Generator Sub-Panel Installation

The existing Cutler Hammer circuit breaker panel.

Dear Mr. Electrician: I want to have a portable generator connection for circuits in my house such as the furnace, the sump pump, and the refrigerator.  How can I install this?

Answer: One simple installation is a stand-alone 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 with a PB-30 generator inlet.  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 and 702.

The Cutler Hammer main electrical panel on the right is nicely hidden in this finished basement, however it is now difficult to add or remove circuits due to a lack of accessibility.
The Cutler Hammer main electrical panel is nicely hidden in this finished basement, however it is now difficult to add or remove circuits due to a lack of accessibility.
This is the inside of the main electrical panelbefore any changes were made for the new generator sub-panel installation with a Reliance PB-30 generator inlet .
This is the inside of the main electrical panel before any changes were made for the new generator sub-panel installation with a Reliance PB-30 generator inlet .
Two large junction boxeswere installed close to the main electrical panel and in the ceiling so that the circuit wires could be spliced and extended to the new generator sub-panel.
Two large junction boxeswere installed close to the main electrical panel and in the ceiling so that the circuit wires could be spliced and extended to the new generator sub-panel.
This is the view inside of the wall looking at the right side of the main electrical panel. The view is from inside of a basement utility closet where the new generator sub-panel will be located.
This is the view inside of the wall looking at the right side of the main electrical panel.  The view is from inside of a basement utility closet where the new generator sub-panel will be located.
Another angle of the large junction boxes that were installed to extend the existing circuit wires over to the new generator sub-panel. The joists are prefabricated I-Joists with pre-punched holes every 12" for wires and pipes.
Another angle of the large junction boxes that were installed to extend the existing circuit wires over to the new generator sub-panel.  The joists are prefabricated I-Joists with pre-punched holes every 12″ for wires and pipes.
Another view of the large junction boxes located in the accessible ceiling.
Another view of the large junction boxes located in the accessible ceiling.
Because the junction boxes are metal, they are required to be grounded by using a green ground screw with one of the grounding conductors under it.
Because the junction boxes are metal, they are required to be grounded by using a 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 upper arrow points to the cable feed that is coming from the generator inlet mounted on the outside of the house. The other arrow points to the interlock device attached to the generator circuit breaker. The interlock prevents the two main circuit breakers from being on at the same time.
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.  The interlock prevents the two main circuit breakers from being on at the same time.
This Square D generator sub-panel is different in several ways from a regular sub-panel. It comes with two main breakers, one for street power and one for generator power. They are interlocked together to prevent both 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
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”.
Here is the Square D generator sub-panelcompleted and labeled.
Here is the Square D generator sub-panel completed and labeled.
The upper arrow points to the 10/3 Romex cable that is coming from the generator inlet mounted outside which will supply power to the generator sub-panel from the generator when needed. The lower arrow points to the 10/3 Romex cable that is coming from the main panel to feed the Square D generator sub-panel on street power.
The upper arrow points to the 10/3 Romex cable that is coming from the generator inlet mounted outside which will supply power to the generator sub-panel from the generator when needed.  The lower arrow points to the 10/3 Romex cable that is coming from the main panel to feed the Square D generator sub-panel on street power.
The two 30 amp main circuit breakers with the interlock device between them. Note the twin circuit breakers for the branch circuits. Notice that the circuit breakers for the furnace and basement lights are connected with a handle tie. This is required by article 210.4(B) of the National Electrical Codebecause these circuits are a multiwire branch circuit.
The two 30 amp main circuit breakers with the interlock device between them. Note the twin circuit breakers for the branch circuits.  Notice that the circuit breakers for the furnace and basement lights are connected with a handle tie.  This is required by article 210.4(B) of the National Electrical Code because these circuits are a multi-wire branch circuit.
The arrow indicates the change from two single pole circuit breakers to one, two pole, 30 amp circuit breaker to feed the new generator sub-panel.
The red arrow indicates the change from two single pole circuit breakers to one 2 pole, 30 amp circuit breaker to feed the new generator sub-panel.
The L14-30 flanged inlet mounted on the exterior of the house. A generator cord is used to connect the generator flanged inlet to a portable generator in order to supply power to the generator sub-panel.
The L14-30 flanged inlet mounted on the exterior of the house.  A generator cord is used to connect the generator flanged inlet to a portable generator in order to supply power to the generator sub-panel.
With the cover down, rain is kept out.
With the cover down, rain is kept out.
This Generator inlet is actually a 30 amp 4 prong twist lock plug
The Generator inlet is actually a 30 amp, 125/250 volt 4 prong twist lock plug (L14-30P).  The generator inlet  is ready for the next power outage when the generator cord will be plugged into it.

You can see photos of an interlock kit installation on a Cutler Hammer Load Center here.