Breaker Ampacity

[SPierce]

I understand you absolutely cannot exceed the breaker rating on any single phase, without the breaker tripping. But, isn't one characteristic or advantage of having a 3 phase panel over a 1 phase, besides having 3 phase power for necessary loads, is having an extra 200 amps at your disposal?

 

[roger]

It may be easier to understand by using KVA instead of amps, because KVA is KVA. For example, on a 24KVA load @ 120V, the rated current is 200A. On a 48KVA load @ 240V, (2 pole) the rated current is still 200A. When you install the load, you are effectively splitting the 48KVA between each phase, then you would have 24KVA on each phase and the voltage also drops to 120V, so the rated current is still 200A, per phase. You could effectively use 2 single pole breakers each rated the same to protect the load. But the total KVA does not increase, it is still 48KVA, and the amount seen on each phase is 24kVA.

IMO, this is the best practical explanation for someone having a hard time understanding the principle, in short, the amount of power supplied on each leg or phase is the same, watts are watts.

Roger

 

[charlie b]

I seem to have missed out on the discussion. I would have liked to jump back in earlier.

Let me start by reminding everyone of the following type of disagreement, for I believe we are in the middle of just such a misunderstanding.

Person One says, ?The sky is blue.? Person Two says, ?No, you are wrong, the grass is green!?

Rick, we are both wrong. The total current is, in fact, neither 200 nor 600. The total current is zero, and will always be zero regardless of power factors, phase angles, or the degree to which the loads are balanced. Kirchhoff?s Current Law demands that it be zero.

That said, I think we need to get our use of terminology settled out, for one of us is talking about the color of the sky, and the other is talking about the color of the grass. I do not like the use of the word ?total,? in the context of currents flowing in more than one phase. I particularly dislike the use of the word ?per? within the phrase ?amps per phase.? These two words, I do believe, are the cause of the misunderstandings and disagreements on display in this thread and in many other discussions as well.

Let us consider a three phase panel with a three phase main breaker that has a load on each phase (note, I did not say ?load per phase?) of 200 amps. Let us consider all loads to be single phase, and purely resistive in nature.
? You have asserted that this represents a total of 600 amps of single phase loads.
? I agree, and I have not said anything that contradicts that assertion.
? I submit that it is inappropriate to describe this situation by stating that ?the total load on the breaker is 600 amps.? Indeed, I believe the phrase ?total load? has no meaning. One reason is that this same panel could have, instead of a bunch of single phase loads, one load that is a three phase load that draws 200 amps from each phase. The main breaker could not distinguish this situation from the single phase loading situation. But I don?t think you would say that this solitary three phase load represents a total load of 600 amps.
? I have asserted (in essence, but using different words) that the current leaving the main breaker (i.e., on its way to the panel?s bus bars) on Phase A will return to the breaker on Phases B and C. I have further asserted that it is all ?the same current.? You have not contradicted that assertion, and I am confident that you would agree.
? I have (begrudgingly) used the statement ?the total load is 200? to describe this situation. Despite my objections to that phrase ?total load,? many people continue to use it.

In conclusion, whenever we hear or see anyone using the phrase ?total load,? I think we should call a ?time out,? and ask the person to clarify what they intend that phrase to mean, before we attempt to answer any questions.

 

[charlie b]

Let me restate, in another way, the sky/grass misunderstanding I tried to describe in my last post. Here again, we are talking about a balanced three phase load that has 200 amps flowing in each phase.
? If, by using the phrase ?total load,? you are talking about the mathematical sum of the three sine waves that represent the current in each phase, that sum is zero.
? If instead you are talking about the equivalent amount of single phase loads that the panel could be serving, that total is 600.
? If instead you are talking about the amount of current flowing in the breaker, as compared to its trip setting (i.e., the original question of this thread), that answer is 200.
? If you use the phrase ?total load,? without stating which of the above concepts (or perhaps there is some other) represents your intended meaning, then I am going to stop and ask you to clarify.

 

[ggunn]

I seem to have missed out on the discussion. I would have liked to jump back in earlier.

Let me start by reminding everyone of the following type of disagreement, for I believe we are in the middle of just such a misunderstanding.

Rick, we are both wrong. The total current is, in fact, neither 200 nor 600. The total current is zero, and will always be zero regardless of power factors, phase angles, or the degree to which the loads are balanced. Kirchhoff’s Current Law demands that it be zero.

That said, I think we need to get our use of terminology settled out, for one of us is talking about the color of the sky, and the other is talking about the color of the grass. I do not like the use of the word “total,” in the context of currents flowing in more than one phase. I particularly dislike the use of the word “per” within the phrase “amps per phase.” These two words, I do believe, are the cause of the misunderstandings and disagreements on display in this thread and in many other discussions as well.

Let us consider a three phase panel with a three phase main breaker that has a load on each phase (note, I did not say “load per phase”) of 200 amps. Let us consider all loads to be single phase, and purely resistive in nature.
? You have asserted that this represents a total of 600 amps of single phase loads.
? I agree, and I have not said anything that contradicts that assertion.
? I submit that it is inappropriate to describe this situation by stating that “the total load on the breaker is 600 amps.” Indeed, I believe the phrase “total load” has no meaning. One reason is that this same panel could have, instead of a bunch of single phase loads, one load that is a three phase load that draws 200 amps from each phase. The main breaker could not distinguish this situation from the single phase loading situation. But I don’t think you would say that this solitary three phase load represents a total load of 600 amps.
? I have asserted (in essence, but using different words) that the current leaving the main breaker (i.e., on its way to the panel’s bus bars) on Phase A will return to the breaker on Phases B and C. I have further asserted that it is all “the same current.” You have not contradicted that assertion, and I am confident that you would agree.
? I have (begrudgingly) used the statement “the total load is 200” to describe this situation. Despite my objections to that phrase “total load,” many people continue to use it.

In conclusion, whenever we hear or see anyone using the phrase “total load,” I think we should call a “time out,” and ask the person to clarify what they intend that phrase to mean, before we attempt to answer any questions.

I, for one, am really glad to see this discussion, because three phase power is something that is pretty new to me since I have been reinventing myself in PV power engineering after having spent nearly 25 years in semiconductor design and FA engineering. It seems to me that one cannot escape the fuzzy (to me, at least, in this stage of my experience) concept of "per phase" current when one is sizing conductors. Yes, Kirchoff dictates that the vector sum (and it seems to me that part of the sky/grass color disconnect in this thread is the difference between scalar and vector arithmetic) of all currents must be zero, but the current in a single conductor is not, no matter if it's a three phase load or single phase loads.

I'll hang up and listen. ;^)

 

[Rick Christopherson]

Charlie, we're on the same page here, but I think it is important to look at what the original poster is asking, and more importantly, why he is asking. What is causing the confusion for him?

This discussion doesn't come up when you have a 3-pole breaker feeding a 3-phase device (i.e. motor). However, this discussion does come up, and is very germane, when you have a 3-pole breaker feeding multiple loads, such as a main breaker in a panel or a MWBC with multiple 1- and 2-pole loads (a common situation with portable power distribution systems).

What sets these situations apart is that it is frequently required to know how many amps are flowing through each bus or conductor downstream from the breaker. Where that current goes after it leaves the breaker is unknown. If all the loads are line-to-neutral without a shared neutral, then none of this current "returns" (bad word, but it works) through the breaker.

Even though you (Charlie) have acknowledged that the statement "the total load is 200 amps" is not correct, it would nevertheless appear to be a very common phrase with many people. This is the phrase that drew my attention in the first place. The common use of this phrase is exactly the reason why people ask the question that the original poster asked. This is the reason why I do my best to use a qualifier of some sort, such as "per phase" or "at 3-phase" whenever it appears there could be confusion.

 

[charlie b]

The common use of (the phrase "total load") is exactly the reason why people ask the question that the original poster asked.

And I know I would be fighting a losing battle to get people to stop using it. It is not my only losing battle.

But I have not given my own answer to the question that I think the OP was trying to ask. So here it is. A 200 amp (2-pole or 3-pole) breaker will trip when the current flowing through any one of its poles exceeds 200 amps by "some amount." The amount needed to trip the breaker will vary with the make and model of the breaker, and with the breaker's settings (if it happens to be adjustable). A current of 205 amps in one leg probably won't trip a 200 amp breaker. A current of 250 amps in any one leg probably will. And if the current in one leg (or one pole, if you prefer) is 250 amps, it does not matter what current is flowing in the other leg(s) or pole(s). If for example a 3-pole, 200 amp breaker that serves single phase loads as a MWBC has current flows in the three legs of 250, 150, and 100, it is going to trip.

 

[Rick Christopherson]

I hope this doesn't come across as nit-picking. It's not intended to be, but it is something I noticed last night during the discussion that I believe may lead to some confusion with other people reading this discussion.

We use the 3-phase power equation so frequently, and there are so many different occurrences of sqrt(3), that it is sometimes easy to forget how this equation is originally derived.[FONT=&quot]

[/FONT]

...The square root of 3 in the example has nothing to do with the current flowing through the breaker. The square root of 3 is because in a 3phase system Vl=Vp*sqrt(3). The equation for 3 phase power is Vl*I*sqrt(3).

The above statements are correct, but the way it's worded (and without the intermediate steps) it may be misleading to some. The hidden factor in the intermediate steps is the multiplication by 3, that results from the summation of the 3 phases to determine total power.

The power for each phase is determined by I*V, however, by convention, this normally represents a phase-current (Ip) times a line-voltage (VL), and one of the two must be converted to the other. For obvious reasons, we convert the line-voltage (e.g. 208) to its equivalent phase-voltage( e.g. 120).

VL / √3 = Vp : Note that this is division by sqrt(3), not multiplication.
208 / 1.732 = 120

The power per phase is therefore:
Pp = Vp * Ip = (VL / √3) * Ip

The total power for the system is the sum of the powers for each phase (PT =PA + PB + PC), which is 3-times the phase power (PT = 3 * Pp) as long as they are balanced (PA = PB = PC). Our total power equation becomes:

PT = 3 * Pp = 3 * (VL / √3) * Ip = 3 / √3 * VL * Ip = √3 * VL * Ip

In my opinion, the reason why this intermediary step of multiplying the phase-power by 3 goes unnoticed is because (3 / √3) = √3, and it simply looks like it was the result of a phase conversion. It is a phase conversion, but it is 3-times that phase conversion.

 

[gar]

110715-1653 EDT

I have not reread all the posts, but I generally agree with Rick's presentation.

Now back to the original post. The answer to the question

Do 2 & 3 pole breakers protect each phase at it's rated ampacity? For example, on a single phase panel with a 200amp main breaker, can you load each phase to 200amps for a total of 400, or 600amps on a 3 phase panel?

Yes.

Consider each pole of the breaker as a single phase independent breaker, then no matter what the source voltage was or what the loads are you can load each breaker to 200 A. In the case of the multipole breaker, if any one of the poles is overloaded, then the whole breaker is tripped. This is thru a mechanical coupling. Electrically each pole is independent of the others.

Can a two pole 200 A breaker protect a single 400 A load? More or less yes. If the internal resistances of the breakers are nearly identical at the trip point, then the breakers could be paralleled and half of the total current would flow thru each half. This has nothing to do with UL or NEC it is just a description to try to help SPierce understand how the breaker works.

.

 

[Leestarkweather]

Try talking to a movie studio electrician if you want to be confused

Try talking to a movie studio electrician if you want to be confused

I recently had to rent a towable genset on short notice. The usual rental yards didn't have the right sizes, so I called a local rental co. that rents to studio/on location film shoot companies. All their gensets are listed as XXX Amp gensets, not in KW. I needed a 100KW genset that would allow me to hook up a 50KW load (single phase 120/240V) with some spare capacity for a comfort Zone. The gensets have a rotary switch that you set the output for (thru zig-zag connections) either 240V 3 phase or 120/240V 1 phase (which has a rated capacity of about 62% of the 3-phase capacity). I finally had to drive over to the yard to look at the genset.

The point being, a 600Amp genset in studio rental terms actually had the capacity to put out 200A on each of 3 line to neutral loads, for a total of 600A 120V that would be connectible. So, I guess in their (studio electricians) world, the 200A per line (not per phase) adds up to 600A load.