How To Wire A Generator Sub-Panel

Wire Your Own Generator Sub-Panel

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Interior of a Square D load center being used as a generator sub-panel. The generator power feed to the panel is installed and connected to a two pole type QO 50 amp circuit breaker
Interior of a Square D load center being used as a generator sub-panel. The generator power feed to the panel is installed and connected to a two pole type QO, 50 amp circuit breaker.

One of the obstacles that I had to overcome in this generator sub-panel installation was the existing radon exhaust pipe shown above on the right.

Knipex high leverage diagonal pliers cutting the knockout rings on the bottom of the electrical panel
Knipex high leverage diagonal pliers cutting the knockout rings on the bottom of the generator sub-panel.

On this particular load center it was easy to remove the knockouts for 1 1/4″.  I banged each ring enough to stick out and then cut them with my Knipex diagonals.  After cutting it was just a matter of moving them forth and back to let them snap off.

Cut knockout rings on the bottom of the generator electrical sub-panel
Cut knockout rings on the bottom of the generator electrical sub-panel.

Not all knockouts are this easy to remove.  It varies by manufacturer.  You can see my post on concentric knockout removal by clicking here.

A completed generator sub-panel with the cover removed showing the interior wiring and the Square D generator interlock. The flexible conduit feed from from the main panel is shown supported by a conduit hanger and threaded rod.

A completed generator sub-panel with the cover removed showing the interior wiring and the Square D generator interlock. The flexible conduit feed from from the main panel is shown supported by a conduit hanger and threaded rod.Although there were less obstacles on the wall to the right of the existing main electrical panel, I chose this location because the existing circuits that were going to be wired into the new generator sub-panel passed overhead.  It was easy to move the circuit cables without having to add junction boxes to extend the cables.  Using the 1 1/4″ flexible metal conduit (FMC) I was able to get around the radon pipe without a problem.  I chose to use a conduit hanger and threaded rod to support the conduit from the ceiling rather than attempt to drill into the rock wall to attach conduit straps.  I screwed a small L bracket to the inside of the joist and used nuts and washers to attached the 1/4″ threaded rod to the L bracket.

In addition to the hanging support, I also installed a conduit hanger screwed to the wood as the FMC exited the main panel.  I used the conduit hanger instead of a strap because the FMC connector was not close to the wood.  The conduit hanger nearly matched the distance from the back board and kept the conduit straight as it came out of the main panel.

Rules regarding the installation of flexible metal conduit (FMC) are described in article 348 of the National Electrical Code (NFPA 70).

A closer shot of a completed generator sub-panel with the cover removed showing the interior wiring and the Square D generator interlock. The flexible conduit feed from from the main panel is shown supported by a conduit hanger and threaded rod.
A closer shot of a completed generator sub-panel with the cover removed showing the interior wiring and the Square D generator interlock. The flexible conduit feed from from the main panel is shown supported by a conduit hanger and threaded rod.

Whenever I do a basement service change or replace an electrical panel I always add a GFCI service outlet and a light with a wall switch for illumination as required by article 110.26(D) in the National Electrical Code.  I have found that it is usually dark in the basement locations of the main electrical panel.

As with all sub-panels this one was wired with a separate equipment grounding conductor that was terminated on a separate ground bar terminal.  Grounds and neutrals are always kept separate in a sub-panel whereas in the main panel they are connected together by the main bonding jumper as per article 250.92 and 250.28 in the National Electrical Code.

Close shot of the inch and a quarter flexible metal conduit angle connector where it enters the bottom of the generator sub-panel
Close shot of the inch and a quarter flexible metal conduit angle connector where it enters the bottom of the generator sub-panel.

An 1 1/4″ flexible metal conduit angle connector was used to bring the flex conduit into the bottom of the sub-panel.  I would have preferred to go into the panel with a straight connector, but the radon pipe had too much of a bulge.  With the angle connector I was able to rotate it a little to allow the flex to go around the radon pipe.  Note the use of the blue plastic threaded bushing to protect the wire from sharp edges.

This panel was fed from the main panel with a 100 amp circuit and fed from the generator with a 50 amp circuit.

Installing The Generator Inlet

6/3 Type NM cable going through a joist in the basement
6/3 Type NM-B cable going through a joist in the basement.

I used 6/3 Type NM-B cable as the feeder cable from the generator inlet to the generator sub-panel.  Above is the 6/3 passing through a floor joist on its way to the outside generator inlet.  I used my Milwaukee angle drill with an auger drill bit to drill through the joists.

Depiction of a 6/3 type NM cable coming through the rim joist in the ceiling of the basement
Depiction of a 6/3 type NM cable going through the rim joist in the ceiling of the basement.
Close shot of 6/3 type NM cable coming through the rim joist with duct seal in the hole to prevent air from coming in
Close shot of 6/3 type NM cable going through the rim joist with duct seal in the hole to prevent air from coming in.

The photo above shows the 6/3 cable as it penetrates the rim joist.  On the other side is the back of the generator inlet.  Duct Seal was used inside and outside to keep air from moving in or out.

To drill through the rim joist I first drilled a pilot hole with a long bell hanger drill bit from the inside out after measuring three times to make sure that I was in a good spot.  Then I used a hole saw on the outside siding to make a clean cut hole and also to not damage the siding.  After I finished drilling with the hole saw through the wood siding, I used an auger bit through the hole made with the hole saw to go through the rim joist.

6/3 type NM cable pulled through the side of the house with duct seal in the hole and the outer jacket insulation removed so that the single conductors are exposed
6/3 type NM cable pulled through the side of the house with duct seal in the hole and the outer jacket insulation removed so that the single conductors are exposed.
Close shot of 6/3 type NM cable pulled through the side of the house with duct seal in the hole and the outer jacket insulation removed so that the single conductors are exposed
Close shot of 6/3 type NM cable pulled through the side of the house with duct seal in the hole and the outer jacket insulation removed so that the single conductors are exposed.

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