I want to install an outside power inlet receptacle for my portable generator, to be used in case of a power failure.

In my house I have two breaker boxes: one is the main breaker box. A second one is installed about twenty inches away from and beside the main box. The main breaker box is wired as it should be -- two separate hot bus bars, a neutral bus and a ground bus. It also has a ground conductor (heavy copper wire) going to an earth ground. I cannot see to tell how or if the ground bus and neutral bus are tied together; I know they should be.

The second breaker box was installed because the main 200 amp box would not support two heat pumps and all the other appliances. Box #2 has two hots and a neutral, I cannot see a ground for the box. I know if it is considered a sub-panel, the ground and neutral should not be connected together. The breakers in the #2 box are all two-pole. The ground wires from the breakers are hooked up on the (ground - or neutral?) lug strips on both sides of the box, depending on what side the breaker is on.

Now, here is my question-concern: I want to power the blower fan only - from one of the heat pumps. It just so happens that the breaker is located in the #2 box, the one with no ground. Therefore I cannot wire the outside receptacle for the generator into this box, so I have the main breaker box to wire it into. I know the #2 box is wyed off of or spliced into the leads that are going to the #1 box as there is only three wires coming into the house, and there are no wires leaving the #1 box to feed #2 box.

If I wire the receptacle to the main breaker box and connect neutral and the ground together to the ground lug strip, will this create a problem with the circuitry? Also I will be using an Interlock, for the lineman's or anyone else's safety.

  • 3
    This is a complex setup, and it's doubtful that some knucklehead on the internet will offer you much help. You should contact a local licensed Electrician, as doing this wrong can cause a lot of damage and/or injuries.
    – Tester101
    Nov 2, 2015 at 18:47
  • 2
    Out of curiousity, why do you want to power only the blower during a power failure? I understand that powering the heat pumps themselves would require a large generator, but is there much advantage in powering just the blower?
    – Johnny
    Nov 2, 2015 at 18:52
  • In reply to Johnny. I have an outdoor wood furnace . I only Need to power up the blower fan from my heat pump system, in order to force air through the heat exchanger from my wood furnace. Nov 3, 2015 at 2:08
  • A picture might be helpful - I'm going to guess that there might be a metallic conduit connecting two boxes in such close proximity, and if so, there's a ground conductor - the conduit.
    – Ecnerwal
    Dec 28, 2016 at 3:35
  • Honestly, when someone says they want to power one hardwired appliance, I think "easy, convert the hardwired appliance to a cord-and-plug-connected one" by giving it a power plug and putting a convenient mains socket right next to it. Then putting it temporarily on "gen" is as easy as an extension cord. Mar 22, 2018 at 20:10

3 Answers 3


I ignored the main power box as a typical portable can only produce a small fraction of the main supply. I put a receptical box inside near where the generator will be outside the house, conduit through the wall and out to the generator. The refrigerator is fortunately near . Then I use temporary extension cords as needed ; a bit of a hassle but but not bad for a couple days a year. I have used it a few times and it was satisfactory ( 2,000 watt portable). I supply a room AC, TV and some lights, phone and PC.


If I understand your description everything in the #2 box is 2 pole or 220/240 here in the U.S. that may be why there is only 1 "ground". You are correct that sub panels should have an isolated ground but it is possible that this is one of the cases where the authority having jurisdiction allowed it.

There are several concerns with home generators, I believe you addressed with your interlock, you must disconnect any devices being fed by the generator from the supply side. My last concern is if all your breakers are double pole that your fan may be also be or be driven by electronics derived from the 220/240 providing the wrong voltage may damage the fan and/or other components. Ok with that said you could tie the ground and neutral together without concern but its not code though.

I tried to add some as I have gotten called away, I did something similar a few years back to circulate the heat from our wood stove through the house. I put in a disconnect to the pump and aux heat coils. My fan speed was electronically controlled so it was all or nothing without the other electronics. This worked for us when we had no power for 5 days but the fan worked. If you look you may find the neutral and ground buses connected in the panel from the MFG (most of the time they come that way), the best way would to be isolate the neutral and add a new ground and have a cross over switch e power 1 side line power the other side so you could not back feed to the line.

  • Sorry I had to run for a minute, I basicly powered my electric furnace and installed a disconect to the pump and aux heat coils so I could drive the fan and circulate warm from my wood stove, this is when I found my furnace had a varriable frequency drive to controll the speed of the fan it was all or nothing without the other electronics working but it would cycle and kept the house warm when we had no power ofr 5 days a few years back,,, hope that is helpful
    – Ed Beal
    Nov 2, 2015 at 20:45
  • Please use the edit link underneath your post if you want to add information to it.
    – Niall C.
    Nov 3, 2015 at 0:45

You gotta really watch it on this one. I have a feeling it's different.

"Two main panels" is common in all-electric homes

It is common for an all electric home, especially in the snowbelt, to need more power than a 200A panel, and be provided with more power, typically 400A. This is NOT a main panel/subpanel situation. Either

  • Both panels are main panels with main breakers, with full 400A service wires to both, often with a shutoff switch outside.
  • Neither panel is a main panel. The "main panel" is the shutoff/master breaker outside. The neutral-ground bond is made there, and each panel should have neutral and ground separated. They will surely have "main" breakers of 200A or similar rating to protect the panel itself, which cannot handle 400A.

That may explain what you are seeing in panel #1 re: the separation of neutral and ground. (it is also a good practice regardless, not least for "second panel/main outside" conversions like yours!)

Heat pumps cannot work in the coldest of winters, so they require "emergency heat" which is plain resistor based heat. Midwestern homes particularly tend to be in low-density suburbs, and the large lot sizes lead to very large homes in one floor - all of which gives the house a great deal of surface area. As a result, it is common for a large home to have dual 70A emergency heat circuits. That's the reason for such large service. I have a friend with that setup, he'd like 2 on-demand heaters for each end of his long home, and he doesn't have the ampacity in the panel!

A 240V-only panel will seem really weird with the neutrals

Since the two emergency heat circuits by themselves will dominate one panel, it's quite possible this is a 240V-only panel.

A 240V-only panel has no use whatsoever for neutral. It still needs ground.

Due to the design of panels, it can be hard to delete (tear out) the neutral bar. It is much easier to delete the ground bar (or never install it in the first place if it's an accessory bar). So you'd convert the neutral bar to a ground bar, which would necessitate leaving the neutral-ground bar in place obviously.

That may explain what you are seeing.

Unfortunately installers are not much for labeling, and expect "the next guy" (i.e. you) to know what you are looking at.

Generator interlock: tricky

In the first part, I described the unusual configuration found in these 400A homes. These are semi-incompatible with generators - at least it's a lot more work.

Your first option is install a transfer switch where the main shutoff is, and have it switch all 400A. This thing will be a monster and may cost a mint.

Your second option is to install main/gen interlocks in either or both panels. What do you do about the fact that some loads you need to power are in one panel, and other loads are in the other panel?

  • Install main/gen interlocks on both panels. There is nothing wrong with splitting a generator's output to have it feed 2+ panels, as long as all the loads you have turned on are within the generator's working capacity.
  • Move loads around so they are all in the panel you are feeding with the generator. You will need the blowers and controls to be powered. Those are the smallest loads (~1-2A and ~5-16A, often @120V) in a heat pump system. Next up is the heat pump(s) proper (20-60A @240V always) and the largest is the emergency heat (70-150A @240V always)... don't move those.

Read the documentation/schematics carefully and make sure there is clear separation between the emergency heat and heat-pump motor proper. Typically those are fairly simple circuits controlled via a contactor, with little other interaction with the controls and blowers. But check.

The lunatic approach (if your gen is big enough)

In the snowbelt, weather usually causes power outages. So loss of power often happens during very cold weather, when the heat pump can't work and the very hungry emergency heat is needed.

The real issue is the temperature at the heat pump's condenser. So consider this: Build a "gazebo" around the heat pump, with room for some other stuff. Have insulated "walls" you can install on demand. And then heat the inside of the gazebo to keep it warm enough for the heat pump to work happily. Now you don't need emergency heat. When the cold snap fades, remove the walls and go back to normal.

Of course, the heat pump is actively chilling the gazebo interior, so you will need a lot of heat. Where do you get it? To start with, the generator. A generator are only 1:3 to 1:5 efficient, and heat pumps are 3:1 to 4:1 efficient - it's almost a match made in heaven. Capture most of the generator's running heat inside the gazebo. Be sure to plumb air intake and exhaust outside.

You will need more. Add any sort of fuel heater which can be setup to vent outside, put that on a thermostat set around 50-60 degrees F. Heat pumps are very efficient when pumping heat from a place that's about as warm as their target. Now, effectively you are making heat using fuel - and simply pumping that heat into the house.

  • Note that this guy does NOT have all-electric emergency heat (see his comment re: the wood furnace he wants to run). So, it's probably even weirder than you're imagining. Feb 19, 2018 at 23:20

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