# Question about 3 phase gennie



## TopTop (Nov 11, 2011)

I found a small 3 ph gennie. Two cylinder diesel, the guy thinks it it is 15kw. He said he can measure output voltage of 4xx, I don't remember if he said 440, 460, 480. The thing looks rough & the original control panel was destroyed along with the exhaust, but he said it only had 200 hours on the clock. He said he thinks it will also output 120 & 208 single phase. 

My house has the normal 120/240 single phase service. One neutral, two 120 volt legs. I know the gennies 208 volt output should run my 240 volt well pump, but I don't know if the 208 is between one hot leg & neutral or between two hot legs like my gas genny and the grid. 

I have a genny interlock on my load center so I can legally backfeed into the panel, which is how I use my gas genny. So my question is, will this little diesel guy do the same & how do I figure out the wiring. I assume the output will be reduced by one third, so if it is 15kw 3ph I will get 10kw single ph. Anyone know about these things?


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## ZoomZoom (Dec 18, 2009)

I'd start by going over the entire setup looking for markings.
2 cyl for 15KW doesn't seem right to me. Every 15KW diesel I've seen is 3 cyl.

Get the mfgr and model number from the unit.

The control panel normally has the wiring diagrams (inside the door) that show how to switch it from 3-phase to single phase. Without that control panel, you're going to have problems. Using the mfgr/model info, you'll want to scan the internet for the wiring diagram. Also, each wire to the control panel should be numbered. The instructions for switching is based on those wire numbers. If the wires aren't marked, you're in even more trouble.

You don't want to run the 3 phase into your home's 120/240. 

Basically, I'd want to know what unit it is, whether a control panel can be obtained and whether the wiring is marked before I'd even touch the thing.


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## Jimmy24 (Apr 20, 2011)

Zoom is correct in that you really need to find some SN# or something. 

Is it an old military unit? And unless it's some sort of off the wall unit, it would not produce 120/208 and 480. It would have to have some sort of dual windings in order to do that. 

As far as running your home, not an issue. Use two of the hot legs and the neutral, roll the thrid one back. And you would retain the buck of the wattage.

120/208 is used by the industry as a "network metering" voltage. Mainly apt complexes.

You measure between legs/phase to phase for 208. Phase to ground will be 120.

Good luck.

Jimmy


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## TopTop (Nov 11, 2011)

It is an Onan, not military. It was used for backup at a community center/swimming pool. It is over 100 miles away, so I won't just go to kick the tires. He said it weighs around 550-600 lbs. Not real big. I am very comfortable working with 120/240 single phase, 3 ph not so much. 

If it is really as simple as splitting out one lead, that I can handle. I thought it had something like a 12 wire output that had to be reconnected in certain configurations for the various voltages, phase & frequency, but never having seen it I really don't know. Bottom line, if I can't get 120/240 single phase out of it then it is useless to me.

Thanks for the responses.


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## Jimmy24 (Apr 20, 2011)

TopTop said:


> It is an Onan, not military. It was used for backup at a community center/swimming pool. It is over 100 miles away, so I won't just go to kick the tires. He said it weighs around 550-600 lbs. Not real big. I am very comfortable working with 120/240 single phase, 3 ph not so much.
> 
> If it is really as simple as splitting out one lead, that I can handle. I thought it had something like a 12 wire output that had to be reconnected in certain configurations for the various voltages, phase & frequency, but never having seen it I really don't know. Bottom line, if I can't get 120/240 single phase out of it then it is useless to me.
> 
> Thanks for the responses.


If it's 208 it's Y wound for sure. No 240 from it. 208 3ph most simple of 3 ph. Use two phases and you have 120/208 single phase.

Spent 30 yrs with electric utility working it. Split many pots.

Jimmy

Jimmy


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## TopTop (Nov 11, 2011)

Jimmy24 said:


> If it's 208 it's Y wound for sure. No 240 from it. 208 3ph most simple of 3 ph. Use two phases and you have 120/208 single phase.
> 
> Spent 30 yrs with electric utility working it. Split many pots.
> 
> ...


So you are saying I can feed my panel with the 120/208 just like I do with my 120/240 gas generator & everything will work ok?

Remember, he said it currently outputs 4xx volts on his meter. He told me it would output 120/208 but I don't know why he thinks that. I will call him again & try to get more info.

Also to Zoom's statement about the two cylinder not being 15 kw, he is probably correct. Looking around the 'net I saw a 12.5 kw 3ph onan with a two cylinder diesel. That guy was also trying to pull single phase & people were telling him to just sell it & buy what he needs.

My main 240 volt load is the well pump but I do have central AC & a wall mount AC that I might want to run as well as the kitchen range.

BTW, my generator circuit is 60 amp @ 240 volts/ 15,000 watts & includes a load meter for each leg. However the biggest gennie inlet box available is 50 amp. I did not want to hardwire my portable gennie, so that is what I used. I can power the entire load center & use load shedding to determine what may run at any given time.


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## ZoomZoom (Dec 18, 2009)

I have an old Onan 3-cyl diesel genset (15KW I believe) buried in one of my buildings. If you can get some more info on the one you're looking at, I'll see if I can dig back to it and get some info on number of wires, wiring diagrams... It may help.


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## Jimmy24 (Apr 20, 2011)

TopTop said:


> So you are saying I can feed my panel with the 120/208 just like I do with my 120/240 gas generator & everything will work ok?
> 
> Remember, he said it currently outputs 4xx volts on his meter. He told me it would output 120/208 but I don't know why he thinks that. I will call him again & try to get more info.


If your equipment will run on 208 (which it should) yes, it will work fine. Make sure your motors, ac, well are ok for 208. Make sure they don't require larger wire. That would be the only hold.

Remember the the term "single phase" is a little misleading. It is actually 2 phase. The old 120 straight voltage service was the real single phase. I won.t go into delta transformer banks and explain that sort of 3 ph. 120/208 Y is the simplest form of 3 ph.

Now having said all that, make sure what voltage the thing makes. Guy says measured 4xx but thinks it makes 120/208.....not sure about him or his test eqoipment....,just saying...

Hopes that helps.

Jimmy


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## ZoomZoom (Dec 18, 2009)

ZoomZoom said:


> I have an old Onan 3-cyl diesel genset (15KW I believe) buried in one of my buildings. If you can get some more info on the one you're looking at, I'll see if I can dig back to it and get some info on number of wires, wiring diagrams... It may help.


I checked it out tonight.
It's a 4-cyl, 15KW diesel at 1800 RPM's.
110/220 only. No 3-phase option.
I opened the control panel. No wiring diagram (but not needed since there is no switching to 3-phase). About 10-15 wires, a circuit board and a tube.

It probably won't help you (sorry ) but I may just pull this one out and get it fired up.


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## TopTop (Nov 11, 2011)

Thanks guys. I really appreciate your input. To clarify, it is actually putting out 4xx now but he thought it could be set up for 120/208. Emphasis on his "thoughts" and he readily admits this is over his head. Just like me. Anyway, I'm thinking I'll just let this slide on by. I may give the guy another call, but I really don't want to bug him if I'm not going to buy it. Seemed like a real decent fellow on the phone. Thanks again.


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## Conelrad (Sep 2, 2009)

The 15kW generator you are looking at may indeed be able to do multiple voltage outputs depending on where it's internal taps are set at. This is usually done via a change board or a rotary switch. Make sure before you put any $ on it.

Most 240-volt demands will run OK on 208-Volt power, and things like a stove will heat a bit less, the A/C unit will usually behave. I have that scenario here at home with no problems.

Keep in mind you not be able to draw the full output as the phase relationship from leg to leg will still be in force as the windings are still 120 degrees apart. 

Some people take a single 120-Volt output of a three-phase plant and run it thru a transformer with a center-tapped step-up secondary to get a proper 120-240 Volt output. Still and at best, you will get about 9 kW overall from that plant for continuous operation.

Au contraire Jimmy, 120-240V is not 2-phase. Two-phase power has the legs 90 degrees apart, not at 180 as in the case of most domestic power. This is a common misnomer, but it is still single phase as the distribution transformer supplying it has but one set of windings.

The advantage of poly-phase electrical systems is the efficiency goes way up, as the power overall never goes to zero as it must in a single-phase system. When one leg is going down, another is on the way up. It sorta has more 'inertia', like a big flywheel to put it in a simpler analogy.

Dennis


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