If all things are equal as far as usage, is it valid to assume that
since amperage ratings for three phase motors are about half that of
their single phase counterparts, that one's electric bill would be
correspondingly about half as well?
No. Power is power and you will be charged for what you use. Current
ratings on three phase motors are not half of single phase motors
either, the current rating will vary with the design voltage and the
current is for three hot legs, not just one (120V) or two (240V). Worse
yet, three phase power at least in the US is typically peak metered
instead of actual KWH metered so you'll get charged according to your
That said, equipment with three phase motors can be a bit more efficient
than comparable equipment with single phase motors since three phase
motors have much better starting torque which means they can be more
closely matched to the equipment's running torque requirements.
I don't know enough about electricity to dispute what you say. I can however
tell you that the electric bill for my shop is usually about the same as it
is for my house. Even in the summer with my air conditioners running. I have
2 large residential air conditioning units that try to cool my 4000 square
foot shop. They don't shut off at all when it's hot out. Plus, I have a shop
full of large equipment, including air compressors and EDM's. The shop is 3
phase and the electric bill is usually within about $25.00 of what my single
phase house is.
Commercial and residential rates are usually different on a per KWH
basis and as I noted, commercial three phase power is typically peak
metered as well. This makes it fairly complicated to compare accurately.
It is a little better, because 3-phase is more efficient at spinning
motors. But as has been mentioned, the loading comes into play with
highest-draw-per-month demand metering, and time-of-day based variable
If you can shift a bunch of the power demand to the lower rate
nighttime hours, like through ice-banking air conditioning that makes
ice at night with cheap electricity, you can cut the bill.
It is if you want one simple breaker and switch to cut off all the
lights in the shop, and run all the lights on one run of modular
wiring - essentially 12-4 BX cable with pre-connectorized ends. You
can stick a whole lotta watts of lighting on one 20A line at 277/480V.
And metal halide high-bay or low-bay fixtures with the right
reflectors for the use give better light and are more energy efficient
per lumen than fluorescents.
Drawback is that if there's a fault you lose everything - when
lighting with 277/480V you really need to buy fixtures with factory
individual fuses, so one bad ballast doesn't knock out the whole shop.
And you can turn off part of the lights - but you have to plan ahead
when wiring it up. Otherwise, the whole area is on one switch and it
costs a lot more to rewire it later...
Good thing is, when the individual fixtures are cord connected and
hung off a hook you can buy extra fixtures and swap the bad fixtures
out yourself. Just get a manlift, and if a new lamp doesn't cure it
you replace the fixture. When you run out of working spares, then you
sit down and (or call someone to) fix or replace the dead ones.
At work we split the lighting between phases as a matter of course. The only
significant issue is that sometimes more than one phase will be present is a
multi-switch box. These are need to be labeled (in the UK) due to the presence
of 415V where electricians might only expect 240V. If you have a lot of
lights, it should be better for you and the utility to use all the phases.
Something else to ask about: adding capacitance to your circuit. Lots
of motors mean lots of inductance, which can goof up a utility's ability
to deliver power to a big shop. AFAIK, they start to charge for the
trouble, and the fix is to put some pretty serious capacitors in
parallel with the equipment. That's all I know about it, but it might
be worth a look.
I really appreciate all the replies, guys.
This is a small, one man shop (mine).
The background to it is that as long as I need 3 phase for some of
my machines, I was wondering just how much I might save by going
ahead and getting 3 phase motors for the other machines that aren't.
Right now, my Bridgeport, B&S surface
grinder, and punch press are 3 phase.
My Southbend lathe & benchtop CNC mill are 120V single.
The Tig machine is 220V single. (I think it's convertible)
Of course, there's the normal bench grinders,
band saw, etc... that are all still 120V, too.
I'm just wondering if it's worth my effort to convert the
others to 3 phase, looking to save on the monthly bill.
Kind of like wondering if it's worth putting 3 phase
in the shop at all, since I have a rotary phase converter...
If you already have a rotary converter and everything is working
properly, it is highly unlikely you would see any benefit utility bill
wise from getting "real" three phase power and converting any single
In a big factory with high duty cycles on all of the machines there
might be some savings, but not for a single man shop.
Worse yet if you go from KWH metering to peak demand metering your bill
would likely go up, even if the per KWH rate was lower since you'd be
charged based on the biggest tool you ran each month and not actual KWH.
No. It won't help your utility bills to change over, because you
are still starting out with single-phase utility power and suffering
the losses from running the phase converter. Plus, you have to go out
and buy new 3-phase motors to convert everything over, and more or
bigger RPCs to run it all, then you have even more riding on the
failure of the RPC. It quits, and you're closed.
You can more than likely do a bit better on the overall power bill,
since you eliminate the load and losses of the RPC - unless you start
too many things at once. On most single-phase services they charge on
simple KWH used, with a multiplier for the size of the service, which
reflects the size of the transformer the utility has to install to run
everything at once... BUT - and it's a big one...
But on 3-phase services they often not only measure the KWH used,
but also the peak KW demand, averaged out over a 1-hour period IIRC.
And that figure gets averaged into your billing rate over a 6-month
period. You have to check with your utility, they are all different.
Go a few days with several other people working in the shop with
you, and you run several heavy loads like the mill, punch press,
surface grinder, air compressor, welder all at the same time, a
properly sized 3-Ph utility feed can easily do it. But you can also
put a big spike of demand multiplier on your bill that will have you
paying a bit extra per KWH for the next few months. You shouldn't get
spanked hard, but the savings will disappear for a while.
I've told a few big condo complexes who are on a 600A switchboard
with a demand meter that it isn't a good idea to have /all/ the relays
for /all/ the outside lighting /all/ triggered by one photocell, so
they /all/ cut in at the same moment and make a big demand spike every
evening... (The hard part is getting them to listen.)
If you can get a 3-Ph service installed without paying big bucks to
the utility for the work (if the 3-phase feeders are already there) do
it. If they start talking $25K charges for new transformer banks
and/or extending the third leg a mile or two, that changes everything.
But I wouldn't convert any existing 1-Ph equipment over to 3-Ph unless
and until the old 1-Ph motor blows up on you.
Thanks for all the info, guys.
I was FINALLY able to get a straight answer out of FPL today.
It seems that if I stay under 8000 KWH per month, I
will be billed at ACTUAL KWH usage, but if I go over
8000 KWH per month, they will install a load average
meter, and bill me at average peak usage per month.
I have no experience on what I will actually use per
month, but have to think it will be less than the 8000
KWH per month as I was told places like laundry mats
and fast food resteraunts are examples of places that
go over that mark.
Anyone have thoughts on a small, one man
machining/welding shop's expected KWH usage?
And, assuming that I am well under that limit, would
it make sense to go ahead and convert the machines
I have that aren't 3 phase, *to* 3 phase?