I have a portable 6500 W generator. I have a main house electrical panel and a subpanel in my garage, which are on opposite sides of the house. My subpanel is on the outside wall of my garage. The main panel feeds the subpanel.

I would like to connect the generator to the subpanel which would then feed the main panel and then I can decide which breaker (main breaker-OFF) to shut off and which to keep on.

Can this be done? What do I need and how do I do it?

  • What is the subpanel for? Is its only purpose to interface to the generator?
    – wallyk
    Commented Dec 21, 2013 at 3:30

7 Answers 7



You'll likely end up backfeeding the grid, which can easily lead to linemen being injured or killed. A generator should never be connected to the electrical system without a proper transfer switch being installed.


I don't see any way this could be done safely, to code, etc.

Problem being there's no way to interlock the MAIN and the Generator if they are on different panels. Without an interlock, it's quite simply not safe (and in a more nuanced view, if the power company becomes aware of it you may not get your power turned back on - they REALLY don't like that.) The fact that you say you will turn it off is just not good enough. If the service folks are in your area restoring power and they hear a generator running, they very often WILL come check.

It IS possible to have a generator input to the main panel, if the panel manufacturer has an interlock kit for the purpose - I have one. It is not physically possible for the MAIN and the Generator to both be turned on at the same time. The only way you could have it go into the sub-panel would be if the sub-panel was interlocked to disconnect from the main panel, so you absolutely cannot feed the main from the sub, no way, no how.

The generator is portable - when you need it, move it, and provide enough cord from a proper interlocked input on the main panel to connect to it. Or, run new wire all the way to where you want it outside the garage.

  • Can you explain briefly what an interlock is and why it can't be placed on a subpanel?
    – Reid
    Commented Dec 21, 2013 at 17:17
  • 2
    @Reid It's basically an OR switch. The panel is connected to the Mains OR to the Generator, but not to both. This means that the generator cannot backfeed into the mains (and put repairfolk at risk). It cannot be on the subpanel because it must come between the mains and the main panel to work.
    – bib
    Commented Dec 21, 2013 at 17:39
  • 1
    That pretty much is the third paragraph. Mine is a big hunk of steel that fits over two breakers, my dad's is a little steel toggle that fits between two breakers. Both can be off, but only one can be on. Far less costly but just as effective and legal as a transfer switch.
    – Ecnerwal
    Commented Dec 21, 2013 at 22:36

There is no way to do this safely, and it's certainly against code. Similar to throwing the main breaker and plugging the generator into household outlets, there is simply no way to make it safe. It's not done because it cannot be done safely.

If you want to connect your generator to the outside of the garage (don't operate it inside the garage) then you simply need to install a transfer switch at the main panel, and run a length of cable from the transfer switch to the generator location. It's not as expensive or as much work as you might think, and it provides provably safe power from the generator to all of the house.

There is no other safe way to do this.


Nope, it's a huge flaming code violation, because a misstep (or miscommunication between two people) will wind up with the generator "backfeeding the grid" and killing line workers.

"But I'm sure we'll do great." No, you won't. The military has studied this: in a crisis, people don't rise to the occasion, they sink to chaos - or, the level of their training. That's why the US military trains so hard.

"But the linemen are supposed to ground out their work area" After they've been up for 2 days straight, and/or called in from a neighboring state and working on systems they're unfamiliar with? Don't count on it.

"But they'll never know" actually smart meters can tell them.

"But there's no penalty" LOL... read your service agreement. They can shut your power off and tell the city, and that hole will cost you at least $10,000 to dig out of, even if there's no $10,000 fine per se.

Run two cables but wait! Relax.

"You can't believe how expensive 6/3 is, I'm not buying TWO of them!" That's right. You're not. Or even one, for that matter.

Look inside your panel, see the neutral bar? That's not tin-coated copper. That's tin-coated aluminum. See the main lugs? Those are aluminum too, and most likely so are the fat wires. Turns out, aluminum lugs play well with copper wire (the reverse was the problem), and aluminum wires work great on them. Large feeders have never been a problem.

The generator feeder.

So, we need two cables. First, the generator feeder. Because of problems with NM and UF jacketing, they are limited in their amperage. Aluminum wire isn't sold in those types, so they get a higher amperage than you'd expect. #6Al is good for 50A if the terminals are rated for aluminum and 75C wire. And the stuff is dirt cheap.

Make sure to get inlet terminals rated for AL wire and 75C thermal - that shouldn't be a problem.

If you want 65A wire, use #4Al. If you want 90A wire, use #2Al. They're not that much more money, really, but just watch what will fit on your inlet terminals. (I'm afraid this does nothing to reduce the cost of the inlet).

"That wasn't so bad!"

The subpanel feeder.

OK, now the subpanel feeder. You can use #6Al there too, but since the stuff is so cheap, I say use #2Al so you get 90A to your subpanel. It lives in a happy price point because of its frequent use for mobile home feeders (in fact "MH Feeder" is the perfect stuff outdoors, or indoors in conduit).

"But I don't want a 90A subpanel, only 60A". Do it anyway, the cost difference is pretty small, and it will give you loads of expansion room for anything you do in the future. Spaces are cheap - and nobody ever came in here saying "Help! I want to add something, but my panel has plenty of extra spaces in it."

And in 2021 we're even seeing people get higher offers, by thousands of dollars, simply because their garage is ready for EV chargers... because they're thinking there might be an EV in their future. (For the homebuyer it's easier to add $11/month to their mortgage today, than lay out $2000 cash right after buying a car). Anyway, for that you'll want 90A (to run an EVSE and whatever else, or dual EVSE's using Share2 protocol).

The bill of materials:

  • Interlock for main panel (US$30-70 typically)
  • 2 breakers
  • Inlet with terminals rated 75C and AL wire.
  • X feet of #6 AL 4-wire cable
  • X feet of #2 AL 4-wire cable
  • 24-space, 200A rated subpanel. Main breaker required only if it's an outbuilding. It can be rated down to 100A - point is, there's no reason to avoid a 200A panel if it's available.
  • various conduit, trenching, cable clamps etc. as needed. Don't put both cables in the same conduit without doing a derate calculation (we can help).

Note that Rigid Metal Conduit only requires 6" of cover). Expensive, but you can trench it with a garden trowel, so saves calling DIG-SAFE to get the utilities marked, and saves ditcher rental.


I work in electrical sales at a large home improvement store. Besides the aforementioned main panel breaker interlock device we also carry a (Siemens) breaker interlock which can be used with two 240 volt (double) breakers back-to-back in a sub panel. One breaker is the sub main fed from the main panel, and the other is the sub main fed from the generator. With the approved device only one breaker can be actuated at a time. Only the circuits that will be handled by the generator are included in the sub panel. The feeders from those circuits are spliced in the main panel and fed to the sub panel. When the power goes out the user has only to switch the sub main assembly to the generator, start the generator and proceed to power up the branch circuits. An effort must be made to properly balance the loads between the two busses of the panel. Technically splicing circuits in a main panel violates the NEC, but it is a common practice, and most inspectors will pass such an installation. In reality, the commercially made generator sub panels must be hooked up the same way.

  • 2
    No, splicing circuits in a panelboard cabinet does not violate the NEC until your panelboard gutter becomes more than 75% full (re-read 312.8 please). Commented Feb 22, 2017 at 12:45
  • I have been told by people more knowledgeable than I am that splicing in the main panel (or even sub panels) can make them messy and difficult to follow where circuits are going, but is not against code unless there are too many wires (i.e. box fill). In other words, it is a good idea to make splices elsewhere, but generally not required.
    – user4302
    Commented Feb 22, 2017 at 16:28
  • @Snowman -- you can also use UL-listed distribution blocks to tidy things up... Commented Jul 20, 2017 at 11:41
  • @snowman my service panels are typically surrounded by a constellation of 4x4" junction boxes, where I have made such splices. It does not help that a thief stole all the breakers out of all the panels in a factory building, cutting the wires at the edge of the panel. But after fixing that, I've found just how useful that strategy is for all sorts of things, like splicing to subpanels, adding GFCI deadfaces instead of expensive GFCI breakers, etc. Commented Jan 22, 2018 at 18:56

If the main 200 amp from street is turned off to prevent back flow, why couldn't one wire a 240/120 generator receptacle below a sub panel? When AC is restored, I unplug the generator. Shut off dedicated double pull 50 amp breaker in garage sub, and turn on the 200 amp mains to street. Works for me......

  • 5
    This may work fine for you, but if anybody who doesn't know the procedure tries to do it, they could end up killing a lineman who's trying to restore power. This is NOT the proper, or safe way to do it.
    – Tester101
    Commented Jul 20, 2017 at 11:22

Yes you can do it if you turn off the main.

  • 5
    Hello, and welcome to Home Improvement. This answer has been given before for this question, and the response was "yes, but you might kill someone". Sounds like there's a serious downside, no? Commented Jan 23, 2019 at 16:56

Not the answer you're looking for? Browse other questions tagged or ask your own question.