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>
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.
Well, the cappucino machine is essential, and the heated
toilet seat. What else do folks need? Oh, yeah, an
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
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.
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:
Good luck with your quest for the almighty electron!
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