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#55101 08/16/05 06:26 PM
Joined: May 2005
Posts: 16
S
Member
The guys just replaced a single phase air handler in a resturant with a new 3 phase AH.

The 3 phase panel doesn't have a seperate bond ground buss bar although the old single phase AH was grounded to the existing ground buss bar in the panel.

To look inside this old 3 phase panel you would think it's a regular single phase type because all the breakers are double pole. The only way to tell is by checking the voltage because it has no 120 volts, both hot legs are 240 to ground.
The new 3 phase Ah is supposed to have 4 wires but there is only 3 coming from the panel.

I haven't seen one of theses 3 wire - 3 phase panels since many years ago when one of my x-partners connected up a vacuum pump on the roof of a resturant with a cheater cord and found out 120 volt vacuum pump motors don't last very long on 240 volts.

Anyway, what would be the proper way to connect a case ground in this situation, just run another wire back to the panel and connect it to the ground buss?

Also, I know I'm showing my ignorance but it's been over 40 years since electrical school plus you just don't see these panels around here anymore (mostly we work with 208/120) but what is this type of 3 phase called?

#55102 08/16/05 07:43 PM
Joined: Jan 2005
Posts: 5,316
Cat Servant
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What you have is a "corner grounded delta." That third leg- even though it hay read "O" volts to ground- is hot as hell!

I'm really surprised this type of three phase service was installed. I've only seen it for well pumps. I don't like it there- and I really object to it for above-ground uses!

#55103 08/16/05 07:56 PM
Joined: Jul 2004
Posts: 9,646
G
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How is the corner "hot as hell" if it is the only grounded conductor. "Hot" in reference to what? The only thing that makes this hotter than 3p wye is 2 phases are 240 above ground instead of 3 being 208 above ground.
(or one phase being 208 above ground and 2 being 120 in "red leg")


Greg Fretwell
#55104 08/16/05 08:08 PM
Joined: May 2003
Posts: 2,876
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e57 Offline
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Reno - Don't see any indication in the thread that would point to a corner ground, could also be an open delta, or just a panel that a neutral wasn't run to. Seen that a few times.

Anyway, SparksNmore, you say "both hot legs are 240 to ground." Whats the third leg voltage? Is there a third leg? Your describtion sounds like there is still a single phase there.


Mark Heller
"Well - I oughta....." -Jackie Gleason
#55105 08/16/05 08:18 PM
Joined: May 2005
Posts: 16
S
Member
The third leg is grounded, if I remember corretly it was "B" center leg because like I said everything looks just like any single phase residential panel.
The only voltage reading you can get is 240 volts.

#55106 08/16/05 08:20 PM
Joined: May 2003
Posts: 2,876
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e57 Offline
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Ah, then yes, you have a "corner ground."

See this thread... https://www.electrical-contractor.net/ubb/Forum15/HTML/000078.html


Mark Heller
"Well - I oughta....." -Jackie Gleason
#55107 08/16/05 08:20 PM
Joined: Jul 2004
Posts: 9,646
G
Member
[q]"To look inside this old 3 phase panel you would think it's a regular single phase type because all the breakers are double pole. The only way to tell is by checking the voltage because it has no 120 volts, both hot legs are 240 to ground."[/q]

That sure sounds like corner delta to me


Greg Fretwell
#55108 08/16/05 08:41 PM
Joined: Jul 2004
Posts: 9,646
G
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You really treat corner delta like it was 240/0/240 single phase. The neutral is white and isolated from the Equipment Ground. This is a 4 wire branch circuit and feeder.
If you did want to address the "hot" concern from Reno you would point out the neutral always carries the same current as the phases, not unbalanced current.
That is why it is not regrounded after the service disconnect.


Greg Fretwell
#55109 08/16/05 10:24 PM
Joined: May 2005
Posts: 16
S
Member
240 volt Corner Grounded 3Ø 3 Wire Delta System.
Pretty interesting reading, thanks!

This is the only system that I know of still in use around here but I'm sure there are more, they're just not our customers.
Looks like we need to run an equipment ground back to the panel.

Thanks again, I've always enjoyed lurking around here, reading "the words of the experts".

#55110 08/17/05 07:40 AM
Joined: Aug 2001
Posts: 7,520
P
Member
Was there ever any accepted convention on phase colors for a corner-grounded delta such as this?

i.e. The grounded phase would have to be white, so which of the regular black/red/blue phase colors was normally dropped?

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