Dear Mr. Electrician: How do I install a generator sub-panel? I want to have a generator sub-panel for circuits in my house such as the furnace, the sump pump, and the refrigerator.
Answer: There are a few types of generator sub-panels available. Some of them come pre-wired from the factory and it is just a matter of connecting the flexible conduit to the main electrical panel and then making all of the electrical connections as per the instructions. That type of sub-panel can only be used directly adjacent to the main electrical panel. NOTE: Text links go to applicable products on Amazon.com
Another type of installation utilizes a standard electrical load center as a generator sub-panel for some previously selected circuits to be powered when the generator is on. Below are photos of a generator sub-panel installation using a Square D generator sub-panel which included two main breakers, an interlock kit, and a circuit breaker hold down kit.
As with any new electrical installation an electrical permit must be obtained from the town building department. Submit a copy of the sub-panel instructions and specifications with your permit application. Call for inspection upon completion.
Some of the applicable articles from the National Electrical Code concerning the installation of generator sub-panels are: 210, 230, 250, 300, 310, 314, 334, 445, and 702. NOTE: Text links below go to applicable products on Amazon.com
Portable Generator Sub-Panel Installation
I have found that some finished basements have made the main electrical panel inaccessible for future renovations and sometimes even for repairs. The Cutler Hammer load center depicted here was boxed in by the wall that was erected in the basement. Fortunately it was not totally enclosed.
I was able to remove the cables of the circuits to be relocated into the generator sub-panel from the top of the main electrical panel and fish them into the ceiling above. There was enough slack on the Romex cables for me to bring them into two 4 11/16″ square junction boxes.
Two large junction boxes were installed close to the main electrical panel in the accessible ceiling so that the circuit wires that are removed from the main panel could be spliced and extended to the new generator sub-panel. From the ceiling mounted junction boxes I installed new cables to the generator sub-panel.
The view above inside of the wall shows the right side of the main electrical panel. The camera angle is from inside of a basement utility closet where the new generator sub-panel will be located. The right side of the Cutler Hammer load center was open all of the way to this closet. Through that space I was able to install a 10/3 Romex cable to feed power to the new Square D generator sub-panel.
The joists are prefabricated I-Joists with pre-punched holes every 12″ for wires and pipes.
Because the junction boxes are metal, they are required to be grounded by using a 10/32 ground screw with one of the grounding conductors under it. All grounding conductors are then joined together by twisting and finished with a wire connector.
The new cables were installed across the ceiling and were brought into the top of the generator sub-panel. There was a lot of room in the ceiling and a plywood board was attached to the concrete foundation wall to mount the sub-panel on. I used an electric hammer-drill to make 1/4″ diameter holes in the concrete and then used plastic anchors and #12 sheet metal screws to secure the board to the concrete.
In the photo above, the upper red arrow points to the cable feed that is coming from the generator inlet mounted on the outside of the house. The other red arrow points to the interlock device attached to the generator circuit breaker.
An interlock device is needed on the generator sub-panel to prevent the utility company power from being on at the same time as the generator. The main utility power circuit breaker and the generator power circuit breaker must be secured to the electrical panel so that they cannot easily be removed or pop out on their own. The circuit breaker panel manufacturer often has a circuit breaker hold-down kit available. Read the label inside of the panel to obtain a part number. This particular panel came with circuit breaker hold-downs from the manufacturer.
This Square D generator sub-panel is different in several ways from a regular sub-panel. It comes with two main breakers, one for utility company power and one for generator power. They are interlocked together to prevent both breakers from being on at the same time. In addition each main breaker has a clamp on it holding it to the busbar as is required by article 408.36(D) in the “National Electrical Code”.
In this small 8/16 sub-panel two Square D #PK2MB circuit breaker retainers were used to hold the main breakers in place. They are mounted on the bottom of each circuit breaker. The Square D #QO2DTI mechanical interlock is used to prevent both circuit breakers from being on at the same time. Another method for larger 100 amp Square D load centers is the Square D #PK4DTIM4LA manual transfer kit which comes with a different type of breaker retainer and the same mechanical interlock.