Breaker Ampacity

[SPierce]

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? Or does the breaker trip when it senses a combined load of 200amps? Not sure why this is difficult for me to wrap my head around, maybe you can help.

 

[charlie b]

The part you need to wrap your head around (and this is not an easy thing to get at first), is that if you load each phase to 200 amps, the total is 200, not 400. On a 3 phase panel with 200 amps on each phase, the total is still 200, not 600.

Let?s stick with the single phase panel, and let?s load it to 200 amps on each phase. The current that leaves the panel on Phase A will return to the panel on Phase B. It is the same current. That is why you don?t add the 200 to the 200 to get 400.

Once you get this concept, the answer to your question becomes obvious.

Oh, and by the way, we don't use the word "ampacity" in connection with breakers. That term only applies to wires, and indicates how much current they can handle without overheating their insulation systems.

 

[skeshesh]

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? Or does the breaker trip when it senses a combined load of 200amps? Not sure why this is difficult for me to wrap my head around, maybe you can help.

You're asking a few things here.

Each phase can be loaded to the rating indicated on the breaker (keep in mind %80 breakers vs. %100 breakers). (This is why it's a good practice to do a sanity check on each phase when loading panels. I've seen a lot of designers just take the total power and divide by Voltage(L-L)*1.732, which may mislead you if there's great phase unbalance.)

The breaker tripping is a different issue and it can get kind of complicated. It could depend a lot on what condition is causing the breaker to trip and the type of breaker in question.

Charlie's got a good point about ampacity I somehow didnt catch that. Also it may help you to think in terms of power (KVA) in addition to current (Amps) when you're thinking about loading panels.

 

[Rick Christopherson]

The part you need to wrap your head around (and this is not an easy thing to get at first), is that if you load each phase to 200 amps, the total is 200, not 400. On a 3 phase panel with 200 amps on each phase, the total is still 200, not 600.

This is not correct, and is actually the reason why there can be confusion on this topic. A 200 amp breaker does in fact permit 200 amps per phase, and the sum of the phases (in magnitude) is 600 amps. It may be convention that we discuss this as 200 amps total, but that overlooks the other situations that may or may not exist. For example, if your 200 amp, 3-phase breaker is supplying a distribution of single phase loads, then you will want to know how much amperage is available for each phase of those loads. The sum of all your single phase loads supplied from this 200 amp, 3-phase supply will be 600 amps (as long as it is balanced).

 

[infinity]

It can be confusing. A 3 pole 20 amp CB can provide 3-120 volt, 20 amp circuits or 60 amps worth of load at 120 volts. The same CB can provide a single 3 phase 208 volt, 20 amp circuit.

 

[david luchini]

This is not correct, and is actually the reason why there can be confusion on this topic. A 200 amp breaker does in fact permit 200 amps per phase, and the sum of the phases (in magnitude) is 600 amps. It may be convention that we discuss this as 200 amps total, but that overlooks the other situations that may or may not exist. For example, if your 200 amp, 3-phase breaker is supplying a distribution of single phase loads, then you will want to know how much amperage is available for each phase of those loads. The sum of all your single phase loads supplied from this 200 amp, 3-phase supply will be 600 amps (as long as it is balanced).

I'm having a hard time understanding your reasoning. If a 200A-3P breaker is connected to 200A Line-to-Neutral loads on each phase, then the breaker still permits 200A, not 600A.

There are two ways to thinks of it. First consider if the breaker was connected to a balanced 3 phase load with 200A per phase. The power (assume 208V-3ph) would be 200A*208V*1.732=72kVA. Now consider the same breaker with 200A line-to-neutral loads...the power would be 200A*120V=24kVA per phase, times 3 phases=72kVA. 200Amps on each phase is 72kVA whether it is a 3 phase load or 3 single phase loads.

The second way to think of it relates to your explanation of the "sum of the single phase loads" when connecting line-to-neutral loads. If each phase is connected line to neutral, then the current on the neutral is In=Ia+Ib+Ic. In other words, the Neutral current is the "sum of the single phase loads." But of course, In does not equal 600A, it equals zero amps (assuming each load has the same power factor.) A circuit breaker does not allow the sum of its rated current per phase, it allows its rated current per phase.

 

[Dennis Alwon]

I always kept it simple. A dp breaker will carry the rated amps of the breaker on each phase. So a dp 200 will handle 200 amps per phase. It is still a 200 amp breaker.

 

[Rick Christopherson]

I'm having a hard time understanding your reasoning. If a 200A-3P breaker is connected to 200A Line-to-Neutral loads on each phase, then the breaker still permits 200A, not 600A.

There are two ways to thinks of it. First consider if the breaker was connected to a balanced 3 phase load with 200A per phase. The power (assume 208V-3ph) would be 200A*208V*1.732=72kVA........

This is more of a rhetorical question to get you to think about it, not an actual inquiry. If a 3-phase 200 amp breaker supplies 200 amps total, then why is there a need for square-root-3 in your example above?

The problem with this discussion is that we use "short-hand" terminology even when we speak. When we say a 3-phase motor draws 10 amps, it is actually "short-hand" for saying the motor draws 10 amps per phase. The net sum of those amps is neither 1 nor 3, but the sq-rt-3 that you used above.

The sum is never 1. That's just a result of short-hand that frequently gets forgotten that it even exists.

 

[jumper]

I always kept it simple. A dp breaker will carry the rated amps of the breaker on each phase. So a dp 200 will handle 200 amps per phase. It is still a 200 amp breaker.

Works for me.:thumbsup:

 

[jumper]

This is more of a rhetorical question to get you to think about it, not an actual inquiry. If a 3-phase 200 amp breaker supplies 200 amps total, then why is there a need for square-root-3 in your example above?

The problem with this discussion is that we use "short-hand" terminology even when we speak. When we say a 3-phase motor draws 10 amps, it is actually "short-hand" for saying the motor draws 10 amps per phase. The net sum of those amps is neither 1 nor 3, but the sq-rt-3 that you used above.

The sum is never 1. That's just a result of short-hand that frequently gets forgotten that it even exists.

You lost me at "amps per phase", I believe Charlie B says they are the same amps.

 

[jim dungar]

...A 3 pole 20 amp CB can provide 3-120 volt, 20 amp circuits...

Absolutely correct, multiple circuits @ 20A each.

... or 60 amps worth of load at 120 volts....

It is not possible to connect this breaker in any manner that will provide 60A @120V to a single circuit. It cannot supply 60A to two circuits (i,e, 45A and a 15A loads) either.

 

[david luchini]

This is more of a rhetorical question to get you to think about it, not an actual inquiry. If a 3-phase 200 amp breaker supplies 200 amps total, then why is there a need for square-root-3 in your example above?

Again, I'm at a loss with your rhetorical. 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 problem with this discussion is that we use "short-hand" terminology even when we speak. When we say a 3-phase motor draws 10 amps, it is actually "short-hand" for saying the motor draws 10 amps per phase. The net sum of those amps is neither 1 nor 3, but the sq-rt-3 that you used above.

The sum is never 1. That's just a result of short-hand that frequently gets forgotten that it even exists.

Now I'm completely lost. I don't see how the net sum of the 10Amps per phase that a motor draws could be 1 or 3 or the square root of 3.:?

If the breaker was fully loaded then the current flowing on the each of the feeder conductors leaving the breaker would be 200A, whether the load was one 3-phase load or three single-phase loads. If a 200A-3P c/b allow can supply 600Amps of 120V single phase loads, then can we put one 72kW, 120V load on one of its poles? That would be a 600A supply. The answer is of course, no.

I don't see where any discussion of the "sum of the amps per phase" applies to a multipole circuit breaker. If you connect only 1 pole, it can't supply more than 200A. If you connect two poles line to neutral, both poles can't supply more than 200A. If you connect one load between 2 poles, the max current is still 200A. And so on...

 

[jumper]

Works for me.:thumbsup:

You lost me at "amps per phase", I believe Charlie B says they are the same amps.

I do believe I just contradicted myself.:ashamed1:

OOPS!

It is 200 amps, kVa will vary according to voltage and number of phases.

It is still 200 amps gosh darn it. (I think :?)

 

[Rick Christopherson]

Now I'm completely lost.

If it's not completely clear, just ask about what is not clear. This might make more sense if you re-read what the OP was asking about.

 

[david luchini]

If it's not completely clear, just ask about what is not clear. This might make more sense if you re-read what the OP was asking about.

I did ask about what was not clear....

Now I'm completely lost. I don't see how the net sum of the 10Amps per phase that a motor draws could be 1 or 3 or the square root of 3.:?

I don't see anything in the OP's question that relates to the net sum of a 10A 3ph motor being 1 or 3 or 1.732. As Charlie correctly pointed out in the first response, each pole of a 200A c/b can carry up to 200A, but the load current is still 200A, not 600A.

The concept of "summing the phases" of the breaker such that a fully loaded breaker can carry 600A doesn't make sense. If you applied 225A to one phase, 100A to the second and 50A to the third, its only carrying 375A, less than the 600A you suggest it can carry. But its surely going to trip.

 

[Rick Christopherson]

As Charlie correctly pointed out in the first response, each pole of a 200A c/b can carry up to 200A, but the load current is still 200A, not 600A.

This is the part that is wrong. The load does not carry 200 amps unless you qualify that with the missing phrase "per phase", which is what this discussion is about in the first place.

The OP was asking whether the 200 amp rating on a breaker meant 200 amps per phase, or if the net current flowing was 200 amps. The answer is 200 amps per phase, which is a net current of 600 amps.

The concept of "summing the phases" of the breaker such that a fully loaded breaker can carry 600A doesn't make sense. If you applied 225A to one phase, 100A to the second and 50A to the third, its only carrying 375A, less than the 600A you suggest it can carry. But its surely going to trip.

Now you are being obtuse. No one was suggesting that the breaker in question could carry 600 amps on a single pole. It is limited to 200 amps per pole, but the total amperage through the 3 poles is 600 amps.

The circuit breaker neither knows nor cares where the current comes from or goes to. It is simply 3 nodes of 3 circuits, and each of those 3 nodes sees an amperage up to 200 amps each, which is a net total of 600 amps. You knew this in your first response, but you are being argumentative for no reason.

 

[david luchini]

This is the part that is wrong. The load does not carry 200 amps unless you qualify that with the missing phrase "per phase", which is what this discussion is about in the first place.

The OP was asking whether the 200 amp rating on a breaker meant 200 amps per phase, or if the net current flowing was 200 amps. The answer is 200 amps per phase, which is a net current of 600 amps.

Reread the OP. He was asking if the 200A rating on the breaker meant 200A per phase for a "total of 600A on a 3 phase panel," or if the breaker would trip when it reached a combined load of 200A.

The correct answer is neither, as Charlie correctly answered in post #2.

Now you are being obtuse. No one was suggesting that the breaker in question could carry 600 amps on a single pole. It is limited to 200 amps per pole, but the total amperage through the 3 poles is 600 amps.

The circuit breaker neither knows nor cares where the current comes from or goes to. It is simply 3 nodes of 3 circuits, and each of those 3 nodes sees an amperage up to 200 amps each, which is a net total of 600 amps. You knew this in your first response, but you are being argumentative for no reason.

Who's being obtuse? There is no such thing as a net total amps of 3 times the rated amps on a circuit breaker. Either the breaker can carry up to a "net total of 600Amps" or it can't." In my example, the net total amps is less than 600A, but the breaker cant' carry it. The statement that a 3 pole 200A breaker can carry a net total of 600A is patently false. The answer given in post #2 is absolutely correct. A fully loaded 200A-3p c/b will not result in a total of 600A on a 3 phase panel...it will result in a total of 200A on a 3 phase panel.

 

[Rick Christopherson]

David, you are contradicting yourself. You need to go back and re-read posts 1 and 2 before carrying on any further. We're both E.E.'s and we both understand this quite well. That's why I suggested you were arguing for the sake of arguing. There is a miscommunication, and you are not noticing the information I was pointing out in Charlie's posting.

 

[kingpb]

Do 2 & 3 pole breakers protect each phase at it's rated ampacity?

Yes.

For example, on a single phase panel with a 200amp main breaker, can you load each phase to 200amps .

Yes, assuming you have sized it according to NEC.

for a total of 400, or 600amps on a 3 phase panel?

No, this is not correct. For sizing purposes it is still a 200A breaker regardless of number of poles, and each pole cannot exceed 200A.

Or does the breaker trip when it senses a combined load of 200amps?

Yes, it will trip when it sees more than 200A on any pole.


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.

 

[david luchini]

David, you are contradicting yourself.

That's quite funny. You should look into a career in politics. :lol:

You need to go back and re-read posts 1 and 2 before carrying on any further. We're both E.E.'s and we both understand this quite well. That's why I suggested you were arguing for the sake of arguing. There is a miscommunication, and you are not noticing the information I was pointing out in Charlie's posting.

I noticed the information you pointed out in Charlie's posting and did not find it agreeable. That is why I responded to it initially. Its really quite simple.

The part you need to wrap your head around (and this is not an easy thing to get at first), is that if you load each phase to 200 amps, the total is 200, not 400. On a 3 phase panel with 200 amps on each phase, the total is still 200, not 600.

This statement is correct.

This is not correct,

This statement is not correct.

I don't know any EE who thinks that if you fully load a 200A 3p c/b, you will have 600A on the 3 phase panel.