If you really want to understand this, you need to bone up on residential power distribution. The difference between 120 and 240 V is more than 2x. With 240V you have two hot legs and a neutral. With 120 you have one hot leg and a neutral conductor. Both need a ground for safety. In your home, about half the lighting a convenience plug circuits are powered on each leg (120V). Big loads like air conditioners ranges and dryers are powered on both legs (240).

You should be able to come up with a way to use a 240 (two leg) transfer switch with the Honda generator with an adaptor cord. Later, with a bigger generator, you ditch the adaptor cord and hook up your bigger generator. I doubt the result of the exercise will be attractive to you.

If this is a simple transfer switch to energize your whole service panel, you must have all the circuits you want to use on one leg. That will mean moving a lot of cables around in the panel.

If this is a transfer switch with circuit breakers designed to run selected circuits, you will have to rework the transfer switch after installing a larger, 240V generator. If you do not you will only be using one leg, which is half the capacity of the new generator.

Hi MightyAnxious,

(Great name btw). The short answer is yes. Honda sells just such a kit so does Gen Tran (see link).

They key thing to understand is what ionized mentioned as "legs". Another way of explaining this is Electrical Phases. Almost all homes in the US are 120v and have two phases. Electrical power to your home is AC = Alternating Current. This power is a wave and it is +ve and -ve and reverses 60 times a second, 60 Hz. Your home has two of these phases hooked up and they are 180 degreese out of phase. This means when one phase (or leg) is max +ve, the other one is max -ve.

In your house most things work off a single phase. Some, e.g. an electrical dryer or well pump may work off both phases. A dryer like this would have a special plug with four prongs on it. Earth, Neutral, Phase 1 hot and Phase 2 hot.

OK now lets get to the Honda EU2000i. Each EU2000i when operating in parallel mode generates a phase. They can "sync" up with each other so that you get the 180 out of phase relationship. They can do this because the Honda EU2000 has power electonics in it, an inverter, which makes this happen.

I would check in with your honda dealer to get a kit for this. The honda web site

http://www.hondapowerequipment.com/products/generators/content.aspx?asset=gg_parallelcapability

Also it may be worth talking to your local RV (Rec Vehicle) place. Many camper types use this sort of two phase set up to go camping, so are familliar with the technology.

warmest regards, Mike.

Here is a link that might be useful: Example parallel hookup

I am aware that some inverters can sync to allow 120/240, 60 Hz operation. They are common for small scale electrical generation from renewable sources. I see no information on the honda power or the other link that indicates that these particular inverters can do that. It would be cool if they could, but I don't see it.

Just a couple of semantic corrections to stinkytiger's post.

First, our alternating current "reverses" 120 times per second. It takes 2 of these reversals to get back to where you started on the waveform, which is therefore measured as 60 Hertz (cycles per second).

Second, North American power is called single phase, not "two phase". While it is true that the two legs of power supplied to most residences are 180 degrees out of phase with each other, standard nomenclature does not call this "two phase" to remove confusion to a true "two phase" power supply that was offered in the early 1900s. (Not sure if it's still available anywhere today).

Kurto, you may be able to clear up something that has bothered me for some time. Why is typical 240 household US electrical service called single-phase and not two-phase? It always seemed, to me, to be two-phase and we must have been stuck with the nomenclature because of some rather silly convention or mistake. I have never seen an explanation.

How can some historically-significant two-phase" be any more "two-phase" than what most of us currently have?

Can I ask a related question on this subject?

I am going to have my generator wired into the house for standby use.

It is a Honda 6500watt inverter series. It has the switch for 120v only, and 120/240v. It has two twist lock outlets. One is three prong, the other is four prong.

I wanted to use an interlock kit made for my house service panel, (200a GE Powermark Gold) GE makes the Kit for their panel, to be able to power the whole house circuits without the extra wiring involved with a transfer switch.

I realize I need to only use specific appliances while the generator is going not to overload it... If it is wiser to go with a transfer switch and separate panel let me know please.

My question: Which cord the 3 or 4 prong is run into the house, and do I set the generator switch for 120v only or 120/240?

I'm going to have an electrician wire it but I'd like to know ahead of time what the difference in the two choices are so I'll know what the electrician is talking about when he gets here.

Also, the Honda has a floating ground. Does the generator need to be grounded outside for the house wiring to be grounded properly, for gfi outlets to work?

Thanks alot for any help!!

Ionized, the thinking goes something like this: The power to your house comes from a single phase of the three phase power that's being generated. We just center tap the 240 volt transformer to provide two 120V legs that by definition are out of phase with each other. These are not derived from two windings on the generator.

Further, one of the key attributes of a multi-phase supply is the ability to supply torque to start a motor without an outside agent. With the two legs of our system pushing (or pulling) in exactly opposite directions, there is no such advantage.

In the original two phase systems, like the generators installed in Niagara Falls in the late 1800s, there were two phases of power, one being 90 degrees ahead of the other. Eventually, we figured out that 3 phases, each 120 degrees out of phase with respect to the others was a much more efficient generation and transmission mechanism.

So in short, think of it from the generator and transmission perspective, and you won't have any problems with the naming.

You should create a separate thread for this since you have some good questions.

I am not a licensed electrician, so my opinions mean squat.

Depending on your jurisdiction, an interlock may be sufficient. But it would be incumbent on you to know beforehand which circuits you will be powering and which need to be turned off. You should then put explicit instructions on the panel where anyone can see them telling which circuits to turn on and which to turn off during generator usage.

Putting these circuits into a subpanel powered via a transfer switch simplifies that a bit since the only circuits powered are those in the subpanel.

With the generator, always use the 120/240. That would be the 4-prong.

And is it a floating ground or a floating neutral?

Wow, Kurto, that is an interesting history. It clears up a lot. From my side of the utility pole, it looks a lot like two-phase, but not from the other side.

Dear Mr. Santim in Pensilvania,

I think for a house backup you need the four prong one. The wires in the four prong are: Earth, Neutral, Live phase 1, and Live phase 2. You sometimes need the two phases because some things, e.g. a well pump needs both phases, a 240v supply.

There is a relationship between (a) Grounding, (b) Bonding or ground and Neutral and (c) if you switch your neutral or not. Rather than confuse you, and since I am not an electrician, I defer to the web sites where the expert can explain this to you. Your electrician should also know this.

In general to bond or not to bond at the generator will depend on the transfer switch / wiring set up that your eletricial will install.

Best, Mike.

Here is a link that might be useful: Cummins Power Generation

"Why is typical 240 household US electrical service called single-phase and not two-phase? "

Because it is single phase.

Two phase power uses 90 degrees of phase differnece, not 180 degrees.

The two phase system could not share any wires between phases, so it needed 4-wires to operate.

3-phase requires only 3 wires (delta configuration) but is often run as 4-wire (3 phase conductors and a neutral in a Y configuration).
Y has an advantage over delta if any imbalance exists.
In a Y system the imbalance is on the neutral, in a delta system it results in circulating currents that can damage transformers.

Any number of actual phases above single phase )2-phase, 3-phase) allows for simpler induction motor design.
You no longer need a start winding, start capacitor, or start centrifugal switch.
you simply apply th evoltage to the motor and it develops starting torque and runs.

On single phase power you need a way to produce a phase shifted voltage to allow starting.
A capacitor is used to produce something less than 90 degrees and the drive the start winding to get things rotating.
This capacitor is usually rather large in value and must deal with the voltage supplied. They are pretty far from ideal and dissipate a lot of power that heats them up.
For this reason they are removed from the circuit once the motor reaches enough speed for momentum to carry the rotor around a full turn.

A run capacitor can also be designed into a motor to help provide some extra power to keep the rotor turning under load.
They are mush smaller values, are less inefficient, and can be left in the circuit all the time the motor is on to help produce a small amount of additional output power.

Let me start out by saying that I am not being negative here. I'm happy because I think I understand something that has bothered me for years.

"Because it is single phase.

Two phase power uses 90 degrees of phase differnece, not 180 degrees."

That is what is confusing to me. For years, I have looked at the wave forms for three-phase power and the wave forms for "single phase" 240V and its components used for 120V and the "single phase 240" does not make any sense. My question is always, why is three wave forms separated by 120 degrees three-phase and two wave forms separated by 180 not two-phase is always answered with the unsatisfying "because that is the way it is" more or less.

With the added information provided by kurto and brickeye I can see that the, apparently, engineering, definition of two-phase is very restrictive. You need to know the jargon to understand it. Knowing some of the history is also pleasing.

In plain English two-phase power should be alternating current of any two different combined wave forms. That is, unless I remember my kindergarten physics all wrong. Instead, the engineering definition is that two-phase power is two alternating-current wave forms that is useful for starting motors. Does that sum it up?

The next question might be, with three-phase power, do the wave forms have to be separated by 120 degrees?

Thanks, brickeye and kurto, for pointing out why 90 degree two phase transmission is inefficient I guess two phases are all that are needed, but three are cheaper for the POCO in the big picture. Thanks brickeye for why start capacitors are removed after starting, what run capacitors do, and why I can't have unbalanced 220V loads on the wild leg of what the POCO calls "120/240 open delta" power at my house.

ionized, yes, it's a convention that we choose to look at residential power as single phase instead of two phase. From a physics stand point, the two legs are definitely 180 degrees out of phase, and could be called "two phase". But to reduce confusion, and because of the way that it works with motors, it's called "single phase".

A generator is basically just a big motor running in reverse. For large generators, there are 3 poles (windings), spinning around a central shaft. There are 360 degrees in a circle, and I suppose you could arrange the poles at any interval, but dividing them into equal 120 degree separation helps to physically balance the generator and the power production.

"In plain English two-phase power should be alternating current of any two different combined wave forms. That is, unless I remember my kindergarten physics all wrong. "

So what?

This is not "plain English" but a description (and definition) from a technical field.

And kindergarten physics is pretty far from any definition of multi-phase electrical systems.

The type of windings required to generate the power also come into play.

Another big issue is that phases cannot be created using simple transformers.

There is no way to create 3-phase power with a simple transformer from single phase.
Rotating machinery is required.

There are nos some solid state 'tricks,' but they are pretty far from a simple transformer.

If you use one of the 3-phases you have a single phase.
If you use a center tapped transformed you can split the voltage and create two legs (en Edison circuit) but it has never been considered two phase.

Technical fields have their own nomenclature.

Usually, jargon from a technical field is unintelligible to people outside of that field. that is bad enough and is often avoidable. In this case, the jargon is misleading. That makes it unusual. Now, if everyone would call it "split phase 120/240" or something like that instead of single-phase, it would be much more satisfying.

What makes it really frustrating is that when I have asked, probably a dozen or more times over so many years, why typical residential power is not two-phase, I get dismissive answers. Maybe the answers came mostly from people that I thought would understand more than I do, but really don't and don't want to admit it.

I just thought of another point for the philosopher-engineers to critique as I re-read the previous posts. I have single phase power at my house (well, most people do) when connected to the POCO. That is because the power originates from a single winding on a generator. When the power goes off and I fire up my back-up generator, I have is two-phase. Is that correct?

Ionized, yes, the back-up generator (really an alternator) usually has 2 poles, and could be looked at as a two phase generator. But there are no devices (of which I'm aware) that use two phase power, when the phases are 180 degrees out of phase. You either use a single 120V leg, which has a single phase, or you connect two 120V legs, which present as a single phase 240V sinusoidal waveform.

So while you can argue that it's misleading to call it single phase, since it's generated (at least on the grid) as a single phase, and it's only used as a single phase, wouldn't it be misleading to call it anything else?

My point of view is still on the user�s side of the meter so if it walks like a duck and quacks like a duck, I think it should be called a duck. What have learned from this thread is that it looks very different from the generation end and from there it honks like a goose. Often I talk with smart people that are looking at the same thing from different points of view from different backgrounds. Sometimes it is difficult to get past the jargon. In this case, if everyone would call the residential power "split phase" there would be no problem from my point of view.

I just thought of another ponderable. If my common generator is two-phase 120/240 and the POCO supplies me with split phase or single phase 120/240, what does my inverter generator do? That sort of takes it full circle to the original posting that I managed to hijack.

Since it is probably really two inverters that are synchronized, it is probably more analogous to a generator with two windings.

All of them do the same thing, supply a usable single-phase electrical supply at either 120V or 240V. Again, there's no use for two phase 120V power (that's out of phase by 180 degrees), except to combine those wires and use them as 240V single phase. By convention, the electrical engineers of North America have decided to not refer to a power system by an unusable configuration, even if it does technically exist.

"...engineers of North America...."

Elsewhere it is done differently?

Laws of physics are universal, but power distribution is done quite differently in other parts of the world.

"In this case, the jargon is misleading."

Only if you do not understand what a 'phase' refers to.

""...engineers of North America...."

Elsewhere it is done differently?"

Very much so.

Europe in particular uses 240 V for routine use.
It is NOT A split phase arrangement, but 240 V on a hot relative to the supplied neutral.

The Edison circuit we use (a split phase system since a single phase is split into two legs) allows 240 V to be available for larger loads without ever having a voltage higher than 120 V to neutral (or earth).
This makes it safer.
The only way to receive a 240 V shock is across the two legs.

Europe with no ready source of copper wanted to keep conductor size smaller, so they elected a higher voltage.

There are even some inverter makers that do not understand that a European 240 V load works just fine on US 240 V power.

I know that power distribution is done differently in other countries. One interpretation of kurto;s post is that engineers in other countries might name a power system by an "unusable configuration". I was trying to verify that. Sorry for being sketchy/lazy.

It is true, I don't have the mathematic ability to understand AC. Just call me Edison.