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PTO Generator
Jim; regarding your question on wire sizing... #2 aluminum conductor will yield a ~2% voltage drop (at 100 amps&240V) for a 90' run. So for your 125' run expect at least a 3% Voltage drop + whatever you may lose through the welder plug that you are using. According to my "35th Edition of Wiring Simplified" the ampacity of #2 aluminum is 70 amps max for Type T or TW line, and 85 amps for types RH, RHW or THW line. These ampacities are for conductors in conduit or buried directly in earth. To sum up: a 24KW generator will really tax the capacity of your #2 aluminum wire. Hope this helps
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No. 2 aluminum is good for 90 amperes
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Thanks Jerri. No, not confusing, informative.It "looks" like I might be able to pickup a used 3ph generator easier and a lot cheaper. I'm just a dumb farmer stuck with trying to get a new generator while every yuppie in the world thinks Y2K is going to bring disaster. Anyway, last spring I ran a 120foot of new underground cable to my shop, three wire w/grd # 2 aluminum. If I can believe my full load amperage would not get near 90amps I was thinking of using it for a generator feed to main panel in the house. Right now I feed the shop with a 60amp breaker in the main panel to a new 100amp panel in the shop, then a 60 amp breaker in the shop panel feeds my welding outlet. That setup will have to go. So, I think if in the main panel I replace the 60amp shop breaker with a 100amp breaker, then in the shop put that feed into a transfer s/w before it goes to the shop panel I can feed from a generator to both the main panel and the shop panel. One question, normally I would think a disconnect or transfer s/w go between the meter and main panel does the above setup violate any standard practices of the trade?VRJim
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Jim,Sorry, can't help with the decision for a 3 phase gen, but I personnally wouldn't head that route, regardless of local availability. To me its an investment, and I'm not one who hastily throws money at something for instant gratification & satisfaction. A little patience will yield what you truely need and want. I'm assuming you've lived this long without it, a few days/weeks won't kill you. I do think that what you mentioned about KW losses & imbalance is correct. Manual xfer switches can either be purchased as complete kits that are wired into your panel, usually containing muliple brakers. Or you can simply have a dedicated, inexpensive braker & plug for your generator, to feed the panel bus. This is similiar to the dryer outlet method, only the wiring from the gen to the panel is heavy enough to safely support the load, and usually short in length, further reducing losses. An auto switch constantly monitors the line voltage and automatically switches the two. Something to think about if you have a wife & kids at home, while you are away. I would love to by an auto generator & switch, but our frequency of need is once a year maybe once every two years. Although that frequency seems to be greater lately, especially with this crazy weather we've been getting. Oh, and my guess about those local 3phase gen sets, being readily available at such good prices is low demand..Think about it..Take my thoughts with a grain of salt, but I hope it helps.
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Jim,OK, where to start....first the transfer switch, no it doesn't have to go between the meter and the main unless you are trying to serve everything on the system. By the way a transfer switch is nothing more than a fool proof way of opening one breaker before closing another, no big deal. As to your other questions, you mentioned a 25 and a 12 kw generator. A 25 kw at 240 volts single phase will produce 104 amperes and I will bet the 25 kw rating is at an 80% power factor so that the generator is really good for 130 amperes. You probably really don't need this much and really couldn't make use of it with the setup as you described so I lean towards a smaller unit. If you go with the 12 kw machine and it is a 3 phase model it would be good for 42 amperes per phase A-B-C. This would work with your 60 ampere single phase welding outlet and things would work as-is. Down side is the one phase would be unloaded unless you set up a dummy load in your shop, which is an idea. This dummy load could be a heater or almost anything just to dissipate the extra energy generated, preferably a 120 volt load so as to use phase A or B or C.I am happy to help you out with this, but I think you need to make some decisions about what size unit, phase configuration and whatever. After that give me a shout and will try to help you work it out. Also, it would probably work better to contact me offline at my email address.Good luck and let me know what I can do to help.
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I would get another opinion on the single phase operation with a three phase generator. We use a three phase generator on our dairy (75 kw pto). We use it for single phase just as much as we do for three phaase, you just don't use the 220V leg that gives you the three phase when you don'tneed it.
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Jerri,I'm confused. I have 220 single phase running into my home. There are the two power legs and the neutral. Most, if not all welding outlets I have seen have the two power legs and the neutral, and neutral to one power leg is one twenty. I am new to this generator thing, but am an electrical engineer. Am I missing something? Oh yes, I do know that the generator must be grounded separately, but that is not the issue here. Help me out cuz I was going to back feed through a 50 amp single phase recepticle to a range breaker and just throw the main too. Building the PTO 3 pt hitch gen set from a unit from Northern and some steel. Almost done, just gotta order the HTD timing belts and pulleys for the right speeds. Hurricane Dennis prompted this, not Y2K. Four days to long to be without power and water. Bill
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Bill, it helps to draw the sine waves to get a picture of what is happening in the various phases. As you are an engineer, a little help might enable you to sketch it out. The key is in the word "phase". First draw a typical cartesian angle coordinate system with the voltage amplitude on the vertical axis and time on the horizontal. Now draw two complete sine waves varying in time. These complete waves repeat every 360 degrees which is every 1/60th of a second.(This is the picture that one always sees in basic physics books.) Draw these two sine waves so that they are in phase as regards time, but opposite in polarity at every instant. In other words, they are mirror images around the horizontal axis - one positive when the other is negative....and always crossing at zero amplitude. This is a picture of "220 single phase". If you think about it, either sine wave with regards to the horizontal axis (neutral or "ground") is a separate leg of household 110v as it repeats the pattern at 60 cycles/sec. To draw three phase current take only one of the sine waves above (which repeats every 360 degrees), and draw another just like it every 120 degrees. So that three distinct waves (phases) can be seen where formerly there was only one. Simply put: In single phase the separate legs have opposite polarity, but are in phase. In three phase the separate legs are 120 degrees out of phase from each other. Many other phases combinations are possible, and sometimes you may encounter a generating system that is different. Enjoy! Roger
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Bill, OK let me make sure I am answering your question. Yes, to most residential structures are 220/120 single phase. This would conist of two phase conductors, a neutral and a ground established at the residence (not from utility). Now for your welding outlet, or any other outlet as that goes, you do want the same setup, two phase conductors and a neutral, if there is a ground great but you are acting just like the utility here. So ground the generator and you are already grounded at your panelboard. As for voltage readings, you should have 220 between your two phase conductors and 120 between any phase conductor and the neutral/ground (these two are really connected together). Hope this all makes sense but if not, feel free to email me directly and I will try to help.BTW are you here in NC? If so, I am located in Durham and small world.
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Jim,First of all, please install a transfer switch for your generator. Utility transformers work both directions, they take the high voltage feed and reduce it to usable levels, but they will also take the low-voltage feed from a generator and step it up to the utilities high voltage (typically 2400-volts or 7200-volts to ground). If the utility workers doesn't get a grounding jumper on the "energized by generator line", he may not be going home to his family that day or ever.Next, I have sold several pto generators in the last year. The 25kw I have available requires a minimum of 35 hp, but typically 2hp/kw is a good rule of thumb. A tractor with a good governer will help since a the load increases the more power the generator takes to turn. I started a 10/16hp (68-72-amp) fan motor on a drying bin with the 25kw being turned by a 110-hp tractor. Let me tell you that tractor lost some carbon build-up with that start, but it got the load turning. If the generator's rotor turns too fast the hertz frequency will be too high causing motor loads to turn too fast also and vice versa when it's slow. This is fine for resistive loads, ea. lights, stove, water heater, but may damage motor load over an extended run time, ea. refrideraton compressors, pumps, fans. The best way to keep the hertz in check for a average homeowner is with a 120v dial-type clock compared to a quartz clock. If the 120-volt clock gains time, the generator is running fast, if it looses time it is running slow.Getting back to generators, a pto generator will get you more KW/dollar usually. I like the idea of having a tractor, which is more apt to be fresh for running. A self-contained gen-set requires maintenance whether it has run or not. You will need too keep fresh fuel around for it and with a diesel, you may need to change between no. 1 and no. 2 when temps drop below freezing. There are factors to take into consideration with a generator like rotor speed and brush or brushless. The one's I sold were brushless and the rotor turned at 1800 rpm. Many brush-type and brushless turn at 3600 rpm, you tell me which one will run longer.
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09-21-1999, 00:00 
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