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05-01-2008, 08:41 PM
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#1
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Junior Member
Join Date: Nov 2007
Posts: 19
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Wire Size for Volatge Drop
My existing service is a 200 Amp 3 Phase 120/240-Delta configuration with C being the High Leg or "stinger" . I've got to get 100 Amps to a barn 750' away. I only need 120/240V single phase to this barn. Can I drop my C Leg and still use 3/0 wire or do I need to upsize my wire even larger to accomodate for voltage drop and the lack of that C Leg. Any help would be greatly appreciated.
Bryan
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05-01-2008, 08:55 PM
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#2
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Moderator
Join Date: Jan 2007
Location: NY State
Posts: 7,510
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This is a very common thing to do, to run a single phase feeder off a three phase panel.
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05-01-2008, 09:31 PM
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#3
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Junior Member
Join Date: Nov 2007
Posts: 19
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Thanks for the reply Petey... BUT, do I have to treat it as a SINGLE PHASE application since I'm only sending an A & B Leg (No C) to my sSub Panel and adjust my wire size to a Single Phase Application? Remember, my distance is 750 feet. Voltage Drop calculators are telling me I need to size my wire to 300KCMIL for 3 Phase and 400KCMIL when I treat it as a single Phase....?
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05-01-2008, 10:22 PM
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#4
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Town Drunk
Join Date: Jan 2007
Location: Baltimore, MD, USA
Posts: 3,716
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Quote:
Originally Posted by brybedo
Thanks for the reply Petey... BUT, do I have to treat it as a SINGLE PHASE application since I'm only sending an A & B Leg (No C) to my sSub Panel and adjust my wire size to a Single Phase Application? Remember, my distance is 750 feet. Voltage Drop calculators are telling me I need to size my wire to 300KCMIL for 3 Phase and 400KCMIL when I treat it as a single Phase....?
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You will have to upsize for sure at that distance.
I would calculate at single phase.
__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution. 
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05-01-2008, 10:33 PM
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#5
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Moderator
Join Date: Jan 2007
Location: NY State
Posts: 7,510
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Two phases of a three-phase service is single phase no matter how you look at it.
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05-01-2008, 10:37 PM
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#6
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Town Drunk
Join Date: Jan 2007
Location: Baltimore, MD, USA
Posts: 3,716
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Quote:
Originally Posted by Speedy Petey
Two phases of a three-phase service is single phase no matter how you look at it.
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Over at Mike Holt's, some poor guy asked why we call 2 legs of a 3 phase system "single phase"
The engineers got hold of it, and the thread is now over 600 posts... 
__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.
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05-01-2008, 10:42 PM
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#7
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Junior Member
Join Date: Nov 2007
Posts: 19
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Thats the way I see it... try telling that to your expert physicist customer. Any body else care to back me up? please???
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05-01-2008, 10:43 PM
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#8
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Junior Member
Join Date: Nov 2007
Posts: 19
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Where's that post?!!!!?? I need it!
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05-01-2008, 10:48 PM
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#9
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Town Drunk
Join Date: Jan 2007
Location: Baltimore, MD, USA
Posts: 3,716
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Quote:
Originally Posted by brybedo
Where's that post?!!!!?? I need it!
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http://forums.mikeholt.com/showthread.php?t=96673
620 posts and counting.
God, I love engineers... 
__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.
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05-01-2008, 10:59 PM
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#10
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Moderator
Join Date: Jan 2007
Location: NY State
Posts: 7,510
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Quote:
Originally Posted by JohnJ0906
620 posts and counting.
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To quote Bob Badger from post #2:
Quote:
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Originally Posted by iwire
Your best bet is to just accept it. 
This question is bound to generate a ton of responses.  |
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05-01-2008, 11:00 PM
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#11
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Town Drunk
Join Date: Jan 2007
Location: Baltimore, MD, USA
Posts: 3,716
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Quote:
Originally Posted by Speedy Petey
To quote Bob Badger from post #2:
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And he hit it right on the head, didn't he?
__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.
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11-24-2008, 04:20 PM
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#12
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Junior Member
Join Date: Nov 2008
Location: England
Posts: 24
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In the uk you will see on test certs . schedule of results that when you do a ir test it asks for the result between phase and phase and not phase and n
are they wrong too....lol
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11-24-2008, 04:22 PM
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#13
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Junior Member
Join Date: Nov 2008
Location: England
Posts: 24
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as for the cable calculation. i would do it as single phase and make sure not to that the volt drop stays within 4% of the supply voltage. if it is not then you will need to up the cable size
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11-29-2008, 12:54 PM
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#14
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Senior Member
Join Date: Oct 2008
Location: Richmond Va.
Posts: 113
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__________________
Just Waking up is a good start
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11-29-2008, 02:45 PM
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#15
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Licensed Journeyman
Join Date: Oct 2007
Location: third world/New Orleans to Mobile
Posts: 5,108
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why is the high leg C phase? see 408.3 (E) doesnt sound like you fit the execption
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11-29-2008, 04:07 PM
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#16
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Senior Member
Join Date: Oct 2008
Location: Richmond Va.
Posts: 113
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Quote:
Originally Posted by nolabama
why is the high leg C phase? see 408.3 (E) doesnt sound like you fit the execption
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I have never seen it as C phace it's always the B phase
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Just Waking up is a good start
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12-17-2008, 02:35 PM
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#17
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Bilge Rat
Join Date: Aug 2007
Location: Fernley, Nevada (near Reno)
Posts: 2,372
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Years ago it was very common to have the high leg phase C.
It was also common to have an existing single phase installation, and add another panel for 3 phase needs. You'd change the meter base, sometimes just the meter mounting block, run another wire from the new panel up the mast, and the POCO would hang another pot on the pole.
Since the existing single phase panel was usually black and red (A and B), the high leg was made to be blue (C). Also, on a feed-through meter, the high leg is phase C.
Rob
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02-08-2009, 11:05 AM
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#18
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Junior Member
Join Date: Feb 2009
Location: canada
Posts: 2
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Quote:
Originally Posted by brybedo
My existing service is a 200 Amp 3 Phase 120/240-Delta configuration with C being the High Leg or "stinger" . I've got to get 100 Amps to a barn 750' away. I only need 120/240V single phase to this barn. Can I drop my C Leg and still use 3/0 wire or do I need to upsize my wire even larger to accomodate for voltage drop and the lack of that C Leg. Any help would be greatly appreciated.
Bryan
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i cant believe u guys still use 120/240 delta.why dont you guys use 120/208 y.That c phase ,what we call a bastard leg just means a hole lot of trouble.Thats not the proper way to do things,there taking the c phase off the middle of the windings to create 208 to ground,off that phase.wow very scary practice.very old school.
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02-08-2009, 11:14 AM
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#19
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Moderator
Join Date: Jan 2007
Location: NY State
Posts: 7,510
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Most times we don't have a choice. It's what is there.
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02-08-2009, 02:27 PM
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#20
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Senior Member
Join Date: Nov 2007
Location: Miami Fla.
Posts: 2,274
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Quote:
Originally Posted by nolabama
why is the high leg C phase? see 408.3 (E) doesnt sound like you fit the execption
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High leg would be at C at meter base. POCO meters are set up this way. High leg at B main disco as per 408.3(E).
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