interlock is accepted locally or only a transfer switch.
• Test: ask for the panel to be labeled and for a test run with the generator on.

Keep the Generator Outside, Whatever You Install

Wiring the house correctly solves the electrical danger. It does nothing about carbon monoxide. According to the CPSC, a portable generator runs outside only, at least 20 feet from the house, with the exhaust pointed away from the home. The CDC asks for the same 20 feet from any doors, windows and vents.

A US report found that portable generators accounted for 264 of 1,604 patients treated with hyperbaric oxygen for carbon monoxide poisoning in three years, and 99 incidents were in a residence (1). After four hurricanes in Florida, generators placed indoors, in garages or outdoors near windows caused nearly all the exposures in a CDC analysis (2).

NIST ran portable generators in an attached garage of a research house for the CPSC. Peak carbon monoxide in the house ranged from under 10 to over 10,000 parts per million across different generators, door positions and ventilation settings (3).

Never run a generator in a garage. After a Maine ice storm, putting a generator in a basement or an attached structure was the strongest risk factor (4). In Kentucky, eight of ten carbon monoxide deaths after an ice storm were tied to generator location (5).

Put battery-powered carbon monoxide alarms inside the house, as the CPSC advises. In a North Carolina outage, 96.2% of the severe poisonings happened in homes with no working alarm (6). Alarms do not replace placing the generator outside.

I place my generator on the gravel pad and keep the cord short of any door or window. I fill the tank only when the engine is off and cool.

What to Remember Before You Call the Electrician

A switch or an interlock makes sure only one source feeds the house. The inlet must match the generator’s outlet. The generator always stays outside, far from every opening. My page on the watts a house needs helps you build the list before the electrician arrives.

I read the pump’s nameplate before I read any box, and the same habit works for a generator’s outlet panel. Read the generator’s manual for the outlets and running watts, then ask the electrician to put the transfer switch, the inlet box and the outlet match on one written quote. If you are still choosing a unit, my roundup of the best generators for home backup offers nine options to check against your load list and connection requirements.

Frequently Asked Questions

Do I really need a generator transfer switch?

You need a generator transfer switch or an interlock kit whenever a portable generator powers house wiring. OSHA says a generator may be attached to a structure’s electrical system only when a qualified electrician has installed it with a transfer switch. If you only plug appliances into the generator with outdoor-rated cords, you need neither.

How does a transfer switch work with a generator?

A transfer switch works with a generator by disconnecting the selected house circuits from the utility first and then connecting them to the generator, with a brief interruption between the two. The generator plugs into an outdoor inlet box, and the switch lets only one source feed the circuits. A manual switch waits for you to flip it, and an automatic one acts by itself.

Can I run a generator without a transfer switch?

You can run a generator without a transfer switch when it powers appliances directly through grounded, outdoor-rated cords. You cannot connect it to house wiring without a switch or an interlock installed by an electrician. Generac calls backfeeding dangerous, often illegal, and something that should never be done.

Is it illegal to run a generator without a transfer switch?

Running a generator without a transfer switch is not illegal when you only use cords to power appliances. Connecting it to house wiring without a switch or an interlock can violate electrical code and utility rules. Ask your local building department how the rules apply in your area.

How much does it cost to have a transfer switch for a generator installed?

The cost to have a transfer switch for a generator installed depends on the number of circuits, the amperage, the inlet box and local labor and permit fees. Ask a licensed electrician for a written quote that lists each part. An interlock kit usually costs less to install, because it adds no separate switch panel.

References

1. Hampson NB, Dunn SL. Carbon Monoxide Poisoning from Portable Electrical Generators. J Emerg Med. 2015;49(2):125-129.
2. Centers for Disease Control and Prevention. Carbon monoxide poisoning from hurricane-associated use of portable generators – Florida, 2004. JAMA. 2005;294(12):1482. doi:10.1001/jama.294.12.1482
3. Emmerich SJ, Persily AK, Wang L. Modeling and Measuring the Effects of Portable Gasoline Powered Generator Exhaust on Indoor Carbon Monoxide Level. NIST Technical Note 1781. National Institute of Standards and Technology; 2013. doi:10.6028/NIST.TN.1781
4. Daley WR, Smith A, Paz-Argandona E, Malilay J, McGeehin M. An outbreak of carbon monoxide poisoning after a major ice storm in Maine. J Emerg Med. 2000;18(1):87-93. doi:10.1016/S0736-4679(99)00184-5
5. Lutterloh EC, Iqbal S, Clower JH, et al. Carbon monoxide poisoning after an ice storm in Kentucky, 2009. Public Health Rep. 2011;126(Suppl 1):108-115. doi:10.1177/00333549111260S114
6. Centers for Disease Control and Prevention. Use of carbon monoxide alarms to prevent poisonings during a power outage – North Carolina, December 2002. JAMA. 2004;291(14):1691. doi:10.1001/jama.291.14.1691

Safety Note: Generator exhaust contains carbon monoxide, which can kill in minutes. Run a fuel generator outdoors only, never in a home, garage, basement, shed or other enclosed or partly enclosed space, even with doors or windows open. The CPSC says to place it at least 20 feet from your home and other buildings, with the exhaust pointed away from doors, windows and vents. Install battery-operated or battery-backup carbon monoxide alarms on every level of the home and outside sleeping areas. Never plug a generator into a wall outlet: connect it to a house only through a transfer switch or interlock installed by a licensed electrician. Refuel only with the engine off and cool, and read the manufacturer’s manual, including for a portable power station.