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Amps is Amps?? #29758 09/24/03 03:39 PM
Joined: Nov 2001
Posts: 75
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Bill39 Offline OP
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We had a lively discussion at work yesterday and I'd like some comments from this forum on the subject.

Suppose I have a service from a Y-connected 3ph-208/120VAC 200amp panel protected by 200amp fuses. I use a standard Amprobe to measure each leg of the panel feeder wires
and it reads 150 amps. Does this mean there is also 150 amps phase-to-phase flowing thru
each fuse, or is it less than that?

One fellow was saying that measuremening the current on one leg will be significantly
higher than what is flowing through the entire system, because of the characteristics of 3ph transformers (vectors, phase angles, etc.) In other words, an ammeter can indicate that the panel's load is more than 200 amps and still not blow fuses. I'd appreciate your input on this.

Thanks

Work Gear for Electricians and the Trades
Re: Amps is Amps?? #29759 09/24/03 04:59 PM
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Ron Offline
Member
Maybe its a terminology issue. A 200A 120/208V, 3 phase service, can have up to 200 amps per phase (leg) of non-continuous load. There is no phase to phase current outside of the transformer.


Ron
Re: Amps is Amps?? #29760 09/24/03 05:34 PM
Joined: Jan 2003
Posts: 4,391
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iwire Offline
Moderator
Bill I am not sure I understand what you are getting at but if the amprobe is around the same conductor as the fuse is installed in they see the same amp load.

Is it possible you are talking about the deference in amps from the primary feed and the secondary load?

If the load on the secondary at 208 volts is 100 amps, the 480 volt primary will only have about 43 amps of load (roughly).

You trade volts for amps, sorry if you already knew this and where talking about something else.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Amps is Amps?? #29761 09/24/03 11:59 PM
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Posts: 202
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jlhmaint Offline
Member
I will take a stab at this. Not sure what your buddy is trying to get at but if you measure 150amps per leg on a 200amp panel then you are drawing 150amps on that 200amp service. The amp meter should not indicate more than 200 amps and a fuse not blow or the fuses are rated to high over the allowable. Unless your buddy is mistaken like some people get and they add the 150amps times 3 legs = 450amps. But thats not how it works if you have 150 on each leg that is your total more or less. just my 2 cents ..

Re: Amps is Amps?? #29762 09/25/03 05:31 AM
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Pinemarten Offline
Member
I think your colleague is referring to total amps on a three phase system.
I(line) X 1.73 = I(total)
The amperage of the three phases individually cannot be added for total amps on a three phase circuit.

Re: Amps is Amps?? #29763 09/25/03 06:54 AM
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pauluk Offline
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Each of the three fuses is in series with the appropriate feeder conductor.

If you measure 150A on a phase, then there must also be 150A flowing through its associated fuse.

Re: Amps is Amps?? #29764 09/25/03 07:19 AM
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Redsy Offline
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How about this...

On a Wye system, line voltage (line to line) is 1.73 X phase voltage(line to ground) , and line current is equal to phase current(the current in a given transformer coil).
On a Delta system, line current is equal to 1.73 X phase current(the current in a given transformer coil), and line voltage is equal to phase voltage.

Maybe he is referring to this concept?

[This message has been edited by Redsy (edited 09-25-2003).]

Re: Amps is Amps?? #29765 09/25/03 01:31 PM
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Bjarney Offline
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Don’t forget that loading a little above rated will not cause immediate overcurrent-device operation. A 220-ampere load on a 200-amp breaker pole or fuse {10% over} may take hours to operate—and also get significantly warmer than most might expect.

Obtain the device manufacturer’s time-current characteristic curve for specifics.

Re: Amps is Amps?? #29766 09/27/03 03:26 AM
Joined: Oct 2000
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Scott35 Online Happy
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Member
Here's another interpretation [Linked Image]

If all loads are L-C (Line to Common Grounded Conductor, or "Noodle"), then:

ØA, ØB and ØC may be all at 200 Amps, and the Common Grounded Conductor will also be at 200 Amps (neglecting THD >10%).

If L-C loads are all Resistive, and each Line is at 200 Amps, the Common Noodle may not be drawing too much Current!
This would be a very rare occasion!

Anyhow, the load Current flowing in a single Conductor will , of course, flow through the Fuse (or Circuit Breaker) also. This is the reason for the OCPD.
Measuring "I" (Amperes) using a Clamp-Around Ammeter (such as the mentioned "Amprobe") on a Conductor, shows the load current on that Conductor at that certain location.

The real deep question to throw back at your co-workers (for a really indepth think-a-thon), would be what's the total L-C load at the Transformer (or main service if Transformer is not reachable).
Figure first by calculating the L-C loads, then check levels via Ammeter at various points prior to checking at the supplying point.
Sweeten the deal with some friendly wagering - such as the closest to the right answer(s) gets a free lunch (paid by the rest of the participating crew!).

Also, consider what Bjarney has mentioned about non LCL loads!
A 20 amp circuit may have 20 amps for eternity, 30 amps for an hour, 40 amps for a few minutes, 100 amps for maybe 15 seconds, and so on!

Discussions like this at work are very good ideas! Best way to learn is to toss ideas around and verify answers with referenced items - such as was done in this forum!

Keep up the activity! Just don't waste too much work time doing this stuff! (gotta throw out that Foreman Attitude once in a while!)

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Amps is Amps?? #29767 09/30/03 04:08 AM
Joined: Aug 2003
Posts: 123
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Pinemarten Offline
Member
I think you are confused Scott.
Doesn't the noodle always carry the unbalanced load. If 3 phases are loaded to 150A each with all devices phase to neutral, then the neutral would carry 150 X 3 but not at the same time.
One phase is rising as the other two are falling, if they all peaked at once or zeroed at once, the fuses would blow, or motors would stop.
If the voltage or amperage were metered at one point in time, it can only total the maximum voltage or amperage.

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