ECN Forum Forums General Topics of the Trade General Discussion Area Amps is Amps??

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.