Trying to understand generator sizing for 3-phase application


At our office the main panel has 3 hot legs coming in. There's one 200A master breaker at the top of the panel, then all the usual 15A, 20A breakers for various office circuits below.
If 120V * 200A = 24,000W, would I need a 24KW 3-phase generator, or would I need 24KW * 3 = 72KW generator (eek!) ... or is my math completely off?
I understand there's more to generator sizing for air conditioning, motor loads, etc., just trying to get a really rough idea of what kind of beast I would be looking at if the entire panel could be powered off the genny. In other words a $5000 - $10,000 beast, or a $15,000 - $20,000 beast. <g>
-- Paul
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If you really have 3 phase comming into the office, you may have a 208 volt system with some 120 volt circuits and need about 42 KW.
Are you really sure it is 3 phase and not a single phase 240 volt system ? If so then you need 48 KW.
It is not likely you will really run all that at one time so you probably need less. Then you have to count how much starting current the AC and other big motors will need to start up.
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Depends on your need and actual loads. Your 200 amp panel may only be used to half that amount. Do you want to power everything or just essential items?
Jimmie
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Well, the cappucino machine is essential, and the heated toilet seat. What else do folks need? Oh, yeah, an occasional PC.
You'd reallly be better served to do some google searching, and talk to generator backup companies near you. We're not there with the ammeter, to see how much oompahpah you really use.
--
Christopher A. Young
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Paul wrote:

Are you sure you have 3 phase power line? Isn't it Edison circuit? Two different thing. 3 phase should have more than 3 wires.
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You may possibly have two hots and a neutral. Seriously, call some generator places near you, and talk to real people, in person. We're just guessing, over the usenet.
--
Christopher A. Young
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Just to follow up - the big mama safety switch by the service entrance says "200 Amp 240 Volt"
Inside the circuit breaker panel it says "200 amp, 3 phase, 4 wire". There are 3 hot wires coming in, plus the neutral. The schematic inside the panel shows each of the 3 hot legs going to alternating rows of breakers. Leg 1 row 1, 4, etc., Leg 2 = row 2, 5, etc., Leg 3 = row 3, 6, etc.
But yes, of course, I'm going to be talking to a pro installer about this... I see that I'll have to dial it down to powering just the essentials.
-- Paul
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Paul wrote:

You may have a high leg delta system, commonly referred to as 240 high leg by a lot of Southern electricians. You could also have a wye configuration with a high leg and Southern electricians will call it 208 high leg. Then there's a balanced wye with 208 on each leg and 120 to ground from each leg. I look outside at the transformers on the pole and when I see 2 large and one smaller, I'm quite sure it's going to be a high leg system. If the three transformers are the same size, it's a pretty good indication that it's a balanced three phase supply. All of the 3 phase generators I've ever dealt with put out balanced 3 phase power. You would be advised to run a survey of your current draw and determine what you want to run off the generator. I installed a number of 15kw Onan 3 phase generators in grocery stores that ran on natural gas. I installed a sub panel that powered every other row of lights, offices, registers, conveyor belts, bar code scanners, automatic doors and bathroom lights. A generator to run the AC and refrigeration too would have been the size of a 40 foot trailer. I once had a customer who owned a beauty shop who's 200 amp 3 phase electrical service was ripped off the wall by a falling tree during a storm, I was able to run everything in the shop including a 7.5 ton AC off a trailer mounted 50kw 3 phase generator for a week while I replaced the old 200 amp service with a 400 amp service. If you have more than one AC unit that you want to run, you might have to use a sequencer and time delays that would keep all of them from starting at the same time. With a little planning and a lot of prudence you wind up spending less money than you think. Here are a few links that will help to explain different 3 phase power:
http://en.wikipedia.org/wiki/Three-phase_electric_power
http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm
http://www.3phasepower.org /
Good luck with your quest for the almighty electron!
TDD
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