How To Wire a Portable Generator

Portable generators provide essential electricity to keep your home running and can be a lifesaver during power outages. However, safely connecting a generator to your house requires careful planning and professional installation.

In this guide, Ask This Old House master electrician Heath Eastman shares two professional methods for connecting a portable generator to a home’s electrical system. Even though this isn’t a DIY project, it’s helpful to be informed.

Professional Portable Generator Installation Methods

Eastman demonstrates two installation methods in the video: using a power inlet box with a manual transfer switch and installing an interlock device. Only a licensed electrician should install a manual transfer switch or interconnect switch. Both can be bought from an electrical supply house.

Power Inlet Box and Manual Transfer Switch

First, Eastman shows a manual transfer switch, which designates specific circuits to be powered in the event of an outage and has to be manually switched from utility power to generator power.

This method involves installing a power inlet box on the exterior of your home and connecting it to a manual transfer switch inside:

1. The electrician installs a power inlet box on the outside of your house.
2. A cable runs from the inlet box to a manual transfer switch inside.
3. The transfer switch is wired to select circuits in your electrical panel.
4. During an outage, you connect the generator to the inlet box and switch the transfer switch to generator power.

This setup allows you to power specific, pre-selected circuits during an outage. It’s a good option if you want to make sure certain critical home systems always have power. 

You can generally expect to pay $1,200–$1,600* or more for a professional to install the switch.

*Cost information sourced from Angi.

Pro Tip: The manual transfer switch is an essential safety mechanism. As explained during a This Old House generator installation, the switch ensures “there’s no way the street power will actually collide with the generator power and cause an issue.” This prevents dangerous backfeed—when generator electricity flows back into utility lines—which can injure utility workers and damage equipment.

Interlock Device Installation

An interlock device offers more flexibility in powering your home during an outage:

1. The electrician installs a power inlet box on the exterior of your home.
2. The inlet is wired directly to your main electrical panel.
3. An interlock device is installed on the panel, preventing simultaneous connection to utility and generator power.
4. During an outage, you connect the generator to the inlet box, turn off the main breaker, and turn on the generator breaker.

With this method, you can power any circuit in your home up to the capacity of your generator. However, more active management is required to avoid overloading the generator. For example, you can manually switch off nonessential circuits, such as your oven or air conditioner, to prevent overloading.

When using an interlock device, electrician Heath recommends a specific sequence: “When the power goes out, you turn everything off, turn the generator on, make your switch here. Now start turning things back on one at a time. So you get your water, your furnace, refrigerator… even a few other receptacles, possibly a TV.” This gradual approach prevents overloading by letting you monitor the generator’s capacity as you add each load.

Pro Tip: Heath, a licensed electrician featured on This Old House, advises keeping large appliances off when running on a portable generator with an interlock: “What you want to keep away from though is maybe the electric oven, your dryer, bigger items, your central air conditioner.” If you accidentally run too many circuits at once, “the generator will actually shut down”—a built-in safety feature, but one that leaves you temporarily without any power at all.

Safety Tips for Using Your Portable Generator

Even after a professional wires your generator, you should know and follow certain important safety best practices:

• Always read the generator’s manual and make sure you follow the manufacturer’s guidelines.
• Educate all household members about generator safety practices, including the importance of keeping the generator outside and away from windows, doors, and vents.
• Never attempt to connect a generator directly to your home’s wiring without proper equipment, a process known as backfeeding. This is illegal and extremely dangerous.
• Use the heavy-duty, outdoor-rated extension cords recommended by the manufacturer.
• Regularly inspect cords and connections for signs of wear or damage and replace them as needed.
• Install carbon monoxide (CO) detectors in your home. Test your CO detectors monthly, and replace batteries and detectors according to the manufacturer’s instructions.
• Let the generator cool before refueling.
• Store fuel in approved containers away from living areas.

Dan Dunn of Dunn’s Equipment in Haverhill, Massachusetts, explains that when a generator is connected directly to a circuit board and the main power supply is restored, the two currents “meet like locomotives barreling toward each other on the same track.” According to This Old House Magazine, such an electrical collision can burn out the generator or start a fire, dwarfing the damage of a mere blackout. A generator transfer switch eliminates this risk by temporarily isolating the circuits receiving generator power from the rest of the house.

Pro Tip: Some newer portable generators now come with a built-in carbon monoxide sensor that automatically shuts the unit down if dangerous CO levels are detected. As highlighted on a 2023 episode of This Old House, this is an important safety feature—but experts still stress: “The most important thing, do not run it in the garage, do not run it in the house, make sure it’s on the outside of the building.” A built-in sensor is an added safeguard, not a substitute for proper outdoor placement.

Legal Compliance

We recommend following these tips to make sure your generator installation complies with all local codes:

• Check local, state, and national electrical codes and make sure your contractor follows all requirements.
• Inform your utility company that you’re planning to install a generator.
• Obtain any necessary permits from your local building department.

Maintaining Your Portable Generator

Regular maintenance helps make sure your generator is ready when you need it. Here are some key maintenance tasks:

• Always use fresh gasoline, as old fuel can lead to engine problems.
• Check and change the oil according to the manufacturer’s schedule.
• Inspect and clean the spark plug.
• Keep the fuel tank filled with fresh gasoline.
• Periodically check and tighten any loose bolts or connections.
• Replace the air filter as recommended.
• Run the generator monthly for about 30 minutes.
• Store the generator in a dry, protected area when not in use.
• Use fuel stabilizers if storing the generator for an extended period.

We recommend documenting maintenance activities and dates to keep track of service intervals. Periodically running the generator prevents moisture buildup and helps identify potential issues before they become major problems.