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

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.

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

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 ..

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.

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.

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).]

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.

Here's another interpretation

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

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.

Pinemarten, are you talking about this part of Scott's post?

I read it a couple of times before I noticed the "not" part.


It would be unusual for Scott to be confused about electric theory, color me jealous.

Of course we could go into non-linear loads but I notice Scott avoided this.

Bob




[This message has been edited by iwire (edited 09-30-2003).]

If you had three individual single-phase loads wired to a panel, each connected phase-to-neutral and one on each phase, each load being resistive and drawing exactly 200A, then you would read 200A on each of the single-phase neutrals.

But because the currents are 120 degrees out of phase they would cancel at the panel, and the current flowing in the feeder neutral would be zero.

In fact in a perfectly balanced system (if such a thing could exist) you could disconnect that feeder neutral and everything would carry on working just fine, with each phase-neutral load still seeing the same voltage.

iwire, I was referring to the last sentence in the paragraph.

Quote:
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!

I was wondering about the 'rare occasion' Scott was referring to.

Pinemarten

I got you now IMO this is a "very rare occasion" to have that kind of balance in a real world transformer.

It works great on paper but not for real.

Paul, I just love telling new guys that with a balanced load you could get rid of the neutral. (I know, not by the NEC)

I'm sorry to leave everyone dangling here on this topic! Forgot to take a look at the replies in this thread until now!

Please launch flames at me for doing that!

Anyhow, to answer the question(s) about the quoted statement (snipped from my previous post - covering the L-C loads), here's the scoop:

When I mentioned the L-C loads being all equal in load current, AND all being Resistance loads, I should have added more details!

Paul explained it very well, but here is an additional follow-up anyway!

On a 3Ø 4 Wire Wye system, if all loads are PURE RESISTANCE type only (no reactance, or 100% Power Factor - all power figured as KW, not KVA), AND the loads are equal on all three Phase Lines to the Common Grounded Conductor (noodle), then the load current on the common noodle of a 4 wire branch circuit (or resultant subfeeder / feeder), will be almost zero - or, in other words, the system is fully balanced.

Example:
If ØA, ØB and ØC had pure resistance loads connected L-C (Line to Common), and each Phase Line had exactly 100 Amps flowing, the load current on the noodle may be as low as zero amperes.

Normally, there is going to be a percentage of the Line Current flowing on the Common Noodle, which might be upto 33 Amps in the afore mentioned example, if the situations are "normal" (explained briefly in the following text).

A "Pure Resistance Load" is referring to some type of appliance or piece of equipment, which draws only True Power (Watts only).
Loads of this nature are Resistors, which are found in Heating applications.
Incandescent Lighting falls in this area too.

Best examples of pure resistance loads are Electric Water Heater elements and Electric Oven / Stove elements.
These have almost "Zero Reactance", so they have 100% power factor, and all power is true power.

Coiled Resistive Elements - such as Heater Windings (Electric Heating) found in Toasters and Portable Heaters; and non-heater applications - such as Incandescent Lamps, have a higher level of Reactance, yet the figure is so low, it can normally be ignored in most cases, and also be figured as True Power.

The coiled Conductor path results in an Inductor with a fixed Resistance (or figured point of Resistance for Incandescent Lamps). As the Current increases intensity, the natural Inductive Reactance effect of the coil takes place, and the corresponding Phase Offset results.
The Inductive properties results in VARs being stored in the element.

As mentioned, the figure is normally so low, it may be dropped, and the load can be "Assumed" to be 100% True Power / 100% Power Factor.

As you can see, only a few load types fall into this category. There are far more load types that are not like this at all - these are known as Reactive Loads (or loads with a Power Factor percentage, also the familiar term of total KVA).

The most well known term is "Non-Linear Loads", but not all KVA loads are "Non-Linear"!

An Induction Motor is a KVA device, but it is a "Linear" Load!

Something like an Electronic Ballast (which produces a certain level of Harmonics) is, in reality, the "Best" description of "Non-Linear Loads".

Now to describe the "Rare Situations" of having a Zero Current load on the Common Noodle on the 4 wire Wye system.

The reason it's such a rare thing is that the loads need to be exactly the same - and continue to be the same!
It's not easy to have 100% similarities between different Resistive elements - they all have a certain tolerance (in the case of High Power elements, tolerances are at least ± 20% the devices rating).
Also, they all need to be drawing the same level of current at the same time. If one L-C circuit has higher or lower draw, the entire figure is changed, and the noodle becomes a current carrying conductor.

It's also difficult to keep the Voltage between L-C (as measured at the element its self), at a steady value. The load current will change when the Voltage changes.

Lastly, the reality of such situations where the well balanced Pure Resistance only loads will be encountered, is very slim!
Not all loads will be used at all times.
Also, there are greater chances that Reactive loads are going to be used in conjunction with, or in place of, pure resistance loads.

In these situations, the Noodle carries (at least) the same level of current as the highest level drawn on any of the Ungrounded Conductors (figure highest L-C load current).

In this case (as this case being the most common scenario), if the loads are mixed, there will be a significant load current on the noodle.
If all loads are Reactive, the load of the highest Phase Line will also flow on the noodle.
If the loads are Harmonic, the noodle will carry the highest Phase Line current value, plus an additional level of current (which is the THD percentage of the loads). On a 4 wire Wye multiwire branch circuit, if the L-C loads were Non-Linear Harmonic Producing ones (with 33% THD), and each Ungrounded Conductor has a load current of 10 Amps flowing, the Common Grounded Conductor ("Noodle") will have 20 Amps flowing on it!

I hope this makes a little more sense!

Scott 35

When somebody seems to be having trouble grasping the idea of the neutral carrying no current in a balanced 3-phase system, I find that it sometimes helps to start by explaining a 3-wire DC system, where the positive and negative voltages are more easily visualized, then go on to a 3-w 1-ph AC system. (The latter should be that much easier for most Americans to follow due to the widespread use of the system for residential services.)

Once someone can visualize what is happening with the unbalanced current from two hot legs 180 degrees out of phase, it then becomes easier to extend the principle to 3 phase and 120 dgrees.

I seem to remember hearing laws allowing the 'down-sizing' of the noodle conductor.
I don't think it is legal in Canada anymore, and I can see why.
When I order wire, I order all black. Once I figure out its purpose, I wrap it in coloured tape.
One wire, one colour, one size, 'do the math', and pick a pipe size.
I leave it to the bean counters to figure out if I wasted any money in the long run.

Hang on a minute fella's!.
In a star-connected system, the Neutral only carries the out-of balance current where ALL of the loads on it are 3 phase loads, and like most of you guys have said, this system of purely balanced 3 phase loads is un-achievable, that's what differs Star from Delta connections(among other things!)
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All Electrical circuits, wether Resistive or not, have a Reactive component to them, even Power Lines have them, a combination of Capacitive Reactance and Inductive Reactance.