120V generator connected to dwelling 120/240v panel

Merry Christmas
For a few reasons many customers are wanting 120V inverter generators. (Quiet, low fuel use, clean power)


Any issue with connecting both panel phases to 120V through a back fed common trip breaker using a pigtail? (no double lug)
We intend to use the standard panel stickers and notices. I do not see a reason to add any but could be wrong.

Given-
The only 3 wire circuit is a dish and disposal 12 AWG with combined neutral load under 20 Amp.
Yes, we have a main interlock device and back fed breaker is held down.

Other than the 3 wire being a possible problem, I cannot find any code / safety issues.
If it’s a code issue, they are allowed to shunt loads by code so can they be instructed to shut that breaker off when on generator? Use a label?

Thanks for the help.
 

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
For a few reasons many customers are wanting 120V inverter generators. (Quiet, low fuel use, clean power)


Any issue with connecting both panel phases to 120V through a back fed common trip breaker using a pigtail? (no double lug)
We intend to use the standard panel stickers and notices. I do not see a reason to add any but could be wrong.

Given-
The only 3 wire circuit is a dish and disposal 12 AWG with combined neutral load under 20 Amp.
Yes, we have a main interlock device and back fed breaker is held down.

Other than the 3 wire being a possible problem, I cannot find any code / safety issues.
If it’s a code issue, they are allowed to shunt loads by code so can they be instructed to shut that breaker off when on generator? Use a label?

Thanks for the help.
IMHO, I would say it is a code violation of the use of the Neutral conductor. 200.4 A
 
IMHO, I would say it is a code violation of the use of the Neutral conductor. 200.4 A
Thanks, I assume you are referencing to the dish disp multi wire circuit. You do not think posted instrucitions to turn off when on gen power will work??
Unless Im incorrect, the neutral from the gen set is a feeder and not subject to 200.04A
 
Last edited:

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I'm not sure it's worth the trouble, but you could have either the disposal or dishwasher (whichever has less priority) powered through a N.O. contactor that has a 240V coil across both phases. As a result, when both busses are supplied with the same 120V source from the generator there will be 0V across the coil and the lower priority appliance will be disabled.
That way the customer could still use the higher priority appliance on generator power and the MWBC neutral will be protected from overload.

More complicated setups allowing either appliance to be selected could be done, but at that point it would be easier to just rewire it with separate 2-wire circuits.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Other than the 3 wire being a possible problem, I cannot find any code / safety issues.
If it’s a code issue, they are allowed to shunt loads by code so can they be instructed to shut that breaker off when on generator? Use a label?

There is a key difference in having the user shut off loads to accommodate the capacity of the generator vs. having them shut off a MWBC. The conductors from the generator are protected by breaker(s), and so even if the user doesn't comply with instructions to limit the amount of load current those conductors will not be damaged. That may not be the case with the neutral in a MWBC if the user does not shut off the breaker as instructed when using a 120V generator.
 
I'm not sure it's worth the trouble, but you could have either the disposal or dishwasher (whichever has less priority) powered through a N.O. contactor that has a 240V coil across both phases. As a result, when both busses are supplied with the same 120V source from the generator there will be 0V across the coil and the lower priority appliance will be disabled.
That way the customer could still use the higher priority appliance on generator power and the MWBC neutral will be protected from overload.

More complicated setups allowing either appliance to be selected could be done, but at that point it would be easier to just rewire it with separate 2-wire circuits.
Thank you. That is a great idea. I have never seen the use of a disposal anyway lol.
 
There is a key difference in having the user shut off loads to accommodate the capacity of the generator vs. having them shut off a MWBC. The conductors from the generator are protected by breaker(s), and so even if the user doesn't comply with instructions to limit the amount of load current those conductors will not be damaged. That may not be the case with the neutral in a MWBC if the user does not shut off the breaker as instructed when using a 120V generator.
I strongly agree. I was wondering if written directions could pass code but ultimately wish to keep it dummy proof if I can. In this case, the multiwire circuit is to 2 fixed loads. Code aside, in theory, the homeowner would have to be washing dishes in the heat cycle, while trying to grind up a tree trunk in the disposal for quite some time before any issue could arise. Still want to do it right so I appreciate the feedback very much.
 
So am I correct in assuming the only issue others have with my lousy install is the presence of the multiwire or is something else lurking that I have totally overlooked?? Please and thank you. (This is a lady with a disabled hubby on a budget so I am trying to work with the gen they already have. I am tring to get them to just get a 120/240v gen but that is still in the wind)
 

tthh

Member
Location
Denver
Occupation
Retired Engineer
I always worry at the guy who comes next who doesn't understand what you did and why. And, there is no way to rely on written instructions being followed hy HO now or in the future. Once in a while I'd leave a note in a box if something non-obvious was there to save a future electrician some troubleshooting/tracing time, but the note you want is different.
 

tthh

Member
Location
Denver
Occupation
Retired Engineer
Maybe a good way for them is a small 120v sub panel and move the critical circuits there.
 

ASIsparky

Member
Location
Philadelphia, PA
Occupation
Electrician
For a few reasons many customers are wanting 120V inverter generators. (Quiet, low fuel use, clean power)


Any issue with connecting both panel phases to 120V through a back fed common trip breaker using a pigtail? (no double lug)
We intend to use the standard panel stickers and notices. I do not see a reason to add any but could be wrong.

Given-
The only 3 wire circuit is a dish and disposal 12 AWG with combined neutral load under 20 Amp.
Yes, we have a main interlock device and back fed breaker is held down.

Other than the 3 wire being a possible problem, I cannot find any code / safety issues.
If it’s a code issue, they are allowed to shunt loads by code so can they be instructed to shut that breaker off when on generator? Use a label?

Thanks for the help.
This is for a portable generator right? If you're worried about the AHJ, just install a standard L14-30 generator inlet as if you are going to be using a 120v/240v (4wire) generator and have the customer purchase an L14-30 to L5-30 adapter for $30 or so. The adapter will be the piece that joins both phases together, and doesn't need to be present for inspection. We all know that a dishwasher and disposal will not overload a 12 AWG wire. As someone mentioned , just combine the MWBC into a single branch circuit with a pigtail if needed.
 

robertd

Senior Member
Location
Maryland
Occupation
electrical contractor
Is there an manual transfer switch or something? How are you feeding the panel from the generator?

>Any issue with connecting both panel phases to 120V through a back fed common trip breaker using a pigtail? (no double lug)
>We intend to use the standard panel stickers and notices. I do not see a reason to add any but could be wrong.

What prevents someone from closing this breaker when poco power is on? They would be closing the breaker into a bolted short.
What happens if a breaker trips, someone other then the HO is looking to see what tripped, and turns that breaker on? How many non-electricians are going to bother to read labels on the panel before switching a breaker?
 
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