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Old 10-24-2006, 09:39 AM   #1
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3 phase electricity


I am being asked a couple questions about 3 phase here that I do not know the answer to. I was under the impression that 3 phase is usually red, black and blue in the US. A friend has a panel in a garage with 3 black wires coming in from the meter and 3 going out to a 3 phase motor. The black wires have blue tape on them. One has one piece of tape, the next has two and the third has three. Is the electrician trying to identify each leg as if it were color coded? Does the sequence matter when wiring up a recpetacle for another 3 phase motor? Do I remember correctly that one of the legs of 3 phase usually carries higher voltage than the others, like 190 or something...

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Old 10-24-2006, 10:36 AM   #2
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3 phase electricity


Actually, scratch the above question. I just went and looked at the motor for them, the motor has been converter to single phase 220. Now, new question, We need to take 220 out of the 3 phase service panel, does it matter what leg we take it from. IS one leg a different voltage? It is my understanding that on this 240 3 phase, one leg will be wild and be anywhere from 170-190v.


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Old 10-24-2006, 11:47 AM   #3
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3 phase electricity


wouldn't that be similar to "Electric Dryer" or "Electric Range" being handle?
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Old 10-24-2006, 02:47 PM   #4
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3 phase electricity


J, I get the impression you are a bit over your head messing with 3-phase. IMHO it is best left to professional electricians to do any work on 3-phase equipment or panels.
In fact, in many places it is againt the law for anyone not licensed and insured to do any electrical work in a commercial setting.

There is 3-phase delta which is 120/240v, with a high leg being 208 line to neutral. The other phases are 120 line to neutral and all are 240v line to line.
Then there is 3-phase Y which is 120/208v. All pahses are 120v to neutral and 208v line to line.

Delta can be touchy. Even though you get a true 240v line to line, which is absolutely fine for motors and such, one leg still may be 208v to neutral. If you connect this to a 120/240v load, such as a dryer, you can fry the 120v items in the dryer.
Basically any 120/240v, 4-wire loads should never be connected to the high leg in a delta panel. This is why you will many times see every third breaker blanked or not being used in a 3-phase panel.
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Old 10-24-2006, 10:35 PM   #5
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3 phase electricity


Pete hit it right on the nose.

I have now idea why this guy has 3-phase to his house, but oh well.

are you certain that it is 240 volts? If it's a normal resi. area then it probably is. which means you would have the high leg which is 208 to ground.

The wires being striped is not normal practice so you'd better check voltages from all 3 phases to ground(neutral). Any 240 volt loads you connect won't matter what 2 legs you tie to, unless the load needs a neutral then you'll have to avoid the high leg.

Any line to neutral load needs 120V(unless you have a 277/480 sytem), so you could guess the result if you fed it with 208 volts.

Motors have a 10% tolerance for voltage leads, so don't just read the nameplate and assume what voltage your dealing with(230V - 10% = 207).

Also by code every that is tied to the high leg needs to be phased with orange tape or an orange wire.

-To give Pete a hard time; 3 phase 4 wire load? never seen it. And if such a thing does exist how would one avoid B phase.
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Old 10-24-2006, 10:48 PM   #6
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3 phase electricity


Quote:
Originally Posted by Sparky Joe View Post
-To give Pete a hard time; 3 phase 4 wire load? never seen it. And if such a thing does exist how would one avoid B phase.
My fault for not clarifying. I mean "single phase 120/240v, 4-wire". That's what I meant by "like a dryer".
A single phase 120/240v load on a 3-phase panel can be tricky.
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Old 10-25-2006, 06:32 AM   #7
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3 phase electricity


Simple enough answer is to say "leave 3 phase alone!" My question is why was a 3 ph motor pulled and a single put in place? If you have 3 ph, be kind of silly not to use it.
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Old 10-25-2006, 09:35 AM   #8
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3 phase electricity


To answer all of the questions:

Joasis - the motor was not pulled, rather a simple and inexpensive conversion was done. The company that makes the machines offers the motor in single and 3 ph. The company does not use different motors, rather a module and capacitor kit. This equipment was bought used, maybe the original owners didn't have 3 phase.


Sparky Joe, its not a house, rather a warehouse in a commercial building. I knew the tolerance of the motor because its actually tagged, probably because as I mentioned, the same motor is used in both applications.

Pete - I appreciate the help. I'm not sure what gave you the impression that I'm over my head, but considering that I know a lot more than these guys do, and they were going to work on this thing regardless of whether or not I helped them, its probably best that I got involved. Besides, I don't really think being over your head is when you have to ask questions to understand something. If I am confused, that's why I ask questions. The people that are over their heads in my opinion, are those who no little to nothing about electricity and even answering a couple questions will leave them confused about most everything else involved. I realize there are risks involved, but I really am careful not to get involved in something I know nothing about.

My question regarding the wild leg was just a function of poor memory, I had worked w/ 3 phase long ago. These guys didn't have a meter and I'm no where close to my house.

Anyway, thank you all for your help. I really do appreciate the incredibly helpful information you all give. As of now, we have tested the legs - I made them buy a voltmeter. We successfully wired the receptacle and the machine works flawlessly. The inspector came (don't ask why he came so fast) and said everything was in order. He did however make a couple of recommendations that some small things be taken care of. Things that were done when the original 3 phase was brought in. Funny thing is, the guys who own the building are telling me he's the same inspector that signed off on the work when it was done in the first place, go figure. Thanks again everyone!
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Old 10-25-2006, 04:28 PM   #9
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3 phase electricity


J, I meant no disrespect with my post. It's just when non-electricians start asking about 3-phase a red flag goes up to us electricians. It is generally accepted that only professional electricians work on this stuff. For the good of everyone. It is far too easy to hurt equipment or personnel by getting something wrong when it comes to 3-phase.

I am a bit concerned. Who are "these guys" who obviously have no clue and are working on your 3-phase system?
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Old 10-25-2006, 05:54 PM   #10
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3 phase electricity


OH pete, I know exactly what you meant. That's why I went to great length to explain, because Iknow what it sounds like when someone is talking about working on 3 phase w/ no experience. I saw taking single phase from 3 phase panel as a little less intimidating. These guys are friends and it is actually THEIR equipment and panel. I was helping because they asked me. I knew they were going to be stubborn enough to try to get it to work, and as I tried to explain, I am usually a very reasonable guy when It comes to knowing my limitations. they may not. I worked for an electrical company many years ago and was accruing hourse under a work-study program. Although I remember a lot of what I learned, I consider myself an amatrure diyer like the next guy. I forget lots too. Thanks again for the help and I definitley took no disrespect from what you said at all. I actually appreciate the notion of your concern. I wasn't refuting you, rather merely trying to explain that I was going to go about this the right way.
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Old 10-25-2006, 07:13 PM   #11
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3 phase electricity


You mentioned they bought a used motor, and I assume it was to save money. These guys should know then that 3 phase motors are 73% more efficient than single phase. They may pony up a bit more money at first, but if the motor is on for any amount of time then they'll be saving a lot more in the long run.
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Old 10-26-2006, 02:40 PM   #12
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3 phase electricity


Quote:
Originally Posted by Sparky Joe View Post
You mentioned they bought a used motor, and I assume it was to save money. These guys should know then that 3 phase motors are 73% more efficient than single phase. They may pony up a bit more money at first, but if the motor is on for any amount of time then they'll be saving a lot more in the long run.

True in most cases, but I'm not sure it would be true here. They bought the machine from a shop as it was closing down. I guess they paid $500 for the machine that costs $2500 new. They actually didn't know it was converted when they bought it either, not until I looked at it.

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