# Multiple Generators



## JohnnyElectric (Aug 30, 2013)

Can I install two standby generators on one residential service?

I have a customer who had a 12K Generac that failed. He bought a 20K Milbank which we installed to feed the existing generator panel after disconnecting the Generac.

He's had the Generac repaired and now he wants to add another generator panel and use the old Generac and it's transfer switch to feed additional circuits in an outage.

I can't find anything that say you can't do this, but I also can't find anything that says you can.

My concern is the neutrals. Because the neutrals of both generators would be tied to the utility neutral, they're tied together, I'm concerned that because the generators are not synced, there could be an issue with two seperately created neutrals being connected. 

After all through lights or appliancesbeing operated, they're also connected to the hot conductors. In the same way losing the on a service can screw up the voltages on the hots, I'm concerned that two different active power sources with connected neutrals can be a problem.


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## JohnnyElectric (Aug 30, 2013)

Losing the neutral on a service I meant to say.


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## Dennis Alwon (May 9, 2009)

I guess there could be an issue with overloading one generator as current from one would go to both generators on the neutral but you could use a transfer switch that switches the neutral making it an separately derived system.


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## BBQ (Nov 16, 2010)

Multiple generators connected to one service with multiple solid neutral transfer switches is not uncommon and not a problem.

All the neutrals are grounded, they are at the same potential (or very close to it) and will be little if any current flow on it.


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## HackWork (Oct 2, 2009)

Dennis Alwon said:


> I guess there could be an issue with overloading one generator *as current from one would go to both generators on the neutral* but you could use a transfer switch that switches the neutral making it an separately derived system.


How would that happen?


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## Dennis Alwon (May 9, 2009)

If both generators are on and the neutrals are all tied together then current will flow in all directions-- NO?


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## HackWork (Oct 2, 2009)

Dennis Alwon said:


> If both generators are on and the neutrals are all tied together then current will flow in all directions-- NO?


I only know the basics when it comes to this, but I always thought that electricity only flowed to the source.

The same way as how when I connect a portable generator to a house and use an interlock or transfer switch the electricity goes back to the generator and not out into the grid via the neutral that is still connected.


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## Dennis Alwon (May 9, 2009)

How would the neutral correct know which source to go to. Electricity will take all paths back. If you have two circuits in a box and tie the neutrals all together- which is not compliant- current will run on both neutrals.

If the switch switched the neutral then this would not happen.


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## HackWork (Oct 2, 2009)

Dennis Alwon said:


> How would the neutral correct know which source to go to. Electricity will take all paths back. If you have two circuits in a box and tie the neutrals all together- which is not compliant- current will run on both neutrals.
> 
> If the switch switched the neutral then this would not happen.


In a 2 generator setup, which neutral do you think current will flow thru and overload?

FWIW, I've seen and worked on many multi-generator systems in hospitals and I don't remember any neutral switching.


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## cuba_pete (Dec 8, 2011)

Dennis Alwon said:


> If both generators are on and the neutrals are all tied together then current will flow in all directions-- NO?


 Kirchoff seemed to think so...


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## Dennis Alwon (May 9, 2009)

HackWork said:


> In a 2 generator setup, which neutral do you think current will flow thru and overload?
> 
> FWIW, I've seen and worked on many multi-generator systems in hospitals and I don't remember any neutral switching.


I don't think it is a very likely thing to happen but I am wondering if it is compliant without a switched neutral in the system. 

Unless the impendence for one generator was so much higher then the other I don't see a serious issue of overload.


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> I don't think it is a very likely thing to happen but I am wondering if it is compliant without a switched neutral in the system.
> 
> Unless the impendence for one generator was so much higher then the other I don't see a serious issue of overload.


It takes a complete circuit to make current flow.

Each ungrounded conductor is connected to its own generator, there is no way in a properly wired system that current will flow from a load supplied by generator 1 to the neutral of generator 2.


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## HackWork (Oct 2, 2009)

Dennis Alwon said:


> I don't think it is a very likely thing to happen but I am wondering if it is compliant without a switched neutral in the system.
> 
> Unless the impendence for one generator was so much higher then the other I don't see a serious issue of overload.


I don't see how it would overload a generator.

I could see, in some rare situation, current from gen 1 flowing thru a neutral with current from gen 2 since, as you mentioned, electricity will take all paths. But I don't see how that would happen to the point of overloading it.


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## cuba_pete (Dec 8, 2011)

The issue I see here, by using two sources in one residence, is that during dual use of the generators and each is running, say, 180 out from each other (since they are not synced)...

...there is a possiblity of the toaster receiving 120V up and the fridge getting 120 down, which woudn't be cool (plasma hot as a matter of fact).

Not to mention the 220V circuits possibly running different phase rotations.

It would be a pain in the arse trying to sort that out in one panel. It would be easier to run separate panels that the electrician knew fed distinct circuits which would not allow the inhabitants to fry themselves.

...then again...there are extension cords...


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## cuba_pete (Dec 8, 2011)

BBQ said:


> It takes a complete circuit to make current flow.
> 
> Each ungrounded conductor is connected to its own generator, there is no way in a properly wired system that current will flow from a load supplied by generator 1 to the neutral of generator 2.


 How about through the connected neutrals in the panel, through the bond to the neutral of the second gennie? I sure _hope_ they all use the same ground.

That's rhetorical...that is how it would happen.


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## BBQ (Nov 16, 2010)

cuba_pete said:


> How about through the connected neutrals in the panel, through the bond to the neutral of the second gennie? I sure _hope_ they all use the same ground.
> 
> That's rhetorical...that is how it would happen.


Its not how it would happen, there is nothing to make the current want to go to the second generator. 

Electrically the OPs installation would be no different than two neighbors supplied from the same utility transformer each having their own generator.


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## Dennis Alwon (May 9, 2009)

BBQ said:


> It takes a complete circuit to make current flow.
> 
> Each ungrounded conductor is connected to its own generator, there is no way in a properly wired system that current will flow from a load supplied by generator 1 to the neutral of generator 2.


Is not generator one and generator two and the power company neutrals all tied together? I do not see how you can stop the current from one going to the other but then I don't deal with this stuff.

The circuits from generator 2 will know not to flow to a neutral in generator 1?


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## BBQ (Nov 16, 2010)

cuba_pete said:


> The issue I see here, by using two sources in one residence, is that during dual use of the generators and each is running, say, 180 out from each other (since they are not synced)...
> 
> ...there is a possiblity of the toaster receiving 120V up and the fridge getting 120 down, which woudn't be cool (plasma hot as a matter of fact).


None of that is possible without a transfer switch failure. The loads are electrically isolated from each other by transfer switches.


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> Is not generator one and generator two and the power company neutrals all tied together?


That is my understanding of the OP.



> I do not see how you can stop the current from one going to the other but then I don't deal with this stuff.
> 
> The circuits from generator 2 will know not to flow to a neutral in generator 1?


In this case the electrons will know the way, they want to go back to their source and only their source.



> If you have two circuits in a box and tie the neutrals all together- which is not compliant- current will run on both neutrals.


Keep in mind in this case the neutrals are only tied to together at the supply end, not at the load end as well.


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## gotshokd666 (Oct 17, 2012)

Yes, you definitely can have multiple generators on the same house. I've done that a few times, and didn't switch the neutrals.


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## Dennis Alwon (May 9, 2009)

BBQ said:


> Keep in mind in this case the neutrals are only tied to together at the supply end, not at the load end as well.


How? if the neutral isn't switched. The example I see is one service with say 2- 200 amp panels. Each 200 amp panel has a transfer switch ahead of it. 

I see the neutral loads coming back to the transfer switches where all neutrals are tied together. I guess I am not seeing the difference.

I see the neutral coming back to each panel but then the neutrals are tied together to both generators. How is that different from two circuits with two neutrals going back to the same source. Does the neutral know what source is feeding the circuit?


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## cuba_pete (Dec 8, 2011)

BBQ said:


> Its not how it would happen, there is nothing to make the current want to go to the second generator.
> 
> Electrically the OPs installation would be no different than two neighbors supplied from the same utility transformer each having their own generator.


Neutral currents from one residence to the other are fairly common (no pun intended), but certainly not objectionable.

The distinction here, which _is_ different, is that both generators and the service are all using the same ground. At least they should be.


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## cuba_pete (Dec 8, 2011)

BBQ said:


> Keep in mind in this case the neutrals are only tied to together at the supply end, not at the load end as well.


I am going to need to see your drawings for _that_ one.


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## hardworkingstiff (Jan 22, 2007)

I look at it this way, all neutrals (PoCo, Gen 1, Gen2) are tied together at the service disconnect/transfer switch. All the neutrals from the house circuits are also connected to the incoming power, so in essence, all neutrals are tied together. 

Let's say all 3 power sources are active. They put potential on the 2 ungrounded conductors and the grounded conductor. Remember though, there is only one path for current from the neutral bar back to each source. So, Gen 1 and Gen 2 have current flowing out of the "hots", going through the load and coming back to the neutral bar. (BTW, the PoCo has the ungrounded conductors disconnect at the transfer switch). 

Now, let's say Gen 1 has an unbalanced load (neutal current) of 10 amps and Gen 2 has an unbalance current of 15 amps. So this 25 amps shows up on the neutral bar looking for it's source. Well Gen 1 is looking to pull back 10 amps and Gen 2 is looking to pull back 15 amps. Each Gen will "suck back" the amount of current it neads to balance the load. 

Now, the current (the actual physical electrons, if you can call them physical) that flows through a Gen 1 load may go back to Gen 2, but the Gens don't care, they just want to balance the current flow with whatever electrons they find.


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## Church646 (Jul 17, 2013)

I've worked at a hospital with two 800KVA generators. They were always in synced. The ATS switches would transfer the line - in this case 3 phase 480. Neutrals were derived from the transformers down the line. Once there was a problem with the sync controls and bad things happened. This is probably an extreme example but the resulting fire burned up both generators and the 5000A switchboard. The fire dept. and insurance companies still don't know what happened. (Fire occurred in 09) I suspect power was inadvertently put on the neutral but I have no proof - its just a guess.


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> Does the neutral know what source is feeding the circuit?


No, but the sources doesn't care, it just is looking for any electron that will fill it's need to balance it's output of electrons. Any electron will do.

I think your concern is an electron leaving source 1 will come back to source 2. Well, it might, but the sources just don't care.


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> No, but the sources doesn't care, it just is looking for any electron that will fill it's need to balance it's output of electrons. Any electron will do.
> 
> I think your concern is an electron leaving source 1 will come back to source 2. Well, it might, but the sources just don't care.


I understand the source doesn't care but my orig. pt was the current would flow to the other gen. thru the neutral and others said it wouldn't. I was trying to understand where my thinking was off. 

Now if the neutral on one genny failed what would happen


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> Now if the neutral on one genny failed what would happen


Nothing except the loads connected to that generator would stop working.


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## BBQ (Nov 16, 2010)

Church646 said:


> I've worked at a hospital with two 800KVA generators. They were always in synced. The ATS switches would transfer the line - in this case 3 phase 480. Neutrals were derived from the transformers down the line. Once there was a problem with the sync controls and bad things happened. This is probably an extreme example but the resulting fire burned up both generators and the 5000A switchboard. The fire dept. and insurance companies still don't know what happened. (Fire occurred in 09) I suspect power was inadvertently put on the neutral but I have no proof - its just a guess.



That is a situation where the generators where in parallel, that is not the case here.


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## cuba_pete (Dec 8, 2011)

BBQ said:


> Nothing except the loads connected to that generator would stop working.


 Not if all of the neutrals were bonded properly to being with. The current would flow through the neutral of the second gennie through ground to the bonded neutral/ground connection the of failed gennie to complete the path.

If the bond was completely failed it would set up a difference of potential between the failed gennie and ground and bad things may occur as the operator approaches and makes the link between ground and the gennie while troubleshooting.

In the meantime neutral/return would be found through ground and, hopefully, circuit interrrupton occurs.


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## SteveBayshore (Apr 7, 2013)

We work on a water plant with multiple generators and multiple transfer switches all being fed from a single service. Over the years more pumps and processes have been added and the service size increased. Rather than making an existing generator obsolite, more generators were added over time. Site now has (1) 500kw, (1) 100kw, (1) 250kw and (1) 350kw that we added two years ago. We installed the 500 about 20 years ago. Next upgrade will include replacing all with one unit.


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> ...t my orig. pt was the* current would flow to the other gen*. thru the neutral and others said it wouldn't. I was trying to understand where my thinking was off.


What current? I'm not sure I understand your concern?


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## Dennis Alwon (May 9, 2009)

BBQ said:


> Nothing except the loads connected to that generator would stop working.


They wouldn't travel back thru the neutral of the other genny?

I guess that is why the utility doesn't need their neutral to be disconnected from the genny.

Obviously me electrical theory sucks


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## BBQ (Nov 16, 2010)

cuba_pete said:


> Not if all of the neutrals were bonded properly to being with. The current would flow through the neutral of the second gennie through ground to the bonded neutral/ground connection the of failed gennie to complete the path.
> 
> If the bond was completely failed it would set up a difference of potential between the failed gennie and ground and bad things may occur as the operator approaches and makes the link between ground and the gennie while troubleshooting.
> 
> In the meantime neutral/return would be found through ground and, hopefully, circuit interrrupton occurs.


I am pretty sharp on this stuff and I am not following you.

Here is a quick and crude diagram of what I think the set up would be.

NET = Neutral

G1 = Generator 1

G2 = Generator 2

MTS = Manual transfer switches

U = Utility supply.

Of course I drew it as two wire 120 volt but adding the second leg would not change things.


Now can you tell me where current will flow improperly?


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## macmikeman (Jan 23, 2007)

I have been laughing at the posts in this thread for ten minutes while reading them. In the op the guy is talking about backing up existing loads at a dwelling. So therefore it is safe to assume that the neutral busbar is sized to the loads to be expected in that dwelling. You could run a generator and transfer switch for each circuit individually and not have any problems in that setup, the neutral busbar will not see any loading in excess of the sum of the loads it serves, which is expected when sizing the service. There will be no "spreading of neutral currents " thruout the common connection at the service neutral. I thought you guys were electricians. Nothing in the NEC to prevent multiple sources, but a fortune in descriptive signage to be located at the premises.


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> I see the neutral loads coming back to the transfer switches where all neutrals are tied together.


The neutrals are all tied together at the supply end, but they are sperate at the load end, you can't have current flow like your example of tying neutrals from separate circuits together at a switch box.


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## JohnnyElectric (Aug 30, 2013)

I'm not an electrical engineer, I'm an electrician. A home standby generator isn't normally used when the utility power is being used. So when during an outage, when the generator is supplying power to the generator panel, there is no unbalanced load from the non generator supplied circuits normally supplied by the utility.

But with two separate generators, not synced, supplying two seperate generator panels, but with the neutrals tied together as that's the way the transfer switches are set up, you would have the unbalanced load from both connected.

I don't know if this is a problem to be considered, or not. But before I do such a thing I want to know for sure.

Because the voltage from the unbalanced load from one generator has the potential to be at a different frequency, and out of phase with the unbalanced load from the other. 

I guess that it isn't different from two neighboring houses running generators in an outage, except that it's happening within the house, which might make a difference as the conflict could happen before it would be dissipated into the earth. 

I've never seen it done before, and there might be a good reason for that.


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## Bbsound (Dec 16, 2011)

This thread is so full of wrong information.......

2 generators each with its own transfer switch, neutrals all bonded correctly will work just fine


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## bkmichael65 (Mar 25, 2013)

...........


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## macmikeman (Jan 23, 2007)

Like I said, just don't forget your multiple source affixed signage.


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## cuba_pete (Dec 8, 2011)

*separate/not separate*



BBQ said:


> I am pretty sharp on this stuff...





BBQ said:


> I am pretty sharp on this stuff and I am not following you.quote]
> 
> I know you are...:thumbsup:
> 
> ...


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## hardworkingstiff (Jan 22, 2007)

Bbsound said:


> This thread is so full of wrong information.......


Maybe you could point out what you are talking about and post the correct information. That way we could all learn.


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## pete87 (Oct 22, 2012)

BBQ said:


> That is my understanding of the OP.
> 
> 
> 
> ...


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## Chris1971 (Dec 27, 2010)

Bbsound said:


> This thread is so full of wrong information.......
> 
> 2 generators each with its own transfer switch, neutrals all bonded correctly will work just fine


I agree, they are operating independent of each other by separate transfer switches. It shouldn't be an issue.


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## JohnnyElectric (Aug 30, 2013)

Okay, think if it this way. Current doesn't follow the path of least resistance, it follows all available paths
proportionate to resistance. It's the reason we separate the neutrals and grounds on circuits.

There is voltage and current flow on the neutral, you don't read it with a meter to ground because they're connected at the source, but open a connection and you will read voltage, so it's there.

If two voltages, out of phase with each other are connected they will flow between each other, right? It's how 240 volt systems work. So if two neutrals from two generators that are not synced are carrying voltage and current from the unbalanced load, are tied together, what would happen?

The transfer switched provided with home standby generators do not open the neutral, so the neutral from the generator is solidly connected to the neutral from the utility, which isn't carrying an unbalanced load when there's an outage and the generator's supplying power. But with a second generator supplying power, and also a second unbalanced load solidly connected to the same neutrals, could bad things happen? 

Because the voltage from that second unbalanced load would almost certainly be out of phase with the voltage from the first one. They would want to flow between each other, at least that's how I see it.

But am I right?


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## BBQ (Nov 16, 2010)

JohnnyElectric said:


> But am I right?


No..


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## BBQ (Nov 16, 2010)

JohnnyElectric said:


> If two voltages, out of phase with each other are connected they will flow between each other, right? It's how 240 volt systems work. So if two neutrals from two generators that are not synced are carrying voltage and current from the unbalanced load, are tied together, what would happen?


Current cannot flow in the situation above because it is not a complete circuit. There is only one wire from each of the three sources (2 gens and utility) hooked together.


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## cuba_pete (Dec 8, 2011)

BBQ said:


> No..


 Damn you, Kirchoff!!!


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## 8V71 (Dec 23, 2011)

cuba_pete said:


> Damn you, Kirchoff!!!


Still having doubts..eh?


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## Bbsound (Dec 16, 2011)

hardworkingstiff said:


> Maybe you could point out what you are talking about and post the correct information. That way we could all learn.


Well, it starts out with basic electrical theory...................


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## ScooterMcGavin (Jan 24, 2011)

Try thinking about it this way:

Take two AA batteries. Tie the negatives together and ground them. From each positive you run it through a lamp back to the negative. Even if one lamp is twice the wattage of the other the current can only flow through the battery to which the lamp is connected. Just because the two negatives happpen to be connected and are grounded doesn't change this.


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## hardworkingstiff (Jan 22, 2007)

Bbsound said:


> Well, it starts out with basic electrical theory...................


Your input is greatly appreciated. Thank you for your insight.


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## captkirk (Nov 21, 2007)

If I heard it correctly kohler will be offering the 20 with a parallel system as an option. If its true it sounds like a great idea...


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## Going_Commando (Oct 1, 2011)

It will work fine. You aren't going to draw more neutral current than is being carried on the ungrounded conductors to the generator, because you are completing the circuit on a coil. A generator will power whatever load is connected to it, and only that much. It's all about the electron flow, bro.


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## Jlarson (Jun 28, 2009)

:blink: < Me reading this thread


:laughing:


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## BBQ (Nov 16, 2010)

Jlarson said:


> :blink: < Me reading this thread


I tried, I really did.




> :laughing:


 Stop laughing at my drawing.


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## HackWork (Oct 2, 2009)

Over at another forum I came up with the awesome idea to run a generator backwards in order to produce gasoline. I think with the help of this thread we could do even better.

If we connect a generator to a house with transfer equipment that doesn't switch the neutral, and run it when the power from the utility is still on, the generator should get a lot of the electrons coming back from the utility over the neutral. The neutral is still connected to all other houses on that tranny, so it should work int he spirit of this thread.

So that means the generator would be running for free? Or making gasoline? I guess that would depend on how much electrics your neighbor is using.


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## macmikeman (Jan 23, 2007)

HackWork said:


> Over at another forum I came up with the awesome idea to run a generator backwards in order to produce gasoline. I think with the help of this thread we could do even better.
> 
> If we connect a generator to a house with transfer equipment that doesn't switch the neutral, and run it when the power from the utility is still on, the generator should get a lot of the electrons coming back from the utility over the neutral. The neutral is still connected to all other houses on that tranny, so it should work int he spirit of this thread.
> 
> So that means the generator would be running for free? Or making gasoline? I guess that would depend on how much electrics your neighbor is using.


I remember that thread. It was a very insightful one. Too bad it got nuked


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## HackWork (Oct 2, 2009)

macmikeman said:


> I remember that thread. It was a very insightful one. Too bad it got nuked


The oil companies sent a spy in to delete it


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## macmikeman (Jan 23, 2007)

Well, I set my reverse still up and produced 9,000 barrels of corn out of 100 jugs of moonshine fuel.


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