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Basic AC Power Systems

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John T
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PostPosted: Mon Apr 22, 2013 4:37 am    Post subject: Basic AC Power Systems Reply to specific post Reply with quote

Okay, there still seems to be some "corn"fusion out there about typical single phase and three phase AC power systems so heres a summary of typical installations:

1) TYPICAL HOUSEHOLD

Thats 120/240 Volt Single Phase Three Wire
Its a single transformer thats tapped in its
center. Its has 120 volt from EITHER outside
leg L1 or L2 to Neutral but 240 Line to
Line. Out at the transformer PLUS at the
service entrance, the Neutral is tied to
Mother Earth, a GroundING Electrode.

2) A TYPICAL COMMERCIAL SERVICE that uses
mostly 120 volt but has some 208 3 phase
HVAC requirement.

Thats a 208 Y 120 Volt Three Phase FOUR Wire
It has three legs of single phase 120 volt
ANY leg to Neutral is 120 but its 208 line
to line. The 208 three phase can be used to
run smaller HVAC loads. Its a Grounded system

3) A TYPICAL OLDER INDUSTRIAL PLANT with lots of heavy three phase or single phase 240 or 480 equipment and a small amount of 120

May be a 240 or 480 Volt Three Phase Three
Wire DELTA. It can serve 240 or 480 single
or three phase equipment. Generally it floats
its NOT a grounded system!!!!!!! In later
years we used them less and less as we
preferred a grounded system

If the utility didnt give us a 120/240 then we
used dry transformers to get it.

4) AN INDUSTRIAL PLANT with lots of 240 single
or three phase and a small amount of 120

That could be the 120/240 volt Three Phase
FOUR Wire center tapped DELTA.

It has three legs of 240 single phase and
240 volt three phase and can supply 120.
It has two legs of 120 to Neutral (via the
center tapped transformer) but the red/high
leg is 208 to Neutral (NOT used)

There are OTHER systems including open deltas etc but in all my years we didnt get into them. I forgot, sometime we used a 480 Y 277 Volt Three Phase Four Wire for heavy 40 3 phase HVAC and we used the 277 for lights. Of course, dry transformers for 120 as needed. Our Primary was 12470 Y 7200 Three Phase Four Wire. NOTE how as above, 12470 = 7200 x square root of 3.

Nuff said, got it?? Any questions??

John T toooooo long retired Electrical Engineer this stuff is gettin more stale alll the time lol SO AS ALWAYS NO FREAKIN WARRANTY
 
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Greg K
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Location: Nebraska

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PostPosted: Mon Apr 22, 2013 6:13 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

Can you explain 2 phase power? I have seen it addressed in the code book, but don't know what it is. Then there is the 120/208 "high leg". Basically a 3 phase delta, it has 2 legs of 120v to neutral and one leg of 208 to neutral, but 208v between any two legs. Good lesson John, and tomorrow someone will ask about grounding Smile
 
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David G
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PostPosted: Mon Apr 22, 2013 6:13 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

John,

The 480V Y is probably the most common in industrial facilities now, with 277 single phase.

they will put in a 277-120 transformer for 120VAC power.
 
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JMOR
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PostPosted: Mon Apr 22, 2013 6:40 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

greg k wrote:
(quoted from post at 10:13:06 04/22/13) Can you explain 2 phase power? I have seen it addressed in the code book, but don't know what it is. Then there is the 120/208 "high leg". Basically a 3 phase delta, it has 2 legs of 120v to neutral and one leg of 208 to neutral, but 208v between any two legs. Good lesson John, and tomorrow someone will ask about grounding Smile


 
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Dr. Walt
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Location: Pahrump, Nevada

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PostPosted: Mon Apr 22, 2013 7:03 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

Two questions; first, does the Neutral work the same as a Ground? and second, can a Neutral be carrying Current? .
 
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David G
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PostPosted: Mon Apr 22, 2013 7:33 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

The neutral should carry current, the ground is only for fault protection. They should be bonded together at one location.
 
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dr sportster
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PostPosted: Mon Apr 22, 2013 7:48 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

High leg /red leg . When you see orange tape on the {most likely panel] B phase and no brown and yellow tape indicating 480 service. Now for the music --- Step back and think before you install those single pole breakers -step back and think before you install those single pole breakers. To any tune you want -sing this song. Do a little dance. Individual spaces need not be marked by code . Use the meter while you dance and sing and not make 120 volt stuff smoke at 208.
 
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Dr. Walt
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PostPosted: Mon Apr 22, 2013 8:49 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

David G:

If the Neutral IS carrying Current, & as you say, the Neutral and the Ground should be bonded at one location - then wouldn't that create a DEAD SHORT ?
 
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David G
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PostPosted: Mon Apr 22, 2013 9:13 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

No, because current must have a return path. The current in the neutral is after the load.
 
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David G
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PostPosted: Mon Apr 22, 2013 10:14 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

The neutral is not considered potential carrying, it should only have voltage on it caused by resistance and current referenced to ground.
 
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John T
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PostPosted: Mon Apr 22, 2013 10:18 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

NOOOOOOO The Neutral (A GrounDED Conductor) DOES NOT I REPEAT NOTTTTTTTTTT WORK SAME AS THE EQUIPMENT GroundING CONDUCTOR.

Neutral (A GrounDED Conductor) is for the normal return current.

Equipment GroundING Conducotr is for fault current ONLY NOTTTTTTT normal return current. Its there to provide a dedicated low resistance current path for fault current ONLY

John T
 
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John T
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PostPosted: Mon Apr 22, 2013 10:20 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

Thanks, heck they already did lol see above

John T
 
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John T
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PostPosted: Mon Apr 22, 2013 10:26 am    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

The Neutral (A GrounDED Conductor) carries return current back from the load to the Panels Neutral Buss. The Equipment GroundING Conductor DOES NOT carry any current back from the load, ONLY fault current if a fault exists.

At the main service entrance and only there the Neutral Buss in bonded to the Equipment Ground Buss but still the equipment ground does NOT carry any current ONLY carries current if theres a fault.

PS for the poster and all here: Its IMPOSSIBLE to explain in a paragraph here what can take books to fill and years of engineering practice and experience to fully comprehend SO DONT FEEL A BIT BAD IF YOU DONT UNDERSTAND IT

John T BSEE, JD
 
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WyoDave
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PostPosted: Mon Apr 22, 2013 2:54 pm    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

And if you have a farm with 8 irrigation services, and 3 farmyards, you get to use all 4 of those setups.

David
 
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DH in Carolina
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PostPosted: Mon Apr 22, 2013 2:55 pm    Post subject: Re: Basic AC Power Systems Reply to specific post Reply with quote

What about a zig/zag transformer system to get a neutral from a ungrounded three phase system. I have only did one of these years ago in a plant using three single phase transformers. DH
 
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