The simplest way to connect a generator to a house is also the most dangerous. A homeowner with a generator and a dead furnace reasons that if the machine can power an extension cord, it can power a wall outlet. A cord with two male plugs seems like a clever shortcut. It is called backfeeding, it is illegal in many places and it has killed utility workers and neighbors.
When a house is connected to a generator the wrong way, the electricity does not stay in the house. It can travel backwards through the service transformer and energize the lines outside, where a lineman working to restore power assumes they are dead. The correct way to connect a generator to a home’s wiring is through equipment designed to keep the two sources apart. This guide explains what that equipment does, the main options and what to ask an electrician. It does not tell you how to install anything, because that work belongs to a licensed professional and the local inspector.
The problem the equipment solves
A house is normally connected to one source of power: the utility. A generator is a second source. If both are connected to the same wiring at the same time, the results range from damage to the generator and the appliances to fire and electrocution.
A transfer device makes sure that only one source feeds the house at any time. When the utility is on, the house is connected to the utility. When the utility is off and the generator is running, the house is disconnected from the utility and connected to the generator. The device never allows both at once.
The main options
Manual transfer switch
A manual transfer switch is a small panel, usually installed next to the main electrical panel, containing selected circuits. When the power goes out, you start the generator, connect it with a heavy cable to an inlet on the outside of the house, and flip the switches that move the chosen circuits to generator power.
Lower cost than an automatic system, and you choose which circuits are powered, so a smaller generator can handle essential loads.
You must be home, start the generator and operate the switch. Only selected circuits are covered, so some outlets and lights will not work.
Interlock kit
An interlock is a mechanical device fitted to the main panel that physically prevents the main breaker and the generator breaker from being on at the same time. The generator connects through an inlet and a dedicated breaker.
Can power any circuit in the panel, within the generator’s capacity, and typically costs less than a transfer switch. It is common in some areas.
Requires careful use. You must shed loads manually so that the generator is not overloaded. Not every panel is compatible, and not every jurisdiction allows it. Whether it is acceptable depends on local code and the utility, so check with your electrician.
Automatic transfer switch
An automatic transfer switch, usually paired with a standby generator, senses the loss of utility power, starts the generator and switches the house over, usually within seconds, and switches back when the utility returns.
No action required. It works while you are asleep or away.
The highest cost, and installation must be done by licensed professionals under permit. It requires an appropriately sized unit and regular maintenance.
Comparing the options
- Operation. Manual transfer switch: You flip the switch. Interlock kit: You flip breakers in sequence. Automatic transfer switch: Automatic.
- Circuits powered. Manual transfer switch: Selected circuits. Interlock kit: Any, limited by generator size. Automatic transfer switch: Selected or whole house.
- Relative cost. Manual transfer switch: Moderate. Interlock kit: Lower. Automatic transfer switch: Highest.
- Installer. Manual transfer switch: Licensed electrician. Interlock kit: Licensed electrician. Automatic transfer switch: Licensed electrician and generator installer.
- Common with. Manual transfer switch: Portable generators. Interlock kit: Portable generators. Automatic transfer switch: Standby generators.
- Watch out for. Manual transfer switch: Limited circuits. Interlock kit: Panel compatibility, local rules. Automatic transfer switch: Cost, maintenance.
Inlets, cords and the generator side
The generator connects to the house through a power inlet box, mounted on an outside wall, and a heavy-duty cord of the correct rating. The inlet and cord must match the generator’s output and receptacle type. A mismatch, such as a plug rated for less than the generator can deliver, creates overheating risk. The electrician will advise on the right ones.
Never use household extension cords for connecting a generator to a transfer switch. Use only cables rated for the load and the outdoors.
Grounding and other details
Generators and transfer equipment involve grounding, neutral bonding and breaker ratings that vary by machine and code. Whether a portable generator needs a ground rod, and how the neutral is handled, depends on its design and the way it is connected. These are exactly the details an electrician is trained to get right. We do not give instructions. If you hear advice online that sounds like a shortcut, treat it as a warning.
Permits and inspections
Installation of transfer switches, interlocks and standby generators normally requires a permit and inspection. The inspector verifies that the equipment meets code, which is also what your insurer may expect. Some utilities require notification of a generator installation. Rules vary by city and utility, so ask your building department and your power company. We do not tell you what they will require.
What to ask an electrician
- Which option do you recommend for my panel and generator, and why?
- Is it allowed by local code and my utility?
- Which circuits will it cover, and how many watts do they add up to?
- What inlet, cable and breaker ratings are needed?
- Will you pull the permit and arrange the inspection?
- How do I operate the system, and will you show me?
- What is the total cost and what is included?
- What is the warranty?
Bring your load list from our generator sizing guide. It makes the conversation concrete.
What you do yourself
Even with the right equipment installed, safe operation involves habits. Place the generator outdoors, far from openings and in the open, never in a garage. Run it, connect it, then turn on loads one at a time, starting with the largest motor. Keep a carbon monoxide alarm working inside. Refuel only when the machine has cooled. Test the whole system under load while the weather is calm, not in the middle of a storm. Our guide on fuel storage and maintenance covers the fuel side.
When a power station avoids the issue
If your needs are modest, a portable power station can power essentials without any connection to the house wiring. You plug devices directly into it and avoid the whole question of transfer equipment. Compare the options in our guide to portable power stations and our overview of generator types.
The short version
Never connect a generator to a house by plugging it into an outlet. A transfer switch, interlock or automatic transfer switch keeps the generator and the utility apart, protects utility workers and your equipment and is installed by a licensed electrician under permit. Ask what it covers, what the local rules are and how to operate it, and keep carbon monoxide safety at the center of every decision.
Mistakes we see most often
- Plugging a generator into a wall outlet with a double-male cord.
- Running a generator in a garage, crawl space or near a window.
- Buying an interlock kit without asking whether the panel and local code allow it.
- Using household extension cords as the link to a transfer switch.
- Skipping the permit and the inspection.
