Paul wrote:
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




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