Detached Garage Subpanel + Generator Back Feed
I'm a couple weeks away from closing on a house and in the process of taking care of some electrical things. First and foremost, my friend, a licensed electrician, is getting rid of a bunch of knob and tube. At the same time, I'm having him upgrade the service from 100 amp to 200 amp and put in a new load center.
Right now, the detached garage is fed in a highly unsafe way with a piece of Romex hanging about 7 feet in the air from the house to the garage. Fixing this is one of the first things I will be taking care of myself.
My current plan is the following:
(1) Bury 2" PVC conduit from house to garage.
(2) Install a 100 amp subpanel in the garage.
(3) Feed subpanel with 2/2/2/6 THWN and properly bond the panel with 2 ground rods.
Not having power for about 5 days after Hurricane Irene has added another item on my list. I plan to get a ~10 kW portable generator and back feed it into the main panel with a 50A breaker. Since I want to do this by the book, I will be installing an interlock on the main panel to prevent the back feed from being on when the main panel is on.
The convenient place for the generator to sit and hook up is near the garage. Thus, I plan to put the back feed receptacle on the exterior of the garage, and leverage the same 2" conduit to run the back feed cabling. I plan to run 6/6/6/10 THWN for the back feed. However, I can get rid of the need for running the neutral and ground if I simply tie them into the garage subpanel.
The question: is this code (NEC 2008) compliant? It's not as if the neutral and ground are switched/fused anywhere, so it doesn't change the "topology" of the circuitry too much.
Whether or not I have to run a separate neutral and ground, I'm also curious about derating. In steady state, there are normally only 3 CCCs, since the genset isn't active. When the genset is active, worst case is 100% of the 50A from the genset go to the main panel and the back to the subpanel. This gives us 6 CCCs (assuming dedicated neutral for the generator) and in this case, the 2/2/2/6 at an 80% derate is way over 50A. The 6/6/6/10 at 75C is also > 50A with the derate.
It is not permissible to feed a generator into the garage subpanel unless the latter is restricted to generator power. You would need to put the transfer switch in the line between the main panel and subpanel or put the or cam/slider interlock on the breaker that feeds the subpanel.
To have both the main panel and subpanel fed by utility power you would need to run separate hot and neutral generator lines up to the main panel where a backfeed together with cam/slider interlock is okay.
For a standard hot-hot-neutral 120/240 volt single phase feed the neutral carries the difference between the loads on each of the hot lines; if both hot lines were supplying the same current for example 50 amps each then there would be zero amps on the neutral. Here the neutral is not counted as a "current carrying conductor" for derating purposes.
So as long as I do keep the cables completely separate is everything okay? In other words, none of the back feed cabling will touch the garage subpanel, it will just utilize the same conduit back to the main panel. Make sense?
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