Question re. emergency backfeed with subpanels

rajraj Registered Users Posts: 5 ✭✭
I have what I hope is a quick question/answer.  Let’s say I am looking at emergency backfeeding a panel (disconnected from the grid, of course) and feeding 120v to both legs. Yes, I can’t run any 240v appliances and would turn off all 240v breakers.  Yes, there could be MWBC issues, depending upon how circuits are wired.  However, let’s say I know that’s not a problem.  Here’s the additional complication, which I’m thinking may still be a MWBC type issue.  My main panel drives 2 other subpanels.  I’m thinking the neutral between the main panel and each subpanel would also be a shared neutral, so it would be overloaded, if both 120 legs are in the same phase?


  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    If you are driving your panel with a "larger" AC genset and wire both L1 and L2 to 120 VAC Hot (and common neutral)... Yes there is the possibility of overloading a neutral in the home.
    • 4,000 Watts / 120 VAC (worst case common neutral current) = 33.3 Amps
    For 120 VAC wired branch circuits--They can be wired as Hot+ (dedicated) Neutral and you have "No" neutral issues.

    However, a common wiring scheme is to use L1+L2+ (shared) Neutral and split the L1+N (to bedroom lighting) and L2+N to bedroom outlets. If you plugged an electric heater into the bedroom outlets, and had 600 Watts of Halogen lighting, then yes, overheating the neutral is a possibility.

    The standard current limits from NEC are [email protected] 15 amps; [email protected] amps; [email protected] amps.

    I guess the terminations are rated for 75C max... Even though the wire itself is rated for higher current (higher temp insulation): (details) (with updated 14/12/10 AWG ratings vs insulation type)

    Anyway 19 amps into electric water heaters for now >65+ years with 15 Amp breakers--No problem except a few years ago one unit the 15 amp breaker was false tripping. Had replaced a few "bad breakers" over the years--But last time these breakers were no longer available new--So required for 10 AWG and 30 Amp breaker. And off course, no false trips...

    My reasoning for 10 AWG and 30 Amp breaker:
    • 19 amps continuous * 1.25 NEC continuous load derating = 23.75 Amps derated branch circuit and breaker -- round up to 30 Amps for any type of 10 AWG cabling and the (then standard small wire AWG ratings).
    So--Your decision here... A generic answer, limit to 15 Amps maximum on the Neutrals (for 14 AWG). Short term running 20 amps @ 14 AWG wiring--Even 65 year old building did not see any issues (I did not check insulation rating for existing wiring--So take it with a grain of salt).

    But it is not something I can tell you is safe to do. If I am running a few refrigerators, lights, and a small sump pump (I used a Honda eu2000i which is ~14 amps @ 115 VAC max output). Usually not an issue.

    If you start plugging in electric heaters/hot plates/etc. in several rooms with a larger genset--Yes, it is an issue.

    There is another (typically minor issue for homes) of running a genset into a panel... Normally, homes have Neutral+Ground bonded in the main panel. Most small gensets and AC inverters have "floating outputs"--So single point AC ground in the main panel is correct. If you "float" a subpanel, then you may have a floating netural question (more posts later, if you want to discuss). Larger gensets and inverters may/usually have N+G bond at their chassis... In theory, you either ground in the panel, or at the genset and carry green wire to ground. If you do both, you have parallel N+G current paths (NEC does not like) and can trip GFI breakers in genset/inverter (if present).

    The other issue which you are aware of--Normally some sort of transfer switch or breaker panel interlock is required when wiring gensets to home wiring.

    There are multi-breaker manual transfer switches for gensets like these:

    Whole house manual genset interface for utility meter socket:

    generator manual transfer switch

    And even neat relay based transfer switches:

    And another class are interlocks for main circuit breaker panels:

    It is all up to you... Some ideas that can make your installation a bit safer and up to code.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rajraj Registered Users Posts: 5 ✭✭
    edited September 2021 #3
    Wow, that's a lot more information than I was expecting!  To be clear, I'm not putting in a subpanel, I was simply thinking about using a 120v generator to feed into both legs of my main panel and then thought that the presence of a subpanel would mean that the neutral between the main box and the subpanel would mean that both separate 120v legs would be returning over that common neutral back to the main panel, which sounds like a MWCB situation.  Of course, the wires to subpanels are much larger, so if only some circuits are turned on, it probably wouldn't be a problem.

    I finally decided maybe it was a bad idea anyway, simply because there are so many strange wirings in this older house that I really can't count out the possibility of MWCB even in one panel!  So, my plan now is to only power one 120v leg and arrange my 3 fridges and one freezer circuits (1 in main panel and 2 others in subpanels) such that they're all on that leg.  Then turn off basically everything else.  It's only for an afternoon.

    Thanks a lot for the quick and very informative response!
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    edited September 2021 #4
    Ok... You forced me to look up MWCB...
    The equities and options exchanges have procedures for coordinated cross-market trading halts if a severe market price decline reaches levels that may exhaust market liquidity. These procedures, known as market-wide circuit breakers (“MWCB”), may halt trading temporarily or, under extreme circumstances, close the markets before the normal close of the trading session. MWCBs provide for cross-market trading halts during a severe market decline as measured by a single-day decrease in the S&P 500 Index.
    Probably not what you are referencing...  :smile:
    Multi-wire Branch Circuit Correct Wiring Diagram

    I am not an electrician... And really don't do will with abbreviation. (and if this was me--I would be "phase tapping" the black leads on the the last recepticale with Red Tape--Carry all colors over from beginning to end for clarity and safety--Otherwise the third recepticale looks like 2x L1-Black circuits on one common neutral).

    Wiring L1 and L2 in parallel... Pretty safe with a 2,000 or 3,000 Watt generator:
    • 3,000 Watts / 120 VAC = 23.3 amps
    And most refrigerator and freezers only draw something like 120-150 Watts maximum running the compressor... And upwards of 600VA or a bit more for starting the compressor... And around 600 Watts for the defrost heaters (for some tens of minutes once every ~24 hours). Your average loads are going to be less than 1,000 Watts--With the occational motor start surge and defrost heater activation.

    The chances of you pulling 20 Amps for more than a few seconds at a time with minimal other loads--Not really likely to happen. And if you can ensure that the major "backed up loads" are splitting loads across multiple (for example 14 AWG) cables--You are probably fine.

    Ran 2x full size refrigerators + 1x full size refrigerator freezer+ largish pond pump (air and water) for 4 days on an eu2000i genset--And never had an overload issue... Since genset was new, took a few hours of run-in at full throttle before it would pickup loads without stalling on ECO throttle.

    This is the old issue--I cannot (and should not) tell you to do things that are, potentially, unsafe. And while you my "trust" information from over the Internet--You need to verify that it is correct and safe(ish) for you.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rajraj Registered Users Posts: 5 ✭✭
    Sorry for the confusion, I should have said MWBC!  Oops!
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    edited September 2021 #6
    I would have had to look up both anyway... Does not help that I was (am?) a bit dyslexic.

    -Bill  :*
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rajraj Registered Users Posts: 5 ✭✭
    Ok, I ran my test.  Disconnected from grid, turned off all breakers, connected my generator to one phase, turned on breakers to other two subpanels, made sure all fridges/freezers were on the same phase and only turned on just those circuits.  Everything worked just fine.  Thanks, everyone, for the sanity check!
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    edited September 2021 #8
    As a safety thing... Electricians will padlock panels off when needed--Such as turning off your utility power breaker/knife switch--And they keep the only key with them--So that only the electrician who turned off the circuit(s) can turn them back on.

    Here are some examples of circuit breaker lockouts (if, for example, your main power is not a knife switch you can simply hang a lock on):

    This is a relatively sane and safe way of ensuring that nobody but you turns on power again after the genset has been turned off and disconnected.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rajraj Registered Users Posts: 5 ✭✭
    Good idea!  Thanks!
  • m151m151 Registered Users Posts: 36 ✭✭
    You could use one of the two sub-panels for your most important loads and use a jumper from L1 to L2 (inside the sub) (disconnect the 2nd leg main feed) for a 120 volt only  panel. This way you can't overload the neutral. It can be powered by an inexpensive main panel  interlock switch. The two panels would have to be close to avoid splicing wires. This would give you a great back-up for other events not just for the afternoon.
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