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#129980 - 12/17/05 09:12 PM clamp meter amp reading  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
I have a question about a reading I took yesterday. Here is the scenario, 3 ph. service, 200 amp, two panels fed from a gutter, I had 80 amps on "A", 17 amps "B", 80 amps on "C" at CT cabinet right before gutter. Now here is the wierd part, I had 1 panel that is a single phase tap from "A" and "B", and there are 58 amps on "A" and 61 amps on "B". How is this possible? How can the panel further down the line have more amps flowing than at the beginning of the service?
Now I have a theory(I guess that is why I posted on this section) that it has something to do with my meter. It is a Fluke 33 clamp meter. It isn't a true RMS meter, and I think it may have something to do with it. Another thought I had was that the nuetral was somehow canceling some of the magnetic field and that was doing it.

Well, any one run across this before?


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#129981 - 12/17/05 09:32 PM Re: clamp meter amp reading  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
Are you sure that you don't just have a 2-pole breaker from a 120/208 DP feeding a 208/240CT transformer to feed a 120/240 single phase panel? Your results would certainly make sense then.
Joe


#129982 - 12/17/05 10:03 PM Re: clamp meter amp reading  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
No, there is no transformer, just a 3 PH 4 wire 120/208, runs from a disco outside, into a large enclosure with CTs, then to an auxilary gutter. nippled to the gutter, are two panels. the main panel feed is tapped from the gutter, and the sub panel is fed from the main panel. The sub is fed from a 2 pole breaker in the main, on A and B phases. Sorry for the confusion, I have always called two legs of a 3 phase panel "single phase", guess it always reminded me of a 3 phase motor that lost a phase.


#129983 - 12/17/05 11:08 PM Re: clamp meter amp reading  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
Well my friend, I guess you get to take out the patent on the power producing breaker. Or your eyes are as bad as mine and you're really looking at a breaker going across C & A. But that wouldn't explain such an imbalance on the B phase. Did you per chance take your B phase reading with your meter at various positions along the conductor? I also wonder if you might not get a similar reading close by with out your pick up loop around the B conductor.
Joe

[This message has been edited by JoeTestingEngr (edited 12-17-2005).]


#129984 - 12/18/05 12:01 PM Re: clamp meter amp reading  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
Tell me about it, I am kinda not underwtanding the imbalance either. Had two sets of eyes on the meter, just so I could take that out of the equation. How could you get a two pole breaker to go across A-C? Maybe I am not visualizing this right, but if i had 2 breakers one on top of the other, lets say A-B, the next two pole would be B-C,and so on. Or am I just not seeing what you are saying?
Now that you say it, I never took any readings other than at the terminals of the subpanel, and in the CT cabinet. Never took any readings by putting the loop close to the conductor, how does that change the readings? change the amount of flux lines it sees?


#129985 - 12/18/05 01:26 PM Re: clamp meter amp reading  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
I would be strongly inclined to think that the two phase panel is tapped from the A and C phases rather than from the A and B phases; just a bit of screwy labeling, including hidden stuff like the way the breaker stabs/connection points are 'interlaced' to alternate phases going down the panel, would make it easy to mistake which phase the sub panel is tapped from.

However there are physically reasonable ways that you could actually see current flowing _between_ the various loads connected together, such that the supply from the mains to phase B is lower than the current circulating on the bus and between loads on the main panel and loads on the subpanel. Your comment about not having a 'true rms' meter opens the possibility of harmonic currents flowing from one load to another.

Even without harmonics, you can see this sort of effect. The classic example is a parallel inductor-capacitor circuit. Remember that current leads in a capacitor and lags in an inductor. If you connect an inductor and a capacitor together in parallel, then the leading current will balance out the lagging current, and the external current flowing through this circuit will be quite small. But the current flowing _between_ the inductor and the capacitor (circulating current) will be quite large.

See: http://www.tpub.com/content/neets/14181/css/14181_33.htm

-Jon


#129986 - 12/18/05 02:39 PM Re: clamp meter amp reading  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
Winnie, That was a very informative post.had not thought about harmonics playing a role in it. I was just sitting here going through it again in my head, and am kicking myself. Of course it could be on A-C, cant beleive I was thinking that it couldn't. I was quite sure I had identified A-B, but your right it makes more sense that it was C-A. I am going back tommorrow, we are putting in another 3 ph panel for some kitchen equipment, I will verify the phases in question. Thanks everyone for your input.

Gary


#129987 - 12/18/05 05:18 PM Re: clamp meter amp reading  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
Something still doesn't sound right to me. Are you sure that you don't have a high phase delta service here? Why would you have a panel with just 2 legs of 208. It's been a long time but I think our power experts here can explain that we could have both 120/240 & single phase 208 avail with a high phase delta. That would explain an imbalance because the "wild leg" would probably be lightly loaded. Be on the lookout for it and measure all three phases to Neutral.
Joe

PS: If so, it would be appropriate, because I seem to remember it looking like a Christmas tree.

[This message has been edited by JoeTestingEngr (edited 12-18-2005).]


#129988 - 01/17/06 11:04 PM Re: clamp meter amp reading  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
Man, I just hate it when people leave you hanging without telling you what they found!!! What do you think winnie?
Joe


#129989 - 01/18/06 12:28 AM Re: clamp meter amp reading  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Sorry for the delayed post; been busy and all that stuff, you know!
[Linked Image]

Although I believe the answer has already been given, I just wanted to toss in something.

Definitely looks (to me) like the Sub Panel is, in fact, connected to ØA and ØC from the Main Distribution (where the Ampere readings were made just outside the CTs).

Bussing in that Main Distribution Panel may have (by the Factory) been setup for the "ØC" leaving the CT Panel Section, to end up in the ØB Position at the Distribution Panel's section (possible, but kind of grasping at straws).
This would be a reality for some gear which may be used on either a 208Y/120V 3Ø 4 Wire Wye, or a 240/120V 3Ø 4 Wire Delta - as the "B and C Phase Position" would be swapped outside of the Metering Section.

Most likely it is simply the position on the Bus, where the Subfeed Circuit Breaker is landing.

If the Gear is using 225 Amp or 400 Amp Frame Sizes, a 2 Pole Breaker might only Terminate to the "AØ" and "CØ" landings on the standoff hardware.
Also, the Subfeed Circuit Breaker may be a 3 Pole, with the Single Phase feeders Terminated to the "Outer Poles" - or across A and C. Some Manufacturers request this type of Termination, when using a 3 Pole Breaker as a 2 pole Unit.

If the Subfeed Breaker is a 100 Amp Frame Size, located on a 42 Space, 250 Amp maximum Bus kit (typical to a common Panelboard), the sequencing of Line Terminations per space locations for 2 Pole Units would be like this:

<OL TYPE=1>


[*] Ckts. 1-3, or 2-4: ØA-ØB;


[*] Ckts. 5-7, or 6-8: ØC-ØA;


[*] Ckts. 9-11, or 10-12: ØB-ØC;


[*] Ckts. 13-15, or 14-16: ØA-ØB;


[*] Ckts. 17-19, or 18-20: ØC-ØA;


[*] Ckts. 21-23, or 22-24: ØB-ØC;


[*] Ckts. 25-27, or 26-28: ØA-ØB;


[*] Ckts. 29-31, or 30-32: ØC-ØA;


[*] Ckts. 33-35, or 34-36: ØB-ØC;


[*] Ckts. 37-39, or 38-40: ØA-ØB;


[*] Ckts. 41 & 42 would be 1 Pole across ØC only.
</OL>

Maybe this is of some assistance?

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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