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Tool Talk Discussion Forum

Re: 3 phase question

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John T

10-24-2013 12:18:51

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Okay, heres the deal, but this is how it was when I was an electrical power distribution design engineer and things may have since changed so no warranty.

SOME equipment is labeled and designed to operate at EITHER 208 OR 240 while some IS NOT, so the correct engineering answer is IT DEPENDS. That being said, much of equipment will still operate at EITHER even if not so labeled perhaps not as efficient or as much output if operated at a reduced voltage.........

The ONLY time I specified 208 Y 120 volt three phase four wire was for say an office complex (NOT an industrial application) where the majority of the load was 120 volt single phase but the HVAC was operated at 208 volt three phase with compressors and air handlers designed for use at 208 and clearly labeled as such.

The more typical industrial service was 480 Y 277 three phase four wire in which case I specified 277 volt lighting and used a dry transformer to get the needed 120 volt single phase

As some correctly stated, on the 208 Y 120 volt three phase four wires system its 120 volt from any leg to Neutral (3 legs of 120) but 208 from any line to other line since 120 x square root of 3 (1.732) = 208.

The thing is I see ALOT MORE 240 volt three phase equipment then I do 208 since 240 is more the standard then 208 although sure, "much" equipment can still work at reduced voltage

If I had my druthers and I needed a lot of three phase 240 (more typical equipment) instead of 208 plus still some 120 Id opt for maybe the 120/240 volt three phase four wire High/Red leg Delta system.

THE CORRECT ENGINEERIGN AMSWER DEPENDS ON THE LOADS AND USES AND HOW MUCH 3 VERSUS SINGLE PHASE EQUIPMENT YOU WILL USE and that cant be answered without first knowing all that. Sure Billy Bob and Bubba can render an opinion or what worked best for their shop/home but that may or may NOT be best for your individual situation

NOTE I do like the idea of three phase but 208 isn't my first choice of voltage, that would be 240 for small shop/farm use.....

Your money your choice, CHECK WITH THE UTILITY PROVIDER and do a little design and leg work as to your loads and uses and the equipment youre gonna operate is my engineering advice ......

John T

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10-24-2013 12:49:01

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 Re: 3 phase question in reply to John T, 10-24-2013 12:18:51  
Thanks John. I understand exactly what you're saying. I was thinking originally for 2 meters. A standard 120/240 single phase to feed the house, and then a 120/240 delta 3 phase for the shop. Then I thought just go with the 120/208 for both and only have one meter. I'm not too crazy about the idea of a 120/240 high leg 3 phase power in the house since that could confuse some electrician down the road with that high leg. I think they'd be more apt to check for it in a shop building. What are the guidelines behind having a 120/240v 3 phase service and only feeding the two 120v legs to the house? Just not hooking up the wild leg? Is that major violation, or something the provider will be against? Kinda thinking out loud.


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John T

10-24-2013 14:49:19

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 Re: 3 phase question in reply to WyoDave, 10-24-2013 12:49:01  
Hey there Dave, I see from your response you certainly understand the benefits or pitfalls of the 208 or red leg system, you're NO Billy Bob!!!

Heres my experience. Actually any half A$$ real electrician knows and well understands the red leg system, they have seen it in industrial applications if they have ever done much work at all and aren't confused or dangerous if all is configured and labeled correctly. Our electricians labeled and red taped any high legs in all panels plus clearly labeled the panelboards.

NOTE for the house you don't have any red leg problem, you run 4 wires to it, the two hots (from that center tapped transformer), Neutral, and Equipment Ground. Theres no red leg even ran to the house, its feed is same as all other houses, simply a 120/240 center tapped transformer THATS NO BIG DEAL every other home in the neighborhood is fed from a 120/240 center tapped transformer!!!!!!!

HOWEVER when I specified that system it was in a primarily industrial application where I had a chance to balance the 3 transformers and there was a lot more 240 loads (both single and 3 phase) then the 120 which was for the office to run on. But if you don't use much 240 those other two transformers are just gonna be sittin there wasting heat energy while the one center tapped transformer that feeds your house doin all the work.

This is why I talked about the choice depending on the loads. I'm just not a fan of the 208 Y 120 for a shop or farm where most of the equipment choices are better suited to 240 then 208, although most will still work

Talk to the utility provider and get a good handle on the equipment youre gonna be using and see if 208 or 240 will work and then make the decision. I don't want to discourage your from three phase, its best, but just take a look at your actual loads before making the decision is my best albeit rusty engineering advice. Again, any real electrician isn't confused by that system so I wouldn't worry that much over that, just be sure all is well labeled

John T

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10-24-2013 15:10:26

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 Re: 3 phase question in reply to John T, 10-24-2013 14:49:19  
Thank again, and for the record I am a Billy Bob. Just been around a lot of 3 phase for irrigation purposes, and it's a little of this, or a little of that. We have 277/480 wye and 480 corner grounded. 120/240 delta red leg. 240 corner grounded. No 120/208 wye though. Almost want to add it, so I have them all. Haha. Thanks again.


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John T

10-24-2013 15:16:32

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 Re: 3 phase question in reply to WyoDave, 10-24-2013 15:10:26  
Since I'm not a fan of floating Delta at our facility I specified the very first corner grounded straight delta that was ever used. Shortly after one of the electricians came to me all excited saying he put his meter on each leg to ground and one (of the 3) of them WAS DEAD, ZERO VOLTS

I say well duhhhhhhhhhhhhhhhh lol wonder how that happened hmmmmmmmmmmmmm

John T

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10-24-2013 15:39:19

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 Re: 3 phase question in reply to John T, 10-24-2013 15:16:32  
I've never been around an ungrounded system. My Dad has mentioned having them, but the power company changed them out before I worked on anything. Thanks for the advise. I'll let you know what I end up doing.


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10-24-2013 12:39:50

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 Re: 3 phase question in reply to John T, 10-24-2013 12:18:51  
John T--in my past experience in working with older Y circuits for bridge power we quite often came across the problem of a HIGH Leg--where one of the legs to neutral was quite higher than 120 V. Would this be of a concern here??

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John T

10-24-2013 14:52:55

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 Re: 3 phase question in reply to dpendzic, 10-24-2013 12:39:50  
In the Y system (208 Y 120 volt three phase four wire) theres no higher leg, all are 120 to Neutral or 208 phase to phase. ITS ONLY IN THE DELTA (120/240 volt three phase four wire center tapped DELTA) where two legs are 120 to neutral while the red/high leg is 208 to neutral BUT THATS NEVER USED ANYWAY......

Got it??????? If not post back

John T

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10-24-2013 17:00:35

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 Re: 3 phase question in reply to John T, 10-24-2013 14:52:55  
THANKS!!--my memory is failing and we did have delta supply circuits---I remember that high leg very well after we blew some relays!!

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