Backfeed 120v to house from 240v garage B-box

Question

^Garage Breaker Box^

Main Breaker in house OFF.

A/C Breaker OFF (the only 240v appliance ).

240v breaker to garage OFF.

Backfeed 120v thru one leg of my 240v breaker box in garage. There is an empty 30a 2 screw breaker. It is an older box and has a brass plate where the common and grounds are attached. Connecting generator power to only one screw ( the red wire side of the box ). #1) will power cross thu the breaker to black sude of the box? #2) can power be pulled from the red wire side of my house breaker box witout worry of MWBCs?

Plan

In House

Main OFF (remain off)

A/C OFF (remain off)

Garage Off

Appliances to be powered OFF

In Garage

30A OFF

Connect Non Running generator to special outlet (power to red )

Start generator

Turn 30a breaker ON

Back in house

Turn garage breaker ON

Power appliances as needed (fridge, freezer & Furnace)

Answer 1

If your objective is to physically locate a generator near (hopefully not in) the garage and to exploit the existing feed wire from the house to the garage as a way to get generator power back to the house, you may not do this.

The very first sentence of your question is the key: "Main breaker in house is off". That is just not good enough. It has to be "Main breaker in house is impossible to turn on while generator is running". And the only way to do that is to install an interlock or transfer switch on the main house panel.

If you want to power the house with your generator you need to install the generator interface (inlet, switch/interlock) on the main panel.

If your generator is 120V, you can move breakers around in your main (and garage) panel so that all the most important 120V loads (fridge, bathroom lights, internet, not necessarily in that order) are on the same phase.

Answer 2

NO! You can't do this!

It's not safe or legal to use a suicide cord. It's not safe or legal to backfeed a panel without a generator interlock. And it's definitely not safe or legal to backfeed a main panel from a subpanel!

I get the desire to make use of the feeder that already runs from main panel to subpanel, but This Cannot Be Done. We've looked and looked. At least not at any sane cost - you can always install a MID/isolation switch and have it electrically operate a contactor in the garage, but you're talking stupid money and a battery system to keep the MID alive when the power is out. And you're still burying data wires, so you might as well just bury power wires, and lay that parallel generator circuit you were trying to avoid.

That's the only sane-cost way to do that. Sorry. Lay a second circuit from where you want the generator to the main panel. If I had better advice I would give it.

Answer 3

A 30A-capable generator that only produces 120V is (a) relatively unusual (in fact, more likely can't actually generate close to 30A) (except I just found there are quite a few designed with "RV outlet") and (b) extremely limited as it can't power any 240V loads and can only power half of your 120V loads unless you bridge it to both hots (and then you run into potential MWBC neutral issues). I highly recommend looking into generators that can produce more power with a 240V 30A output.

There are two absolutely key issues here and one secondary issue:

Inlet vs. Outlet

An ordinary receptacle has holes or slots in it for prongs from a plug. That is an outlet. It is use for sending power out of the receptacle. When you are sending power in to a receptacle then it must be the opposite, an inlet with prongs on it, which is used with a plug that has matching holes or slots in it. Here is a 30A example, Leviton brand, from Amazon:

Note that you can use an ordinary extension cord (as long as it is sized appropriately for the particular circuit type) because the male end will go into the outlet on the generator and the female end will go into the inlet on your house.

For safety (especially carbon monoxide concerns) the inlet should be mounted on the outside of your house or garage, not inside, as that will help make sure you actually run the generator outside.

Generator Interlock

An interlock is a device, typically a couple pieces of specially designed metal that only allow one of a pair of breakers to be on at a time. This is required (unless you have an automatic transfer switch which would include an equivalent automatic mechanism) to use a generator in place of utility power because otherwise someone will go through the sequence of steps incorrectly (stuff happens, particularly when there are storms going on and/or people are worried about getting the power on to keep refrigerators and/or HVAC running) and potentially zap a utility worker.

Correction: If your generator inlet is connected through a subpanel then the subpanel feed must have an interlock in the main panel. Which effectively means the only loads that could be on the subpanel would be loads powered only when on generator power. Which basically makes the subpanel useless in most circumstances. Which means running a fresh 10 AWG cable (in which case I recommend /3 for future expansion to 240V even though /2 will do for 120V) or 10 AWG wires in conduit from the main panel to wherever the inlet is installed (i.e., even if the inlet will be near an existing subpanel).

Ground/Neutral

Based on the description, it sounds like you have a single bar for neutral ("common") and ground in the subpanel. That is not allowed any more. This is an important issue in general, independent of the generator installation.

If the subpanel is very old and there is no separate ground wire (or metal conduit as ground) between the main panel and subpanel and that was legitimate at the time the subpanel was installed then that is OK. Otherwise, you need to separate ground and neutral in the subpanel by installing a separate ground bar (if there isn't one already installed), removing the neutral/ground bond (if there is one) and moving wires as needed so that neutral wires are only on the neutral bar and ground wires are only on the ground bar.