determine legs in 3 phase

I'm going to be setting up shop of sorts in a commercial space with some
dodgy electrical service that's been tinkered with over the years. It has
208 volt 3 phase, which I wish to extend into my space.
There should be a A, B and C legs, but how is this order determined by
testing? I've not seen any electricians carry around an oscilloscope, so
there has to be an simpler way. Grab a small 3 phase motor and put
alligator clips on it and fuss around until it spins in the right
direction?
Reply to
Cydrome Leader
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Pretty much .
Reply to
Terry Coombs
As long as you can sort out the 3 hot legs and the neutral with a voltmeter, why do you care?
If you really need to know you can use one of these:
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Or make one with something like this:
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Oriental makes the smallest standard 3-phase motors I'm aware of. Bodine also makes a few sub-fractional 3-phase motors, but not as small as Oriental's.
Reply to
Ned Simmons
Pretty much. I have a meter with some functions for that sort of application, but I never use them. I just hook up a motor and see which way it spins. Swap any two legs to reverse. Even on VFD outputs. I can see some mechanical applications where that might be a problem, but I have not run into one yet.

Reply to
Bob La Londe
Why do you want to label each leg? Or, do you just want all three legs where you want to use power and make sure that a wire IS part of the 3-phase?
Reply to
Tom Gardner
I know which wires provide 3 phase, just not their phase. If I can identify and mark all this first I don't have to do this test ever again if I add outlets or anything like that.
I can't imagine they trial and errors this every time wire up a new device.
Reply to
Cydrome Leader
"They" do just that. I can't recall ever using, or seeing someone use, one of the phase rotation gizmos. Nor can I remember the last time I saw a piece of equipment that came with info that would guarantee getting the phase rotation right on the first try.
If reverse rotation is absolutely unacceptable for even a second or two, you remove a belt, disconnect a coupling, etc. Hydraulic pumps are often the most difficult thing to deal with.
Reply to
Ned Simmons
Where we really, really cared for the vacuum pump motors, we'd toss a phase rotation indicator on the control panel. About $115 in 1983 dollars. Likely can find something cheaper now. I see a hand-held clip-on in the $80 range, or a few more if you want it from McMaster.
Reply to
Ecnerwal
I have never seen the phase relationship re rotation marked on a motor. However, every CNC machine I have connected always admonished in the manual to connect the three phase properly so that the motors would rotate in the correct direction. So I would always wire up the coolant or lube oil pump motor and check to see if rotation was correct. This has always worked for me, and I have wired up a lot of machines, CNC and manual. Eric
Reply to
etpm
No 'Phase Tape' on the incoming lines?
Reply to
Michael A. Terrell
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Reply to
Jim Wilkins
It's determined by wiring all the sockets alike (and you can check the wire colors at the breaker panel and at the socket, if there's any doubt). Few, if any, electricians will mix their wiring color conventions.
If you know electronics... set a flip-flop on A, reset it on B, and if the output more often LOW than HIGH, the order is A-B-C; if the output is more often HIGH than LOW, the order is A-C-B. If someone wants $100 for the tester, it's because of the fancy don't-shock-me probe wires.
Reply to
whit3rd
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Well ... at work we had a power supply (solid-state electronics) driven from three phase, and with something like 200A and 50V output. It came with dire warnings that connecting reversed phase to it would instantly kill it.
*is* no motor to look at -- then a phase tester is worth while. IIRC, I found a circuit for one using resistors, caps and neon pilot lamps in one of the annual "circuits" books from an industrial electronics magazine.
Enjoy, DoN.
Reply to
DoN. Nichols
I've not ripped off the panel yet to see exactly what surprises are back there. The last panel had about a half dozen double tapped breakers. The electric company seems to like to mark their wiring around here on the utility poles, but past that, there doesn't seem to be any real standard for marking wiring other than neutral is white, and anything else could be hot. I know orange tape on hot can sometimes mean it's special, but again, there's no real way to trust what the last person did.
Do the non-lazy electricians take the extra 15 seconds to just write A B and C on wiring or inside panels?
Reply to
Cydrome Leader
Even the power companies get it wrong.
I worked for a large computer service bureau in the 1970's. One day I watched a transformer turn red and blow out a top insulator. Three transformers feeding 3-phase to the building. We had a combination of Burrought and IBM main frame computers.
The local power company quickly came and replaced the bad transformer. With power restored, the first computer to boot up was an IBM 360. All the tape drives unwound their tapes. The motors were 3-phase and were now wired wrong. Power company redid their wiring and all was ok.
Paul
Reply to
Paul Drahn
If you are concerned with rotation issue - just swap two and it reverses.
There might be a 'wild' leg, might not. Put wild on lights and heaters...
Use the two good and a wild for real 3 phase. If you put a computer on a wild phase you can have issues...
Martin - 377 three phase in shop from my 208 single phase.
Reply to
Martin Eastburn
Some can - tin can - machines run on 3 phase and direction wrong will mess up the machine - needing new parts. There are simple phase units that use neon bulbs to view. They use RC networks to sort it out.
Martin
Reply to
Martin Eastburn
Exactly. The Phase Tape is a fuse. If it blows - something is wrong. If it doesn't and all works - take out the tapes and make permanent connections.
A local college installed a brand new CNC machine - a big one. Large industry here that uses big machines. Foundry and 50' gears...
So some one - I hope not a master electrician - wired up the unit on a major panel. He only got a phase and ground swapped. The ground line back to the machine and then in the back routed to earth ground - shorted the main transformer for the campus. It was out for 18 hours.
Heads rolled on that one.
Martin
Reply to
Martin Eastburn
Don't recall which port it was ( might have been Sasebo Japan), but while we were WestPac in '72 we managed to get shore power hooked up reversed . Senior Chief MM grabbed me as I was headed for shore and said "I've got over a million dollars worth of powerplant running backwards , get us back on ship's power!!" The whole damn ship (destroyer , 300 feet long and 42 wide) jumped when that breaker engaged . Made a helluva noise .
Reply to
Terry Coombs
I always did! Especially when wiring a 3-phase outlet. I couldn't think of a faster, more sure method. I will be watching this thread.
Reply to
Tom Gardner

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