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My main panel is in my basement and it has a 60 amp breaker that goes to a sub panel in my pole barn. The pole barn sub panel provides power for the pumps and blower motor of my outdoor woodstove. Last winter I lost power for a week and had to run two extension cords from my 30 amp generator; one to the main panel in my basement to power the furnace and the other to the sub panel in the pole barn to power the woodstove.

I want to get a hookup for my generator so that I don't need all that sassafras if I ever lose power in the winter again.

While it would be easier to run the generator by the pole barn and backfeed the main panel, reading other questions makes me believe that is neither safe nor legal. So, I think I need to get a transfer switch or interlock kit setup on my main panel for the generator. However, I still need to power the woodstove from the pole barn sub panel.

My question is, can I use a 30 amp generator connected to my main panel to power the 60 amp sub panel in my pole barn? The only thing I want to power in the pole barn is the woodstove, which is on a 20 amp breaker in that sub panel.

References:

Is it reasonable to create a 30amp branch circuit for the purpose of feeding the panel from a generator?

Can I connect a portable generator to a subpanel to backfeed my house?

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4 Answers 4

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Having a panel that's rated for more amps than you're going to put through it isn't a problem. (Trying to put power through something that's rated for fewer amps is a big problem.)

For the rest, that's outside my expertise.

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  • Nothing unsafe nor illegal has been proposed. An unsafe and illegal method others have proposed in the past has been specifically discarded in advance and removed from consideration. The second part of your answer seems to be based on vague and baseless fears.
    – Ecnerwal
    Commented Sep 27, 2014 at 20:33
  • OK, edited to just make the statement. I absolutely admit to vague fears in this setup, baseless or not, despite being pretty comfortable with house wiring. I know my limits.
    – keshlam
    Commented Sep 27, 2014 at 20:36
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    Generator salespeople do love to sell autostart generators and transfer panels where you have to pick all the loads to be powered by the generator in advance, and the generator has to be oversized so that it can run them all if blindly auto-started. There is no loss of safety (but a considerable reduction in salespeople commissions and your costs) in using a simple interlock, if you are fine with having a power outage for a while, and manually controlling the loads while you are having a power outage so that you can use a smaller generator and not run everything at the same time.
    – Ecnerwal
    Commented Sep 27, 2014 at 20:49
  • @Ecnerwal: True enough. Electrically, as long as the generator and the line are physically prevented from ever being connected to the house simultaneously, and you don't overload circuits or the generator, and either the generator's putting out true sine or none of your electronics will object to the higher frequencies, the system should work. I'd still consider the interlock to be something a pro -- or at least someone who doesn't have to ask questions about it on the Web -- should handle installing, so I really am not comfortable going beyond that point.
    – keshlam
    Commented Sep 27, 2014 at 21:51
  • While all three answers are applicable, this answer was first.
    – CatShoes
    Commented Sep 30, 2014 at 12:01
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Yes, if you make sure to shut down enough things (if not, you'll trip breakers, so you are still good, from the safe and effective front.) I have a 40 amp manual generator interlock installed in a 200 amp panel. I neither need nor want a generator big enough to run everything at once, or automatic switching. I do want (and I have) the ability to power any one thing (or combination of things) that the generator can safely supply, without having to fool around with hack rewiring jobs under poor conditions or absurd extension cord tricks. That panel will eventually have a 100 amp feed to another building, and the generator input will be able to feed it (so long as the actual draw over there is controlled to be below the generator output, of course.)

Exactly what your options are will depend on exactly what your panels are, and what the panel maker has for approved interlock devices, but generically, it is possible. The legal and important safety end of "safe and legal" is having the interlock so you absolutely, positively, cannot backfeed power to the grid. The overcurrent part of safe will be taken care of by the generator output breaker and/or the panel input breaker for the interlock. Obviously the 60 amp breaker will be highly unlikely to trip if a 30 amp generator is feeding it - but if you draw 40 amps for some reason, the generator breaker will trip. The 60 amp rated wiring will of course be just fine with 30 or less amps running on it.

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An interlock kit would certainly work, and be safe and legal. What's nice about them is that you can choose what is going to run before you connect the generator. Also, in cases like yours you can alternate loads so that you can run critical and non-critical loads, albeit not at the same time.

You CANNOT, as you already know, connect a generator to a sub-panel and backfeed a main panel.

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Sure, feeding the generator into the main panel is no problem. You can install a proper interlock into the main panel (assuming that is possible), and that will automagically power the subpanel. You won't have to do anything special. It will just happen.

After you install the interlocked generator breaker, run that out to an inlet. Now if you don't want to pay $70 for an inlet, a poor man's inlet is simply 1 foot of cord coming out of a junction box with good and proper strain reliefs on the cord. Location should be chosen carefully to keep the cord out of the mud, snow and other threats.

Now, with a 30A generator I don't know whether it is a 120V generator or a 240V generator. If it's only 120V, then you'll need to do some careful planning to make sure all your critical circuits are on the same pole of power.

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As far as your problem of wanting the generator to be at the shed, there is no easy answer for that. There is no legal way to have "2-way traffic" on a single feeder. Because you would need to interlock the main breaker and the generator breaker, so what will you have? A 100 foot long Bowden cable? LOL Not gonna work.

Unfortunately the only advice I can give you is to run a second parallel feeder cable. There's no better answer. What you can do, though, is make sure it is aluminum to save a ton of money. (or alternately recycle the original feeder to be the generator line, and make the feeder the aluminum. 6 AWG aluminum is good for 50A, or 2 AWG aluminum is 90A and large enough that even the most superstitious cities recognize that it is safe.

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