Three-Phase Power from a Generator

Sawmill operators discuss the technical issues posed by running power-hungry three-phase electric equipment off a generator, as compared with the costs of getting utility power to the site. August 30, 2007

Question
Let me first say I realize need for electrician before installation. I need general guidance and advice. I need to supply power to a 125 volt 3 phase motor, a 75 HP 3 phase motor and a 50 HP 3 phase motor. 3 phase service from the power company is out of the question. I have the option to buy 300 KW diesel Genset supplying 120/208 volt 3 phase and 1040 amps. All motors are rated 208 230/460 with max amp being 320. Does the power from generator run into a breaker box just like power company service would? What size box would I need -1000 amp? Any ideas or suggestions are appreciated.

Forum Responses
(Sawing and Drying Forum)
From contributor A:
The breakers are to protect the wiring. Whatever size wire you plan on using will determine the breaker size. If you plan on using the entire 300 KW then you need to get a wire size that can handle that current and then a breaker to protect the size wire. That will be the main lead in and connect to the main breaker. The same applies to the breakers that go to your equipment. You will need to size the wire to handle the 75HP motor and the 50 HP motor. Then this will determine the size of the breaker you need to protect them.


From contributor B:
Does your Genset have a main beaker capable of handling the entire load, in its control panel? If it does you can feed to a splitter trough rated high enough, at least as large as your main breaker or a breaker panel. You can then size the secondary breakers to match your individual motor, amperage requirements. You are talking some major loads here. If you can also get some of the panels, breakers, cables, etc. used, in good condition, you will save yourself some coin.

From contributor C:
You can buy phase inverters and make 3 phase out of single phase and, in most places, the power company has to give you whatever you need in single phase. Running a generator can be pretty expensive these days. A really good one will give you 12-13 kWh per gallon of oil. Depending where you are, that could be many times the price of single phase. Unfortunately, every power company has a different rate structure and it is critical to look at that before going too far with a Genset

From contributor D:
I assume you have a 125 HP motor, and not a 125 volt motor? I agree with contributor C on this, though you are getting into some big motors now. When I looked into a Genset, I calculated it was going to burn 7-10 gallons of diesel an hour - @ $2.75/gallon - $20/hour. Granted, you would burn #1 heating oil without the road tax, but it’s still expensive.

What about setting up a phase converter out of a big motor? Put it on a soft start/VFD and run everything off that, as contributor C is suggesting. It’s a big load you're running, but it’s going to be cheaper than a Genset, and more reliable. Using a soft-start lowers the in-rush load at start up, and once it’s up to speed, the demand isn't too bad, certainly better than $20/hour. You can get big used motors for scrap value.

As far as wiring goes, you would want to run everything at as high a voltage as you can, 460+, to cut down on the installation costs. Doubling the voltage, 240 to 480, halves the amperage, which determines the conductor size. So you want to run a big set of wires from the Genset phase converter, at as high voltage as possible, into a distribution/breaker panel, then smaller wire to each circuit, only what you need. If you need to step the voltage down, get a small transformer to put at the location or machine where it’s needed. Keep your panel close to the source so your large conductor run is short as possible. Wire cost can be amazing. And Contributor B raises a great point - you can buy all this stuff surplus for pennies on the dollar.


From the original questioner:
Yes, it is 125 HP motor. I would much rather use a phase converter but the electrical draw is too much for my utility lines. The power company said no can do because it will dim the neighbor’s lights. They mentioned some type of device that limits in rush on startup, but I priced that at over 40k. The Genset is rated at 10gal/hr, so 20 bucks an hour is not too bad.

From contributor B:
What type of machines are you running off these motors? Is there a control panel on the machines with magnetic starting? You would need some method like that to control the startup. A 125hp motor at 440 v will be drawing something in the neighborhood of 140 amp. The starting load could be close to double that for an instant. Someone correct me if I’m wrong on these figures. I worked for a utility and was an electrician. My brain is a bit rusty sometimes. As I suggested before, if your generator has a breaker that will control 1000 amps, feed directly into a splitter trough of the same rating as your main breaker, 1000 a. Then use fused knife switches rated large enough to feed each motor. These feed directly to the control panel of the individual machines. If you are feeding the trough at straight 440 volts 3 ph you could install a dry core transformer to step the voltage down to 120-208 for lighting, small motor load etc.

From the original questioner:
To contributor B: Thanks. Your response is full of useful information I need. At the minimum I have a better idea of educated questions to ask when we get underway with this. The motors operate a sawmill. 125 on headsaw, 75 on hydraulics, 50 on edger. Will use regular utility service for lights, setworks etc.

From contributor C:
If you are running 250 HP of motors in a sawmill, you will be using probably about 40,000 kWh per month. This will cost you about $8000 a month to run a generator.

If you are in an area where power is reasonable, you might buy the power from the utility for $3200 a month. If the utility needs $50,000 to bring you power and you subtract the price of the generator, you can pay back the investment pretty fast. And have a lot fewer headaches. Some areas have lower cost electricity than that and some more so you would have to do the arithmetic. Of course, I am guessing at the time your equipment will run but if you are putting in all that, I assume you will be using it. Sawmills (not including kilns) normally have a 30% load factor, that means that equipment runs at full load 30% of the hours in a month. Could be more, could be less. But look at it carefully.


From contributor E:
I’m no expert, but from all I have ever heard, a generator can't compete with the power grid. I know that you said that the power Co. has said no can do, but I would almost bet that for half of what a new Genset would cost, the power company could be made to do.

Some other things to think about are the life of the Genset and the steady upkeep involved. The engine only has so much life even with overhauls before you hit the point of diminishing returns and the power grid will for the most part always be there. Your electrical service equipment - panels and breakers, fuses will have a longer life than the engine. Some things could fail, but at a lesser rate than the engine.

If you already had the Genset I would say that it would be a good start or stop gap until grid power was available. Or if it was your intention to be semi mobile, say set up in a remote site until the woods around you was milled and then move to the next location and never have to worry about the grid. Now don't get me wrong, I am not trying to rain on your parade and I don't know for a fact about the math and all of the cost. These are just the conventional wisdom or wive’s tales that I have heard. If your production is good enough to bear the burden of a Genset because of location or labor or shipping, then by all means go for it!

Two of my biggest goals are to have everything mobile to a degree and to be self sufficient. Never know when I might want a change in scenery. By the way, back to the original topic. I would think that the Genset would have at least a single breaker or fuse set big enough for the total output capabilities of the unit. A subpanel to feed your equipment and a transformer and another subpanel for those circuits would not be that big of a deal on paper, it is the paper money that will be the big deal.

Another issue is that the power company or local inspectors may watch you like a hawk. Dual services and generators are a big risk in their eyes. Complete and distant separation is the name of the game. It may take the involvement of an industrial type electrical contractor to wade thru the red tape to get you the power that you need from the grid. There are tricks and terminology that only some of the truly connected can work through, and there is no substitute for the "good ole boys" club sometimes.


From contributor B:
From a utility's point of view, they will supply you basic single phase power, provided it is going past your place. If it is some distance away they would expect you to pay the cost of the line extension. The same would apply if three phase power is available going past your place. It is possible that the line would need to be rebuilt back to a main feeder line to accommodate your quite large load. EG, line wire size might have to be increased. They might expect you to pick up the rebuild costs or a portion of them. You would also likely be responsible for the cost of three phase bank of transformers at your mill. In our utility the customer could buy his own transformers, and in return get a better rate of power. It will depend on how close you are, what the costs will be. Each utility will have a bit different policy.
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