Fact-checked by Marcus Hayes, Master Electrician (License #CA-EL-89241), 15 years generator installation experience.
Connecting a portable generator to your house requires a transfer switch or interlock kit — never plug
a generator into a wall outlet. This guide covers the three legal methods, their costs, and the safety
rules you must follow. Written by a licensed master electrician.
Key Takeaways
Never plug a generator into a wall outlet — this is backfeeding, and it is illegal and lethal to utility workers.
An interlock kit is the cheapest legal method ($50-$150, DIY-installable) and prevents the main and generator breakers from being on simultaneously.
A manual transfer switch ($200-$500 + electrician) powers specific circuits and is safer for selective backup.
An automatic transfer switch ($500-$3,000+) is required for standby generators and switches automatically during an outage.
Your generator needs an L14-30R outlet (30A 120/240V) for most transfer switches; a 14-50R for 50A units.
The NEC requires a transfer device for any generator-to-panel connection. No exceptions.
Three ways to connect a generator to your house
The three legal methods are: (1) an interlock kit ($50-$150, DIY), which prevents the main and generator
breakers from being on at the same time; (2) a manual transfer switch ($200-$500 + electrician), which
powers specific pre-selected circuits; and (3) an automatic transfer switch ($500-$3,000+, electrician
required), which detects outages and switches automatically. All three require a permit and inspection.
Method
Cost
Installation
Best For
Interlock Kit
$50-$150
DIY (permit required)
Budget whole-panel backup
Manual Transfer Switch
$200-$500 + labor
Electrician
Selective circuit backup
Automatic Transfer Switch
$500-$3,000+
Electrician
Standby generators
Never Backfeed Your Panel
Plugging a generator into a wall outlet (backfeeding) energizes the utility lines outside your house.
If a utility worker is repairing the line, they can be electrocuted. Backfeeding is illegal under
the NEC, can result in criminal charges, and has killed utility workers. Always use a transfer
switch or interlock kit.
Step-by-step: Installing an interlock kit
To install an interlock kit: (1) turn off the main breaker, (2) install the interlock plate on the
panel cover, (3) install a generator breaker in the panel, (4) install an inlet box on the exterior
of the house, (5) wire the inlet box to the generator breaker, (6) pull a permit and schedule an
inspection. The interlock physically prevents the main breaker and generator breaker from being on
simultaneously.
1.Turn off the main breaker in your electrical panel before doing any work.
2.Install the interlock plate on the panel cover so the main and generator breakers cannot be on simultaneously.
3.Install a generator breaker in the panel to receive power from the generator inlet box.
4.Install a generator inlet box on the exterior of the house to connect the generator.
5.Run wiring from the inlet box to the generator breaker in the panel.
6.Pull an electrical permit and schedule an inspection with your local building department.
An interlock kit is the most popular DIY method because it allows you to power any circuit in your
panel through the generator breaker — you are not limited to pre-selected circuits like with a manual
transfer switch. The trade-off is that you must manually manage your loads: turn off the main breaker,
turn on the generator, then selectively turn on individual circuit breakers one at a time, staying
within your generator's wattage limit.
Frequently Asked Questions
How do I connect a portable generator to my house?
You connect a portable generator to your house using a transfer switch or interlock kit installed on your main electrical panel. The generator plugs into an inlet box on the exterior of the house, which is wired to the transfer switch. You never connect a generator directly to an outlet — this is called backfeeding and is illegal and lethal.
Can I plug a generator into a wall outlet?
No. Plugging a generator into a wall outlet is called backfeeding, and it is illegal under the NEC. It energizes the utility lines outside your house, which can kill utility workers repairing the lines. You must use a transfer switch or interlock kit that physically prevents the generator and utility power from being connected simultaneously.
What is an interlock kit?
An interlock kit is a metal plate installed on your main electrical panel that physically prevents the main breaker and a generator breaker from being on at the same time. It costs $50 to $150 and can be installed by a homeowner. It is the cheapest legal way to connect a portable generator to your house panel.
Do I need an electrician to install a transfer switch?
Yes for a manual transfer switch ($200-$500 plus labor). An interlock kit can be DIY-installed if you are comfortable working in an electrical panel, but you must pull a permit and have it inspected. An automatic transfer switch for a standby generator requires a licensed electrician. Never work in an electrical panel without turning off the main breaker first.
What outlet do I need on my generator for a transfer switch?
You need an L14-30R outlet (30A, 120/240V) on your generator to connect to a standard transfer switch or interlock kit. Some generators use a 14-50R outlet (50A) for higher-wattage units. Check your generator's outlet list before buying a transfer switch — the inlet box must match the generator outlet.
Can I use extension cords instead of a transfer switch?
Yes, for small loads. You can run heavy-duty extension cords from the generator directly to appliances (refrigerator, lights, TV). This is safe and legal, but you cannot power hardwired appliances like a furnace blower or well pump without a transfer switch. For whole-house essential backup, a transfer switch is required.