Dear Mr. Electrician: How do I install a main breaker generator interlock kit on my main electrical panel?
Answer: A main breaker generator interlock kit is great if you want to supply power to your entire circuit breaker panel and can be an economical choice compared to installing a separate external transfer switch. NOTE: Some text links below go to applicable products sold on Amazon and EBay.
Below are step by step photos depicting a main breaker interlock kit installation on an older Cutler Hammer main circuit breaker panel. On the last page is a short YouTube video of this installation.
Before beginning the work of installing an interlock kit on your main electrical panel, you should obtain an electrical permit from your town’s building department. Submit a copy of the interlock kit installation instructions with your permit application. In addition I always write a short “Scope of Work” and include that with my permit applications so that the inspector has a clear understanding of the work being done before hand.
I had submitted documentation about the interlock kit and the PB30 generator inlet along with my application for an electrical permit for this job. I also made a simple drawing depicting the location of the generator inlet in relation to the basement electrical panel. The work was inspected by the town electrical inspector.
When the work is completed you will arrange to have an electrical inspector look over the job to make sure that it is compliant with the electrical code. This is very important to ensure that your installation is safe and not hazardous to someone connecting the generator during a power outage.
HOW TO INSTALL A MAIN BREAKER GENERATOR INTERLOCK KIT
Above is what the Cutler Hammer circuit breaker panel looked like prior to the installation of the interlock kit. Fortunately there were five available spaces in this 30 circuit panel. The interlock kit for this particular Cutler Hammer panel requires three spaces on the top right.
To install a main breaker interlock kit you will need a few extra spaces in your main electrical panel as a two pole 30 amp or two pole 50 amp circuit breaker will need to be installed adjacent to the main breaker.
The new two pole breaker will be connected to the incoming power from the portable generator. Usually two or three spaces are required for the generator two pole circuit breaker.
Main breaker generator interlock kits are available for many new electrical panels from the original manufacturer. However for older electrical panels it is sometimes necessary to use an interlock kit from a third party manufacturer. The interlock kit depicted in the photos on this page is from a third party manufacturer (www.interlockkit.com). It is approved by a certified testing laboratory for use on this particular electrical panel.
The homeowner had typed up a panel directory for most of the circuits years before this installation. It came in handy when identifying critical circuits to run off of the portable generator. A circuit tracer is handy for tracking down circuits throughout the house.
Prior to the start of the job of installing the interlock kit you should identify some of the circuits that you will definitely want to be powered by the generator. Circuits such as the refrigerator, heating system, and well pump are most commonly used for generator power. You should also identify the circuits nearest the main circuit breaker as they will need to be relocated to allow room for the generator circuit breaker.
This is the location that the homeowner chose to have his portable generator sit and operate during a power outage. The open door is the entrance to the garage
The hole in the wood siding was made using two spade drill bits. First a 7/8″ bit was used to drill a 3/8″ deep hole to allow for the button connector. A 1/2″ drill bit was used to drill all the way through into the basement.
Pick a location outside of your house to mount the generator inlet. It should be in an area that is convenient for you to wheel the generator over to and should also be easy enough to install the permanent electrical wiring from the generator inlet to the main electrical panel.
Do not operate a portable generator near a window or inside of the house as the exhaust fumes from the generator are deadly and can kill the occupants.
For a 30 amp generator inlet use #10 wire in conduit or use a 10/3 cable with ground as depicted in the photo above. For a 50 amp generator inlet use #6 wire or 6/3 cable. In the photo above I used two wood spade drill bits to make the hole.
I used a larger bit first and drilled into the siding approximately 3/8″ to allow space for the cable connector. Then I drilled with a smaller bit to accommodate the wire only. A small hole will keep out rodents and outside air.
After pulling the wire through the hole and stripping it back, push duct seal into the hole and leave a small mound on the surface. The cable connector will push into this and make a tight seal. If you use a button type cable connector you would attach that to the inlet box first and just pull the wire through.
If you use a two screw Romex type connector you would put that on the cable first after removing the locknut. Do not over tighten the clamp screws as that could cause the wires to short circuit each other.
I usually cut the clamp screws short after attaching the connector using my Knipex High Leverage Diagonal Pliers or my Klein Lineman Pliers. This makes it easier for the cable connector to fit into the hole in the house that the cable is coming through.
Remove the 1/2″ knockout on the back of the generator inlet box and bring the wire through the hole. When the threads of the Romex connector come through the hole, screw the locknut back on tightly.
A button type Romex connector was used above to prevent the 10/3 Romex from being pulled out. The generator inlet box was secured to the wood siding using #8 x 1-1/2 inch pan-head sheet metal screws.
UPDATE: Since this installation was completed, the design of 10/3 Romex cable has changed. It is now flat instead of round. Consequently you will need to use a 3/4″ Romex connector or button.
Now you can attach the inlet box to the house. For a wood or vinyl siding house I use #8 or #10 sheet metal screws. For a solid masonry house I use plastic anchors with #12 sheet metal screws.
Note how the number 10 bare copper ground wire is shaped in the rear of the generator inlet box and then wrapped around the green ground screw using the included clamp.