Hehe, guilty as charged. It's hard not to appropriate some of his idioms. They're just too darn catchy and convenient.

 

Hehe, guilty as charged. It's hard not to appropriate some of his idioms. They're just too darn catchy and convenient.

Yeah he's petty good.

 

As far as powering more things in your house - you could probably utilize the full capacity of your genset by bringing out the third leg and neutral as a separate feeder and splitting it into two 120V, 20A branch circuits. You just wouldn't be able to backfeed this into your household wiring.

 

As far as powering more things in your house - you could probably utilize the full capacity of your genset by bringing out the third leg and neutral as a separate feeder and splitting it into two 120V, 20A branch circuits. You just wouldn't be able to backfeed this into your household wiring.

Yeah I thought of that. My idea was to just connect the 120 into both sides of the transfer switch, but then the stove dryer, and everything else that's 220 wouldn't work.

 

Very simple answer, WYE connected generator. So you will have 120V from any phase to neutral, 208V between phases and still a limit of 52.5 amps absolute. Power output is 15kw/root2 or around 9kw but could be pushed slightly higher as long as individual line amperage doesn't exceeed 52.5/1.414 or about 35 amps. Math may be a little rusty but that is what you have without converting to DC and back using a VFD capable of 25kw 3 phase load and assuming said vfd can be operated single phase out!
Your limits are 120v single (5kw), 208V single at under 10kw or with appropriate VFD 15kw 120v single (7.5kw) 15kw 240v. I do not know where you can get a VFD programmed to output that way but they should exist as it is only software driven. Another option is an autotransformer if you require 240v but still limited to 10kw at 240, 5kw to neutral.

 

Very simple answer, WYE connected generator. So you will have 120V from any phase to neutral, 208V between phases and still a limit of 52.5 amps absolute. Power output is 15kw/root2 or around 9kw but could be pushed slightly higher as long as individual line amperage doesn't exceeed 52.5/1.414 or about 35 amps. Math may be a little rusty but that is what you have without converting to DC and back using a VFD capable of 25kw 3 phase load and assuming said vfd can be operated single phase out!
Your limits are 120v single (5kw), 208V single at under 10kw or with appropriate VFD 15kw 120v single (7.5kw) 15kw 240v. I do not know where you can get a VFD programmed to output that way but they should exist as it is only software driven. Another option is an autotransformer if you require 240v but still limited to 10kw at 240, 5kw to neutral.

Thanks!

 

Your post of the generator nameplate does not indicate its 3 phase, does the nameplate have 3 phase on it anywhere? if not I suspect it to be 1 phase only. Most generators built by Onan are 1 phase also the voltage 120/240 tells me you should look again at what you have.

 

208V is the phase to phase voltage on 3 phase with 120V being the phase to neutral voltage. 120/240V would indicate single phase. This relationship is root 2 times the sum of the individual voltages to neutral due to 120 degrees phase between the three voltages. FWIW this is a common industrial spec "small" generator. One of my operations that I started had two 800kw/1200kw peak 600v 3 phase units. We would use little autotransformers (10-50kva) to address lighting and control circuit loads. Often it was difficult to find competent industrial electricians and then I would step in. Otherwise I was owner or partner and personally inspected, approved and signed off. Look at the west end of Manitoulin Island to find where I spent '80 and '81 before the big guys forced us out.

 

Square root of 3. 15000/208/root(3)=41.64A. Max load between A-B/B-C/C-A is 8661VA unless the rather generous nameplate amperage is to be believed; in which case it would be 10,920VA. Upsized windings to accomodate unbalanced loads perhaps?

EDIT: They added 20% capacity to accomodate 0.8 power factor. Must be an older alternator. Nice. 10,920VA.

 

Slight math error, full load amperage / root2 or 1.414 to get max individual phase to neutral. 52.5/1.414=37.12A for single phase to neutral or another single phase ie 120 or 208V. Almost 50 years since 3 phase math by Avro Arrow engineers at college. This was one of the easiest subjects they taught!

 

Slight math error, full load amperage / root2 or 1.414 to get max individual phase to neutral. 52.5/1.414=37.12A for single phase to neutral or another single phase ie 120 or 208V. Almost 50 years since 3 phase math by Avro Arrow engineers at college. This was one of the easiest subjects they taught!

Root(2) is used for converting AC RMS to AC peak & vise-versa. Root(3) is used for 3 phase calculations.

120*root(2)=170Vpk. 120*root(3)=208VAC line-to-line... 277*root(3)=480VAC line-to-line... 100A phase = 173.2A line in a 3-wire delta, etc.

 

Right, should have used root3 and generator is connected WYE thus the 120/208 rating. Still we have overcomplicated a relatively simple question. Personally I would approach this as 15kw 3 phase will give 10kw in a single phase configuration and if the 208V was too low, use an autotransformer to get what you need. One College instructor got frustrated and cancelled a 3 hour lab to teach us how to do quick approximations and get close when close mattered. Anyone who missed it was lost for the semester!
My approach is use quick conservative figures then if necessary work out the essential details. You have a small mine with 1.1MW installed motor load but most are 25-30% larger than mins due to starting and reserve needs. Your 800KW set runs at 650-700KW with the plant at peak production. Worst were conveyors with motors twice as big as necessary which caused power factor to be a consideration.

 

Right, should have used root3 and generator is connected WYE thus the 120/208 rating. Still we have overcomplicated a relatively simple question. Personally I would approach this as 15kw 3 phase will give 10kw in a single phase configuration and if the 208V was too low, use an autotransformer to get what you need. One College instructor got frustrated and cancelled a 3 hour lab to teach us how to do quick approximations and get close when close mattered. Anyone who missed it was lost for the semester!
My approach is use quick conservative figures then if necessary work out the essential details. You have a small mine with 1.1MW installed motor load but most are 25-30% larger than mins due to starting and reserve needs. Your 800KW set runs at 650-700KW with the plant at peak production. Worst were conveyors with motors twice as big as necessary which caused power factor to be a consideration.

10kW serving line-to-neutral loads only - 12.6kW if using 52.5A. E.g. 120V loads.
8.66kW serving line-to-line loads only - 10.9kW if using 52.5A. E.g. the OP's heat pump.

Do not use autotransformers to derive 240V for a 120/240V split phase system; You'll get wacky voltages from line to neutral (152V!). The derived voltage from inserting an autotransformer(s) into two legs of a three phase system will be suitable for 2-wire line-to-line loads ONLY.

 

Correct, autotransformers must reference to neutral. Lots of experience with them but I omitted that part. My problem is that I often assume others have a higher level of knowledge or experience than I should. Many of the shortcuts I take make no sense without a thorough knowledge of theory and practice.
Anyone want to delve into CD ignition systems! 100+ amps thru a 10 amp SCR but that is not what causes them to fail. Hint, dv/dt!

 

Correct, autotransformers must reference to neutral. Lots of experience with them but I omitted that part. My problem is that I often assume others have a higher level of knowledge or experience than I should. Many of the shortcuts I take make no sense without a thorough knowledge of theory and practice.
Anyone want to delve into CD ignition systems! 100+ amps thru a 10 amp SCR but that is not what causes them to fail. Hint, dv/dt!

Neutral reference has nothing to do with it. Add 32V to 208V derived from two legs of a four wire wye system and you'll get 152 volts line-to-neutral on one leg. Totally unacceptable for supplying a 120:240V generator inlet. You'll fry half the loads in OP's house. The only way to pull it off is with a transformer at every single load that needs strictly 240V, and that sort of installation is not suitable for this application unless OP wants to wire up transformers to his clothes dryer, water heater, etc. every time the power goes out.

 

Technically it is two autotransformers of a three phase set, wye connected or two autotransformers each connected to neutral. Then select the appropriate tap. Lots of ways to do this and I would always grab the simplest or cheapest solution.
Home shop, 600V 3 phase 3hp mill, 240 single into a lighting transformer to 600V to a VFD and we have 600V 3 phase. Dirt cheap and works well but derates VFD by 1/3.

 

Okay, then add 16V to A and 16V to B if you prefer to do it that way. That's still 132 volts line-to-neutral and still unacceptable for a 120/240V generator inlet. The simplest and cheapest solution is to just supply the house with 120/208 and forget all notions of bucking/boosting a 120* phase-shifted system to achieve 180* phase-shifted voltages.

 

Different approach, we already have 120V to neutral for small loads but some loads require 240V. Isolate and use an autotransformer to drive them but they will see a higher voltage to neutral reference if needed. If not needed One autotransformer between phases is all that is necessary. Lots of solutions, pick the appropriate one.

 

The only way to pull it off is with a transformer at every single load that needs strictly 240V, and that sort of installation is not suitable for this application unless OP wants to wire up transformers to his clothes dryer, water heater, etc. every time the power goes out.

Use 120/208 or trade for a single phase genset.

 

Just use the 208 and if possible turn the exitation up a little. 120V devices generally run fine at 130V and unless it is a strict motor load it should run fine at 225V which is what 130 single phase would be. Personally if mainly resistive loads I wouldn't worry about driving with only 208V. A 10% voltage error is generally easily within design tolerances, 208V is 15% so close enough to get you thru in a pinch. Turn single phase up to 130 and the error is less than 8% either way so tolerable under most circumstances. FWIW my brother ran a 440V sand washing plant with a 600V genset for 5 years without any electrical failures. Not recommended but all he had available under the circumstances.
I just hope we have not confused most of the people following this thread with excessive detail. Many after reading will come away with a partial understanding of single vs three phase and know when they don't know.
My experience leads me to the cheapest reasonable solution for the job. I don't cut corners with safety. A Ministry of Labour electrical engineer and I had a fairly intense discussion about grounding which after an incident at a competitors resulted in adopting my method of grounding equipment which was copied from what I saw in another jurisdiction. Nothing new under the sun!