1. No. The phases have to be synchronized (exactly 180 out of phase) for this to work properly or be legal. Yamaha does have a dohicky that you can use to slave these two together (they call it the combo kit) that will make it work.

2. Absent the dohicky to synchronize the generators, the voltage between the two legs likely will not be 240. This means your 240V loads won't work properly at all and you can overload the neutral on 120V only loads.

plus and minus?

read a book or something please before you kill yourself or burn your house down.

Wow, wow, WOW!

PLEASE call a pro. PLEASE.

From this post it is SO apparent that you really don't even begin to know what you are doing.

This is not meant as an insult, but this topic is too important to play around with.

This is directed to Ronnatalie:
Thanks for your thoughtful response. I have the item you referred to as a dohicky and combo kit. For the two generators, the Yamaha parallel running kit you referenced does not provide a 240 volt output but still outputs only 120 volts. The kit ties the two together together but increases only the amperage. It provides one 30 amp circuit as opposed to two 13.3 amp separate circuits. I will discuss the issue with Yamaha technical support to confirm that Yamaha does not offer another device that will allow the two generators in parallel to output 240 volts.

Please confirm that your number 1. answer of "no" means that one generator cannot be safely connected to the plus (right) side of the power panel and the other to the minus (left) side of the power panel.

For the two individuals failing to address my original two questions, I fully intend to confirm every answer received herein with a licensed electrician and likely an electrical engineer. I hope they both will have read a book and also prove to be a pro as Ronnatalie has with his initial response.

Ray, if you can get a licensed electrician to proof your work great. I hope he is thorough. It would be even better if he were there with you as you do the work.
I'm not sure what an EE will do for you. They can be pretty clueless when it comes to construction/building electrical work.

PLEASE stop referring to each side of your panel and "plus" and "minus". This is HIGHLY incorrect and only serves to confuse things.
NO, one genny cannot be on one leg of your service and the other to the other leg. You can use the parallel kit but you will still only have 120V. You would have to jury-rig a transfer switch or main-breaker interlock kit to feed your whole panel with 120V as opposed to 120/240V.
A MUCH BETTER solution would be to simply get the proper, and larger, generator.

Also, I truly FAIL to see how our responses were somehow unprofessional. I am a professional electrician, and I am good at what I do. I am NOT a professional message board free answer giver.

What is it you do for a living? Maybe you can give us some advice?

I am NOT a professional message board free answer giver.

I love the folks who don't like the answer and then take umbrage at the person giving the answer. I suppose most fail to realize how serious this stuff can be.

If you have an electrician and an electrical engineer, why get advice from strangers on an Internet message board?

The answer to the problem is exactly, what you said - get a properly sized generator.

I'm not familiar with the Yamaha product, but I seem to remember that Honda has a kit for this purpose.

No, you cannot connect two individual 120vac generators to create 240vac as you describe.

Though I've not seen it up close, the Honda kit for their small generator (2kw) synchronizes the generators and provides a neutral.

you're right... I shouldn't have even posted.

Please continue, you're in the running for a Darwin award.

"1. No. The phases have to be synchronized (exactly 180 out of phase) for this to work properly or be legal."
I agree completely. Unless the two generators used are identical AND connected precisely as the manufacturer specifies, the results will absolutely not be satisfactory and probably will be disastrous. I have never attempted such a connection.
A larger generator will cost much less than the total cost of two smaller ones plus the synchronizing device.

Yes, and in this case, the sync kit only runs them in true parallel (doubling the 120V amperage) without providing the ability to feed 240V loads. I was incorrect in my assumption earlier.

No, it does not appear that these options offer a 240 VAC output combination.

However with "inverter output" generators, the syncing of the phase is a really simple problem to solve electronically - home solar grid tie systems do it all the time by just "sniffing" the incoming AC line phase and matching the inverters phase to match it.

As busdriver pointed out, use of non-inverter generators would be a total crap shoot, unless specifically designed to do this - which I don't ever recalling seeing.

Can you expand on the term "sniffing" the AC incoming line phase in order to match the inverters phase to it generators?

The inverter in the generator isn't "sniffing" anything. They don't have a line in, so they don't care. The main reason that these small generators users an inverter is it allows the AC generated to be decoupled from the speed of the motor which makes the motor control and design.

In fact, no "sniffing" is required here. Just a simple tying together of the underlying oscillators.

Yes, it would be easier to design away to sync the inverters, since they can sync them together to run in parallel, it should be not so hard to sync them exactly opposite. Still this is nothing someone who barely knows what electricity is will be able to hack themselves.

By the time you get all that done, you might have well bought a 240 generator.

The sampling of the line frequency is used for renewable sources such as wind and solar so that the inverters they use will be synchronized with the power company. It would be very counterproductive for those sources to feed to the grid non-synchronized power.

For God's sake, just buy a 120/240v generator and be done with it.

"Can you expand on the term "sniffing" the AC incoming line phase in order to match the inverters phase to it generators?"

I think Ron's and Bus Driver's replies more than answered the question. Bus Driver's use of the word "sampling" is also much more correct than my causal term of sniffing. And, I agree 100% with btharmy's reply!

Just to clarify, my example was of how technology can make problems that were very difficult to solve even 10 years ago, are now very simple from a design standpoint.

Grid tie inverters used for alternative energy "sell back" must meet three very critical criteria. 1 - Match the exact phase of the AC grid. 2 - Meet the exact voltage of the AC grid. 3 - Shut the inverter down immediately in the loss of the AC grid - obviously to prevent back feeding.

Semiconductor manufactures have addressed this by offering single IC solutions and reference designs that make this a simple and inexpensive task. There's no reason generator mfgs could not use the same off the shelf solutions to offer options like the OP (just from the generator standpoint here) suggested.

It appears that many providing comments don't have a clue as to the manufacturing technology of the two generators identified. Let me attempt to enlighten you. Then you may understand why I plan to use them for some level of emergency power.

1) Each weighs 44 pounds and therefore can me moved simultaneously to any location without the help of another individual or lifting device.
2) The decibel level of each generator is 53. What that means is that both can be operated on full load and speed between two individuals having a conversation without raising their voices. Show me a large heavy generator that outputs 120/240 volts that has a decibel level of 53 and I will buy it. Most have a decibel level of 75 to 90. Such a generator cannot even be operated in most campgrounds due to the noise level. And what about respect for your neighbor's that will listen to the 120/240 output generators after bedtime with his windows raised.
3) The Yamaha generator will run at a speed as deemed necessary by the load. At 1/2 speed/load it consumes 1.1 gallons of gasoline in 11 hours. The generators proposed by some providing comments herein run constantly at full speed while consuming more fuel in an hour than the Yamaha EF200is consumes in 11 hours.
4) The Yamaha generators in parallel will run my 15,000 BTU RV air conditioner for about 6 hours at a cost of about $4.

Obviously, that low cost, 150 to 200 pound, gas guzzler that causes a loss of hearing will run a few more items in the household.

You can not practically connect these to your panel.

If you are in such love with these units, get some extension cords.

Ronnatalie:
According to other electricians, either one or both generators in parallel can be connected to one side of the power panel safely while also meeting code. Practically, they can be connected to the power panel as well as any 120/240 volt generator as long as the grid power is locked out and no 240 circuits are activated. Obviously, the downside is the limited circuits that can be activated simultaneously. That same downside to a lesser degree exists with your preferred 120/240 v generator.
Keep in mind that the Yamaha's output about 1/2 to 2/3 of the power of your preferred noisy gas guzzlers. Extension cords are not practical given the potential to connect through the power panel. Once I have had it connected by a licensed electrician I will update you. If I am wrong I will admit that too and let you know. I do appreciate your assistance. Your comments have been helpful in my tabling the idea of connecting them on both sides of the power panel. I believe some of the others commenting feel that their profession is one that can't be learned by a neophyte. By the way, I discussed the issue with a a relative last night that was grandfathered as a licensed electrician many years ago. He had no idea of whether the generators could be safely connected to the power panel. My point here is that there are electricians and there are jack legs who practice the trade. I am sure you know some of both types.

You seem to be obsessed with implying that all larger generators are "noisy gas guzzlers".

Since you seem to be SO educated about these things I find it surprising that you are not familiar with the Honda EU4500 or EU6500, or the full line of Yamaha inverter generators, including the EF4500 and EF6000. All are EXTREMELY quiet, provide an adequate amount of power for a small to average home in an emergency situation, and are VERY fuel efficient.

ANY of these can be had for about the same price as two smaller ones using the interconnect kit, which as you can see is ABSOLUTELY NOT an ideal setup for backup use.
Keep the small suitcase units in the field where they belong.

Then again, I get the distinct impression that you have your mind made up already and are just looking for justification.

You need to do your homework as to the price, quietness and efficiency of the Honda and Yamaha generators referenced. What is your definition of quiet and efficient??? When you have done your homework then tell me specifics and convince me that you know something I don't already know about them! Have you seen or heard one run??? Do you know what a decibel level of 53 sounds like versus 75???

Indeed, it appears that approbation is sought rather than advice.

That's *NOT* what you said. You said you wanted to backfeed a double pole breaker. There is no PRACTICAL let alone SAFE or LEGAL way to do this with these generators. Yes, perhaps you can move 120V loads to some sort of transfer switch on the thing, but again, you are going to end up spending more money for a less efficient, less reliable solution.

Ronnatalie:

Let's assume:
1) The breaker/s in the first two slots on the left side of the power panel are relocated downward.
2) Install A 30 amp double pole breaker in the first two slots of the left panel now vacant. This breaker will serve as a generator breaker. If later I connect a 120/240 v generator the other 120V leg will be connected with no other changes in wiring.
3) Install a mechanical interlock at a price of $69. Now, at no time can both the main power breaker and generator breaker be closed at the same time. See the following link: http://natramelec.com/genswitch/ch/ch5/ch5.htm.
4)Connect the 120 volt input line to the left leg of the generator breaker with the main breaker and all breakers of the left and right panels in the off position.
5) Start the generator.
6) Turn the generator breaker to the on position.
7) Close breakers in the left panel while not exceeding the output of the generator nor generator breaker. Never close any double pole breaker other than the generator breaker.
Obviously, one must calculate the usage of all items on the circuits closed.
Now, is it safe and will it pass code. Efficiency is of no concern. Why is this unreliable as compared to extension cords? Of course, one could install a sub panel rather than using the left panel with some closed and some open circuits.

You must put in the listed hold down for the backfeed breaker.

"Indeed, it appears that approbation is sought rather than advice."

Yup. It's pretty obvious.

OK, I'll play along just a bit longer.

Yamaha TWO EF2000 with parallel kit compared to ONE EF4500

Cost:
$2445 vs $3400 - EF2000's win
$.61/watt compared to $.76/watt

Fuel consumption:
EF2000 - .105 gal/hr x 2 = .210 gal/hr
EF4500 - .296 gal/hr
EF2000's wins by slight margin, almost negligible, although the EF4500 gives 500 more watts, I'm calling it even.

Sound:
EF2000's - 51.5 - 61 dBA each
EF4500 - 58-60 dBA
I'm calling the 4500 the winner.

Overall, even given the price I would call the EF4500 the clear winner, especially considering the simplicity of use and added wattage. Noe of that silly parallel cable kit.

I have used both Honda EU2000 and EU4500. Both are AMAZINGLY quiet. Even the EU6500 is VERY quiet.
To carry to a site or field work, the EU2000 cannot be beat. To use it to feed a home panel, even two in parallel is not even worth considering IMO.

Then again, I don't expect you to give any of this a second though. You already have your mind made up. I did all this simply to satisfy my own curiosity.

"According to other electricians, either one or both generators in parallel can be connected to one side of the power panel safely while also meeting code. "

I would be curious to know which NEC code it is that allows for a 120v generator to be attached to a 240v panel.

I also wanted to mention that if you only power one side, only every other 120v breaker will get powered (depending on the panel). This is why you can put a double breaker in and get 240v. The first 120v breaker is on one leg and the next breaker is on the other leg.

Petty-Racer:
You seem to have a problem making comparisons with your preferred Honda generators and mine:
First, the cost of my two were just under $2000 including the parallel cord. That makes your initial comparison in error my a factor of 22%. Today they can be purchased for significantly less than $2445 each.

Have you ever purchased anything using Amazon or ebay where they can currently be purchased for less than $2100 with the parallel cord? You have herd of internet buying before or do you always purchase everything from your local hardware store or Sears????????

As far as fuel efficiency, your own stats prove that the EF4500 uses 3 times (300%) more fuel than the EF2000. Yet, you believe its an even race. Where did you learn your higher 3rd grade math???????

Given the first two comparisons and your gross errors, the other comments/comparisons don't justify consideration.

Obviously, I like the generators I own and you appear to like the Hondas. Let's agree not to waste each others time again. If you care to offer positive input as Ronnatalie and weedneister have professionally done I'll consider what you have to say. Have a good day.

The description of what you want to install (panel interlock, 30 amp breaker, assumed power inlet plug) is what I have today. The correct way to do this is to wire both sides of the breaker and use a 4-prong NEMA plug (L1, L2, neutral and ground).

Then your question becomes can you connect L1 and L2 to a single 120v source. People have talked about this here before. Both sides of the panel would be driven by the same 120v source. The 240v loads would see 0v line to line, but 120v line to neutral. I would not want to predict how 120v/240v loads would react to this.

I am NOT recommending nor sanctioning this kind of connection.

"Petty-Racer:
You seem to have a problem making comparisons with your preferred Honda generators and mine:
First, the cost of my two were just under $2000 including the parallel cord. That makes your initial comparison in error my a factor of 22%. Today they can be purchased for significantly less than $2445 each.

Have you ever purchased anything using Amazon or ebay where they can currently be purchased for less than $2100 with the parallel cord? You have herd of internet buying before or do you always purchase everything from your local hardware store or Sears????????

As far as fuel efficiency, your own stats prove that the EF4500 uses 3 times (300%) more fuel than the EF2000. Yet, you believe its an even race. Where did you learn your higher 3rd grade math???????

Given the first two comparisons and your gross errors, the other comments/comparisons don't justify consideration.

Obviously, I like the generators I own and you appear to like the Hondas. Let's agree not to waste each others time again. If you care to offer positive input as Ronnatalie and weedneister have professionally done I'll consider what you have to say. Have a good day. "

MAN you are an arrogant F-, aren't you.

LOOK at what I wrote. Don't just glance over it.
All the numbers I gave were for BOTH EF2000's. NOT just one.
Second, I used Yamaha's list pricing as a guideline. You don't have to be an a$$hole and ask I know what Amazon or Ebay are.
Continuing on with exposing your own gross errors, the fuel comparison is .210 to .296, which I acknowledged the 2000's were better. But HOW is this 300%???

Lastly, I NEVER said I had a preference of Honda over Yamaha. I used Yamaha stats since that is what you have/prefer.
How about you READ and comprehend what is written, or just ignore me. Either way, why be a douche about it?

This final message is directed to those individuals who were certain that two Yamaha generators having an output of only 120 volts cannot be safely installed on a residential power panel. As I stated before, it can and has been done by a master electrician with 40 years of experience. The generators have been run in parallel using the manufacturer's parallel cord. Each side of the power panel has 120 volts. By the way, the neutral leg has not been overloaded! A Cutler Hammer mechanical interlock cover has been installed making it impossible to close the main breaker and generator breaker at the same time.

I have simultaneously run a refrigerator, ceiling fan, window fan, curling iron, two bathroom exhaust fans and lights in a bedroom, bathroom and closet. At no time do the generators go into an overload mode nor does the generator breaker trip. Wattage use was verified for all items prior to activating the circuit. The total wattage of all items (including surge start-up) is less than 2900.

The cost of the installation including labor was about $400.
If a 120/240 generator is later used, it can be used with minor connections and disconnections (no rewiring).

What did you do about your 240v loads?

If you are referring to the 240A breakers, they are all opened/off along with all other unused 120V circuits. .

If you REALLY wanted 240 volts out of those generators, a center-tapped step up transformer would be your huckleberry.

I would suggest that you mark those 240v breakers to be turned off during generator usage so that someone doesn't come along and turn them on by mistake.

This happened to a friend of mine. Long story...

After reading this all I can say is WOW.. Electricity is one of the most unforgiving forces we subject ourselves to and it is always best to play it safe when around these forces. This is why we wear motorcycle helmets and seatbelts, safety glasses, fall protection etc. Everyone has brain dead moments that every cylinder is not firing 100% all the time and we take measures to help protect ourselves and others from that.

Don't give safety a second glance or you might need an am-bu-lance... well or possibly a fire truck.

Can you make this work like many other things YES... but you would be much much better suited with a 240v unit.

You are an error or two from throwing enough money away to buy a really nice 240v inverter generator or worse.

If it's an ambulance, you still got a chance.
If it's a hearse, it's gotta be worse.