dumb question - 200 amp, 3 phase, 208 volt

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roger said:
So, to simplify all of that for someone entering the trade and asking the OP's question, we can say "200 amps per phase or a total of 600 amps combined" is the answer.
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roger said:
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I think not. I would never want to start a new person off on the wrong track, by using the phrase "amps per phase."

Rather, I would tell the new person that you have 200 amps of load on a three phase system. I would add that each phase, or leg if you prefer, is carrying 200 amps (at a phase voltage of 120V), and the system as a whole is carrying 200 amps (at a system voltage of 208V). That is not, by itself, going to satisfy the new person's curiosity. Indeed, it will confuse, before it educates. But the important thing to say is that three phase systems are going to be confusing for a while, and that the best approach (as some have already said) is to do all addition in terms of watts, and to convert to amps only at the end.
 
infinity said:
Charlie what about this example:
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infinity said:
I have a three pole 20 CB from a 208Y/120 volt panelboard, I run a MWBC to feed three 120 volt heaters that each draw 20 amps. How many total amps am I using? I say 60 amps at 120 volts. Shut one off now I'm using 40 amps at 120 volts. Turn the second one off and now I'm using 20 amps at 120 volts. What am I missing?
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Thanks for a very good question, Rob. It allows me to make a distinction that I think is important.

Let me change your example such that I have three 20 amp heater loads, each coming from a different circuit within the same panel. For the moment, let us say that we do not know what phase is used to power any of the heaters. Now ask your question again. My answer is that you have three 120V loads, each comprising a 20 amp heater. You ask for the total, do you? I say the total is 7,200 watts. Sorry, did you ask for total current? My mistake. The answer is that “total current” has no meaning, since we don’t know which heater is powered by which phase.

Now let me change the situation, and put all three loads on the same phase, Phase A, but let me use a single breaker (with appropriately sized conductors and overcurrent devices). With all three turned on, you have a total current, on that Phase A breaker, of 60 amps. Turn off one breaker, and the total load becomes 40 amps. Turn off one more, and the load is 20 amps.

Why are these two situations different? It has to do with what happens to the current after it leaves any one of the loads. In the second situation, 60 amps leaves the breaker. 20 of those amps take the first exit (i.e., the first of three parallel paths, this one going to the first heater), and the other 40 continue down the line. 20 more amps take the second exit, and the last 20 continue to the third heater. Once the amps leave the individual heaters, they combine again (in the neutral leg), and add up to 60 again before they get back to the source.

But in your example, and in the OP’s example, the current that leaves the source via the Phase A breaker will go through its load, then pass through the Phase B and/or Phase C load, and return to the source via the Phase B and/or Phase C breakers. So the current you measure in Phase A cannot be added to the current in Phase B, since the one became the other.

I thought up the “move a dollar from pocket to pocket” analogy on the spot, as I was typing my earlier comment. I really like how well that explains the situation, and I think I’ll write a book about it. :wink::grin:
 
richard.bessey said:
All of our loads are single phase, 120 volts. None of them are 3 phase. I checked the amps of each phase, and they are close to the same:
Phase 1 - 30.5 AMPS
Phase 2 - 46 AMPS
Phase 3 - 35.5 AMPS

I remember someone told me once this is bad and not efficient, and causes our electrical bill to go up, but I believe that is due to the inefficiency of a 3 phase motor due to un-equal load. In our case with all single phase loads, that makes no difference. Can anyone confirm or deny that?
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Significantly unbalanced voltages will adversely affect the efficiency of a three phase motor. However, at the currents you measured on a supply with a capacity of 200A per phase, I don't think it would cause concern.

Back to your original point...
It is evidently a 4-wire system, 208V line to line, 120V line to neutral.
So yes, you could take 200A from each phase to neutral.
But in the event that only one phase was loaded to maximum rating and the others off load you'd need to be sure that the neutral conductor is rated at not less than the phase conductor.
 
charlie b said:
But the important thing to say is that three phase systems are going to be confusing for a while, and that the best approach (as some have already said) is to do all addition in terms of watts, and to convert to amps only at the end.
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I don't agree.
Amps matter.
The supply and all connecting components are rated in terms of Amps, not Watts.
 
Besoeker said:
I don't agree.
Amps matter.
The supply and all connecting components are rated in terms of Amps, not Watts.
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You didn't disagree with me. Amps do matter; I never said otherwise. What I said is that you do the math in watts, not in amps. The manufacturers may give you their ratings in amps, but you need to convert to watts (or VA), before you take the next step. When you have all the watts added up, and when you have applied the appropriate demand factors, then, and only then, do you convert back to amps. Then, and only then, will you have the correct value of amps to use in selecting conductor sizes and panel ratings and service ratings and the rest.
 
charlie b said:
You didn't disagree with me. Amps do matter; I never said otherwise. What I said is that you do the math in watts, not in amps. The manufacturers may give you their ratings in amps, but you need to convert to watts (or VA), before you take the next step. When you have all the watts added up, and when you have applied the appropriate demand factors, then, and only then, do you convert back to amps. Then, and only then, will you have the correct value of amps to use in selecting conductor sizes and panel ratings and service ratings and the rest.
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Conductor sizes depend on Amps. Not Watts.
That's pretty basic stuff, Charlie.
 
Besoeker said:
Conductor sizes depend on Amps. Not Watts. That's pretty basic stuff, Charlie.
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As if I didn't know. :mad: Read my posts again. You calculate watts first, and convert to amps at the end. That, too, is pretty basic stuff. :cool:
 
charlie b said:

As if I didn't know. :mad: Read my posts again. You calculate watts first, and convert to amps at the end. That, too, is pretty basic stuff. :cool:
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If I know that the current rating of a motor is 540A, I'd size the cabling for 540A. The kW rating is a secondary issue in terms of cabling, contactor, overload settings.....
 
LarryFine said:
But, would you ever call it "540 amps per phase?"
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Absolutely, if you were up on a lift and put a clamp around each phase of a three wire circuit in a pull box of which you didn't know where it went to or came from and the meter reading was 540 on conductor A, 540 on conductor B, and 540 on conductor C, what would you tell the ground man the readings are?

You could say 540 amps on phase A, 540 amps on phase B, 540 amps on phase C, or you could shorten it and say 540 "amps per phase", both ways would be understood and be correct.

Roger
 
roger said:
You could say 540 amps on phase A, 540 amps on phase B, 540 amps on phase C, or you could shorten it and say 540 "amps per phase", both ways would be understood and be correct.
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Well, we have at least one forum member (the OP), and I have observed many others who posted similar questions, who did not understand. They all saw the phrase "per phase," and wondered if that meant you need to add up the currents. Take another look at post #1. The question talks about "200 amps per phase," and asks if that means the total is 600 amps. If someone were to believe that the total really is 600, then they might size the conductors for 600, not for 200, and that would be a gross waste of money and other resources.

That is why I take issue with the word, "per." It can cause confusion, and it offers an easy opportunity to get the answer wrong.
 
richard.bessey said:
Wow, so much response from a simple question, but I understand it, mostly. The 3 phases and how the relation from phase to phase varies in total output.

Anyway, one final question.

All of our loads are single phase, 120 volts. None of them are 3 phase. I checked the amps of each phase, and they are close to the same:
Phase 1 - 30.5 AMPS
Phase 2 - 46 AMPS
Phase 3 - 35.5 AMPS

I remember someone told me once this is bad and not efficient, and causes our electrical bill to go up, but I believe that is due to the inefficiency of a 3 phase motor due to un-equal load. In our case with all single phase loads, that makes no difference. Can anyone confirm or deny that?

I don't think we need to move our loads around so they are all the same, but if it would save on our electric bill, we will do it.
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You will have higher losses in the conductors with unbalanced loads for two reasons:

1. Loss is proportional to the square of the current, so reduced losses in the lightly loaded phase wires do not offset the higher losses in the heavily loaded phase wires.

2. With unbalanced loads, you will have current in all neutral conductors. If the loads were balanced, there would be no current in the neutral between the source and the point where all of the three phases come together. Current in the neutral conductor causes losses.

With the loads you're talking about, there won't be enough difference to pay for much labor, but there will be some increased losses.
 
richard.bessey said:
Anyway, one final question.

All of our loads are single phase, 120 volts. None of them are 3 phase. I checked the amps of each phase, and they are close to the same:
Phase 1 - 30.5 AMPS
Phase 2 - 46 AMPS
Phase 3 - 35.5 AMPS
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OK, to cut to the chase, you evidently have a 4-wire system.
Phase 1 is running at 15% of rated capacity.
Phase 2 is running at 23% of rated capacity.
Phase 3 is running at 18% of rated capacity.
 
I wish there were some forum for folks who understand phasors or at least trigonometry (or even physics). Unfortunately Steinmetz notation does not have emoticons. How much of this squabble would go away if we had an electronic cocktail napkin to draw vector diagrams.

What if that emoticon menu (to the right as I write) had schematic symbols instead of silly cartoons.
I can make a frowny-face or a grinning-smiley but I can not make a wye or delta. DOH!
 
rabtrfld said:
What if that emoticon menu (to the right as I write) had schematic symbols instead of silly cartoons.
I can make a frowny-face or a grinning-smiley but I can not make a wye or delta. DOH!
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You can do it in AutoCad, copy it as a jpeg, upload it to an image hosting site, and link to it........
Yes, tedious, I know.
:D
 
QUOTE=richard.bessey;944768]
I have asked all of the local electricians I know, and never got a straight answer, maybe its because of the way I ask the question, I am not certain.[/QUOTE]

I am more interested in who you were asking the question. I hope it wasnt people you were working with. It sounds like you were asking the question to one of those 8 ball things.
 
rabtrfld said:
I wish there were some forum for folks who understand phasors or at least trigonometry (or even physics). Unfortunately Steinmetz notation does not have emoticons. How much of this squabble would go away if we had an electronic cocktail napkin to draw vector diagrams.
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Well, first of all it's not a squabble, (at least not in my mind) it's a discussion and there are a few different things being discussed which seems to be the problem.

But, if you want vector diagrams showing what is happening at the source, click here page 3 and 4 probably have what you are looking for.

Roger
 
My 2 cents: I agree with Charlie that there have been many questions just like the original post on this thread and that the term "amps per phase" can be taken the wrong way by many. Moving to ban the term is a bit over the top. I think it falls into the category of slang terms causing miscommunication. We are never going to be able to get rid of slang terms or the amps per phase, IMO.

In the 540 Amp 3 phase load example, if I measured 540 Amps on A,B,and C, I would say the line current is 540 Amps.

To Besoeker, it is possible to size panels, switchgear, and conductors with current rather than Volt-Amps, but if you sit down to design the electrical system of a building, you will find that it is much easier to start with volt-amps and then convert to amps when everything is added together. Mistakes are made when one just uses current alone, especially in a 3 phase system where phase angles are more involved.
 
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