Question re. emergency backfeed with subpanels

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 14awg@ 15 amps; 12awg@20 amps; 10awg@30 amps.

I guess the terminations are rated for 75C max... Even though the wire itself is rated for higher current (higher temp insulation):

https://www.ecmag.com/section/codes-standards/sizing-conductors-part-xxi (details)
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (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:

https://www.homedepot.com/p/Connecticut-Electric-30-Amp-8-Space-10-Circuits-G2-Manual-Transfer-Switch-Kit-EGS107501G2KIT/100669964?MERCH=REC-_-searchViewed-_-NA-_-100669964-_-N&

Whole house manual genset interface for utility meter socket:

generator manual transfer switch

And even neat relay based transfer switches:

https://www.solar-electric.com/pmts-50.html

And another class are interlocks for main circuit breaker panels:

https://interlockkit.com/

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

-Bill

Ok... You forced me to look up MWCB...
Probably not what you are referencing... 

https://www.electrical101.com/multiwire-branch-circuit.html


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.

-Bill

I would have had to look up both anyway... Does not help that I was (am?) a bit dyslexic.

-Bill 

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):

https://www.amazon.com/Circuit-Breaker-Lock/s?k=Circuit+Breaker+Lock

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.

-Bill