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A completed generator sub-panel with a Square D generator interlock between the utility power main circuit breaker and the generator circuit breaker

How To Wire A Generator Sub-Panel

Dear Mr. Electrician:  How do I wire all of my circuits to a generator sub-panel? Answer:  First get a sub-panel suitable for your application.  Some generator sub-panels are prewired and you just make the correct connections with the existing circuits.  Those types usually have limitations as to the number of circuits connected to the generator. It is possible to make a generator sub-panel out of an ordinary load center circuit breaker panel using an interlock kit.  You just need to get the correct accessories.  In the generator sub-panel circuit wiring installation pictured below I used a standard Square D Type QO, 125 amp, 24 circuit, single phase load center as a sub-panel with a separately purchased Square D Interlock Kit made for this purpose. I also had to purchase separately a grounding terminal bar for all of the ground wire terminations.  Most load centers do not come with a separate ground bar unless it is designated as a sub-panel.  This panel did not have a main circuit breaker installed by the factory.  I installed the QO 100 amp main breaker and a QO 50 amp generator circuit breaker.  NOTE: Text links below go to applicable products on Amazon.com. For this particular generator hook up installation the client wanted many circuits to be powered by a portable generator.  Those circuits included an electric range, electric water heater, oil burner for heat, microwave oven, bathrooms, and kitchen outlets & lighting.  My first thought was to install an interlock kit on the main breaker in the existing circuit panel that would have provided the homeowner access to all of the electric circuits. I chose to install a generator sub-panel rather than install an interlock kit on the entire main electrical panel because at the time of this installation I could not find an interlock kit for the existing panel that would be approved by the electrical inspector.  The panel manufacturer does not make an interlock kit for this particular panel.  There is a company called Interlockkit.com that does make an interlock kit for this main panel.  However the State of New Jersey, where this installation is located, was not accepting the test laboratory certification that the manufacturer was using for their products. The manufacturer has since changed testing laboratories and some of their products have been re-tested by the new lab and are now acceptable to New Jersey.  If you are in New Jersey and need to use an interlock kit from this company, contact the factory by phone to learn if the product that you need is approved.  Click to see some examples of interlock kit installations done by me.   Generator Interlock Kits on Amazon Steps Before Generator Sub-Panel Installation You need to apply for an electrical permit ahead of time for wiring a generator sub-panel.  When I submitted the permit application for this job I included a copy of the installation instructions of the interlock kit, as well as the model number of the load center.  I also included a typed, one page "Scope of Work" briefly detailing...
Here is the Square D generator sub-panelcompleted and labeled.

Generator Sub-Panel Installation

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. From the ceiling mounted junction boxes I installed new cables to the generator sub-panel. Amazon Sells Generator Sub-Panels The right side of the Cutler Hammer load center was open all of the way to a closet with plenty of clearance a few feet away.  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...