shop electrical question

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?
Just curious.
Thanks,
Weyland
Reply to
Weyland
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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 largest load.
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.
Pete C.
Reply to
Pete C.
My 3-phase is 1/3 the bill with lighting at @$1,200 and all 3-phase @$600...OUCH! There are 2 air conditioners on the split-phase that spike-up the summer's bills. Is 3-phase lighting worth it?
Reply to
Tom Gardner
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.
Reply to
Dave Lyon
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.
Pete C.
Reply to
Pete C.
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 rates.
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.
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Reply to
Bruce L. Bergman
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.
Mark Rand RTFM
Reply to
Mark Rand
Used to be, but no longer so. Compare T8 or T5HO lamps to Metal halide. The CRI and lumens/watt and lighting distribution is better on the T8's or T5HO's.
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Reply to
ATP*
Weyland,
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.
Bill
Reply to
Bill Schwab
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...
Thanks,
Weyland
Reply to
Weyland
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 phase machines.
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.
Pete C.
Reply to
Pete C.
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.
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Reply to
Bruce L. Bergman
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?
Thanks.
Best,
Weyland
Reply to
Weyland

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