generator backfeeding question

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
BLUF: on a typical medium-size genset that has both 240v and 2 - 120v outlets, are the 120v just opposing legs of the 240v like in a traditional home panel? Or do they operate differently?

Lets pretend that a guy is going to backfeed power to his house w/ a, say, 7500w generator, the type w/ a couple 120v 20A lines and a 240/120 that has the 4-pin twist-lock (40A).
Meanwhile (assume there is a clean disconnect from the incoming power to avoid killing a lineman) as luck would have it there is no good way to do the obvious backfeed connection using the 240v supply because the house doesn't have a 240v appliance connection to connect to. Or heck its just a small house/shed/roach cart/whatever and only has 120v appliances.
The better-than-nothing solution is to plug one of the 120V gen lines into a 20A outlet in the house. Downside here is that aside from only getting a max of 20A of uice, it only feeds half the circuits (the ones on the same leg as the supplied line).
But can you connect the other 120v gen circuit into an opposing leg on the home panel? Are they comparably out of phase right?

EDIT - I just realized an empirical test would be a continuity measurement between the + of both 120v gen circuits in question....?
 
Just put an ac volt meter between the hot legs. It won't tell you if they're exactly 180* out, but it'll be close.

Back feeding two different 120v circuits doesn't require them to be 180* apart unless you're hoping to pick up 240v somewhere (rvs, for example).

Most of what you're describing is dangerous as shit, though.
 
RatLabGuy said:
BLUF: on a typical medium-size genset that has both 240v and 2 - 120v outlets, are the 120v just opposing legs of the 240v like in a traditional home panel? Or do they operate differently?

Lets pretend that a guy is going to backfeed power to his house w/ a, say, 7500w generator, the type w/ a couple 120v 20A lines and a 240/120 that has the 4-pin twist-lock (40A).
Meanwhile (assume there is a clean disconnect from the incoming power to avoid killing a lineman) as luck would have it there is no good way to do the obvious backfeed connection using the 240v supply because the house doesn't have a 240v appliance connection to connect to. Or heck its just a small house/shed/roach cart/whatever and only has 120v appliances.
The better-than-nothing solution is to plug one of the 120V gen lines into a 20A outlet in the house. Downside here is that aside from only getting a max of 20A of uice, it only feeds half the circuits (the ones on the same leg as the supplied line).
But can you connect the other 120v gen circuit into an opposing leg on the home panel? Are they comparably out of phase right?

EDIT - I just realized an empirical test would be a continuity measurement between the + of both 120v gen circuits in question....?
Click to expand...
Does this hypothetical home not even have a clothes dryer?

Id be willing to bet the 2 120v outlets are the two legs of the 240v. Where else would the power come from. Measure hot to hot on the outlets to find out. 0ish v means they're the same leg. 220ish they're different. Also, better just ask @Ron
 
I would be 100% certain that your panel has 2 leg busses, and that regardless of the house not having a 240v appliance (read - gas), a generator plug and interlock could be installed relatively inexpensive.

Also fwiw I do not endorse this whatsoever but if you feed 120v onto a 2 pole breaker with the main breaker off, you just parallel your output in half and have 120v all over the house.

Edit: also, if with a meter you read that there’s 120v of potential between each 20a outlet to ground and 240 between each, there ya go. Otherwise if it’s the same phase there will be 0 volts.
 
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Ron said:
The simple yet most accurate answer is - it depends.
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this right here is what I expected and is why I asked :D
awheelterd said:
Does this hypothetical home not even have a clothes dryer?
Click to expand...
336wheeler said:
I would be 100% certain that your panel has 2 leg busses, and that regardless of the house not having a 240v appliance (read - gas), a generator plug and interlock could be installed relatively inexpensive.
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This came up in debate w/ a neighbor while we were all down after the storm that came through tuesday (I just got mine back this afternoon) discussing ways to backfeed in an emergency and how it isn't always simple (and yes, dangerous if you don't know your wiring).
Houses here have NG. In my case, the only 240v lines in the whole house are AC - which is hardwired - and the stove - which has a plug that is inaccessible to remove and plug in a backfeed. So there is nothing to unplug and plug into that already has wring for high amperage or both legs of the panel. that's what made me think of it, a traditional homemade "cord of death" into a dryer isn't an option.

Given that the hypthetical scenario is "emergency", and not having the means to properly run wire, tapping into the panel at the breakers properly doesn't really fit.
336wheeler said:
Also fwiw I do not endorse this whatsoever but if you feed 120v onto a 2 pole breaker with the main breaker off, you just parallel your output in half and have 120v all over the house.
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I'm sory, I don't quite understand how you'd do that. You mean split the hot wire from the 120v of the gen and connect to both of the taps on a 240 breaker? Or I guess technically even just feel into 1 leg at the tap and jumper over to the other. Haha the image of that gives we willies.
 
shawn said:
Most of what you're describing is dangerous as shit, though.
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Of course it is. All electrical work and wiring is dangerous.
And that danger is mitigated by being informed and doing the right thing - based on an interwebs education from forums and social media.
 
shawn said:
Says the guy who asked how to find out if something was 240v.
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I'd rather ask a dumb question than make a dumb assumption!
 
This thread is why I simply keep 2 drop chords at the generator. One cord goes to the basement freezer, one into the kitchen for the fridge and/or microwave ect.
 
What perfect timing. Our power went out at 2350, and I had the generator up and running the essentials by midnight. Ran the 2 cords, yanked the rope a few times and off to the races.
 
JSJJ388 said:
This thread is why I simply keep 2 drop chords at the generator. One cord goes to the basement freezer, one into the kitchen for the fridge and/or microwave ect.
Click to expand...

I do the same thing ..... just with 1 cord into the house and 1 into the basement (freezer).
The only difference is I use gen rated cords.
Biggest reason is, it's an old house that does not have a disconnect.
Power comes in from the pole straight to the panel.
It's called a double-split, double-6 or something an to kill power to the house you shut off the top breakers on each side.
 
Lizooki said:
I do the same thing ..... just with 1 cord into the house and 1 into the basement (freezer).
The only difference is I use gen rated cords.
Biggest reason is, it's an old house that does not have a disconnect.
Power comes in from the pole straight to the panel.
It's called a double-split, double-6 or something an to kill power to the house you shut off the top breakers on each side.
Click to expand...

That's on my list to get some heavy cords.
 
I used to pull out my panel and backfeed the whole house with my pto generator and tractor, now In the new house we put in a generac stand by unit runs the whole house and low maintenance
 
@RatLabGuy a 2 pole breaker with a jumper between the poles, one 120v leg feeding one side and a jumper feeding the other. Same voltage and power potential, but you split all the power between both bus bars.

I’ve done this in a couple off-grid homes that have an electrical panel but no 240 loads, and 120v inverter. You can either jumper the main lugs, or jumper a 2-p breaker.

It can’t run a 240 load but will accommodate for all the home’s 120 loads. Otherwise backfeed 240 thru a plug or gen hookup.

Electricity is sketchy; interface with it carefully.
 
336wheeler said:
@RatLabGuy a 2 pole breaker with a jumper between the poles, one 120v leg feeding one side and a jumper feeding the other. Same voltage and power potential, but you split all the power between both bus bars.

I’ve done this in a couple off-grid homes that have an electrical panel but no 240 loads, and 120v inverter. You can either jumper the main lugs, or jumper a 2-p breaker.

It can’t run a 240 load but will accommodate for all the home’s 120 loads. Otherwise backfeed 240 thru a plug or gen hookup.

Electricity is sketchy; interface with it carefully.
Click to expand...
But doesn't this also half what you can get for max current? At least vs the proposed situation.
If the genny has a 20A fuse on each of the 2 outlets, and you take 1 of those and tap it to both legs, you can still only pull 20A total across the whole house before it pops that fuse.
Whereas if you do as noted above - and run 2 separate wires, 1 from each genny circuit to each leg of the house, then you have 40A to work with (20A on each leg).
 
I was being tongue in cheek with 'it depends' but its also true.

There really is no standard on portable gens. Ive seen (what I would consider ideal) and 2 outlets pulled 180 degrees (just roll with it - not perfect) out and 2 separate 20A circuits....and Ive also seen models where the two outlets are pulled off a single alternator lead and daisy chained.

If you have 2 separate 120 outlets on the genny and you run two drop cords in theory you get full output of both "circuits"...

Also remember a 5000 watt gen is only good for 41A at 120.

And as voltage sags amps increase for a given load.
If anyone has one of the old school tank onans...go meter its 120 and ask yourself why your meter is reading 131V....less than 10% variance and...
 
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