dumb question - 200 amp, 3 phase, 208 volt

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OK, this really seems like a stupid question, but I have asked it numerous times and never got a straight answer.
We have what the pros refer to as 200 amp 3 phase service, 208 volts.

I just used my amp meter and recorded how many amps we have going down each phase. I added them all up, 112 amps.

Now, my question is, does 200 amp 3 phase service mean we get 200 amps total when you add up the use of each phase
OR
does 200 amp 3 phase service mean we get 200 amps down each phase, totallying 600 amps?

Can the pro's here clarify for me please. I don't know if we are at 56% of our capacity or 19%. 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.
 

iwire

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Massachusetts
I agree with Roger but suggest you think in terms of watts not amps.

A 200 amp 120/240 service can provide 48,000 watts of power.

240 * 200 = 48,000



A 200 amp 208Y/120 service can provide 71,968 watts of power.

208 * 200 * 1.73 = 71,968
 

jghrist

Senior Member
I also agree with Roger, but would add that you are making a mistake by totaling the current. It's just 200A in each phase. Adding the three phase currents mathematically to get 600A makes no sense.

Actually, if the power factor in each phase is equal, and you add the currents vectorially, you will get zero amps (the neutral current).
 

charlie b

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200 amps per phase.
With respect, Roger, you just stepped into my campaign to forever eliminate that phrase from everybody?s vocabulary. Call it a pet peeve, if you wish, but ?amps per phase? just grates on my nerves.

(Now standing on my soapbox)
Attention, please, everybody: The phrase ?amps per phase? has no meaning, and should be forever banned from our vocabulary.
(Now stepping down from the soapbox.)

My reasoning is simply that the word ?per? tends to imply that there is an addition about to take place, that two or more numbers are about to be added together. When you speak of current flowing through wires, and when you at the same time speak of Phases A, B, and perhaps C, there is no addition taking place. We never, never add the amps from Phase A to the amps from Phase B, and we never (or at least never should try to) obtain the total load on a panel or service or whatever by adding amps. We add loads in terms of power, and convert to amps as the final step in the calculation.
 

zog

Senior Member
Location
Charlotte, NC

With respect, Roger, you just stepped into my campaign to forever eliminate that phrase from everybody?s vocabulary. Call it a pet peeve, if you wish, but ?amps per phase? just grates on my nerves.

(Now standing on my soapbox)
Attention, please, everybody: The phrase ?amps per phase? has no meaning, and should be forever banned from our vocabulary.
(Now stepping down from the soapbox.)

My reasoning is simply that the word ?per? tends to imply that there is an addition about to take place, that two or more numbers are about to be added together. When you speak of current flowing through wires, and when you at the same time speak of Phases A, B, and perhaps C, there is no addition taking place. We never, never add the amps from Phase A to the amps from Phase B, and we never (or at least never should try to) obtain the total load on a panel or service or whatever by adding amps. We add loads in terms of power, and convert to amps as the final step in the calculation.
"Infinite resistance" is the one that gets me going, should also be banned.
 

roger

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My reasoning is simply that the word “per” tends to imply that there is an addition about to take place,
And with mutual respect, I don't see it as meaning an addition is necessarily about to take place. I see it as meaning for each or every, even if there is only one selected or chosen.


From Dictionary.com


1. for each; for every: Membership costs ten dollars per year. This cloth is two dollars per yard.

"Per year" is singular, when the year is over ten dollars was the total cost that my wallet would bear, no more.

Roger
 

charlie b

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From Dictionary.com 1. for each; for every: Membership costs ten dollars per year. This cloth is two dollars per yard.
Those two examples would tell us that after three years the membership would have cost us thirty dollars, and that three yards of cloth would cost us six dollars. In our profession, the phrase "200 amps per phase" used for a system that has three phases is going to be, absolutely going to be interpreted, by some, as meaning that the total for three phases is 600 amps. Witness the original question of this thread; witness many other examples that have been posted on this forum.

My point is that the value in avoiding that phrase is that we won't be led into the trap that it contains. You and I and many others can see the phrase "200 amps per phase" and be perfectly aware that it does not mean "600 amps total." Persons new to the profession will not know that. I think we can teach them to understand current better, if we discourage the use of that phrase.
 

roger

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Those two examples would tell us that after three years the membership would have cost us thirty dollars, and that three yards of cloth would cost us six dollars.
That would be correct.

In our profession, the phrase "200 amps per phase" used for a system that has three phases is going to be, absolutely going to be interpreted, by some, as meaning that the total for three phases is 600 amps.
And that would be correct as well. There would be a total of 600 amps @ 120 volts or 72,000 watts available.

Witness the original question of this thread; witness many other examples that have been posted on this forum.
And 200 amps per phase or 600 amps total would be the correct answer as the OP questions in the third sentence

My point is that the value in avoiding that phrase is that we won't be led into the trap that it contains. You and I and many others can see the phrase "200 amps per phase" and be perfectly aware that it does not mean "600 amps total."
But once again it does mean 200 amps (24,000 watts) "per phase" totaling 600 amps (72,000 watts) can be delivered from all three phases.


Persons new to the profession will not know that. I think we can teach them to understand current better, if we discourage the use of that phrase.
I agree with you that we need to make sure that things are explained in detail and hopefully understood by those entering the trades, and in this discussion I think "per phase" is a correct term.

Roger
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Since I work with systems that have arbitrary phase count, I regularly use the concept of 'aggregate amps' to provide a comparison between two systems. 'Aggregate amps' is simply amps (per phase) times number of phases.

-Jon
 

charlie b

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Electrical Engineer
I just address this one:
But once again it does mean 200 amps (24,000 watts) "per phase" totaling 600 amps (72,000 watts) can be delivered from all three phases.
Saying ?24,000 watts per phase, for a total of 72,000 watts,? is valid. But saying ?200 amps per phase, for a total of 600 amps,? is not valid. That is because, in the context used herein, a ?watt? is a ?watt? is a ?watt,? but an ?amp? is not an ?amp,? nor is it an ?amp.? That, speaking mathematically, is the difference.

What I mean is that if one phase is using energy at a given rate (i.e., the watts expended in that phase), and if another phase is using energy at a given rate, then the total rate of using energy can be found by adding watts to watts, with the answer being expressed in watts. There is no difference between a watt being expended in one phase and a watt being expended in another phase. That is not true with amps.

When we use the word ?amp,? we are not giving a complete physical or mathematical description. To be complete, we need to say that,
(1) In Phase A, we are measuring a current of ?200 amps at a relative phase angle of 0 degrees,?
(2) In Phase B, we are measuring a current of ?200 amps at a relative phase angle of 240 degrees,? and
(3) In Phase C, we are measuring a current of ?200 amps at a relative phase angle of 120 degrees,? so that,
(4) The total amps is given by 200 (angle 0) plus 200 (angle 240) plus 200 (angle 120), with that total being equal to zero amps.

Consider a three phase heater (so that we don?t have to talk about power factor). The watts used in the Phase A element are transmitted into the air, and vanish from the electrical component forever. So too do the watts used in the Phase B element, and so too do the watts in the Phase C element. In that sense, it is easy to see that the total watts used in the three phases will be equal to the sum of the watts used in each of the three elements.

However, the current that leaves the source via Phase A does not disappear from the electrical system. Rather, some of it returns to the source via Phase B, and the rest returns to the source via Phase C. The 200 amps you measure in Phase A is not in addition to the amps you measure in the other two phases. Rather, the amps you measure in Phase A will become the amps you measure in the other two phases. You don?t add them because they are the same amps.

It is like saying that you can take a dollar bill out of your left pocket with your left hand, transfer it to your right hand, and place it into your right pocket, and by doing so you now have a ?total of 2 dollars.? And why not? After all, your left hand was moving a dollar, and your right hand was moving a dollar, so weren?t you moving a total of two dollars? I could wish that getting rich would be so easy, but in the end you still have only one dollar, whichever pocket it resides within.
 

roger

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Electrician
I just address this one:
Saying ?24,000 watts per phase, for a total of 72,000 watts,? is valid. But saying ?200 amps per phase, for a total of 600 amps,? is not valid. That is because, in the context used herein, a ?watt? is a ?watt? is a ?watt,? but an ?amp? is not an ?amp,? nor is it an ?amp.? That, speaking mathematically, is the difference.

What I mean is that if one phase is using energy at a given rate (i.e., the watts expended in that phase), and if another phase is using energy at a given rate, then the total rate of using energy can be found by adding watts to watts, with the answer being expressed in watts. There is no difference between a watt being expended in one phase and a watt being expended in another phase. That is not true with amps.

When we use the word ?amp,? we are not giving a complete physical or mathematical description. To be complete, we need to say that,
(1) In Phase A, we are measuring a current of ?200 amps at a relative phase angle of 0 degrees,?
(2) In Phase B, we are measuring a current of ?200 amps at a relative phase angle of 240 degrees,? and
(3) In Phase C, we are measuring a current of ?200 amps at a relative phase angle of 120 degrees,? so that,
(4) The total amps is given by 200 (angle 0) plus 200 (angle 240) plus 200 (angle 120), with that total being equal to zero amps.

Consider a three phase heater (so that we don?t have to talk about power factor). The watts used in the Phase A element are transmitted into the air, and vanish from the electrical component forever. So too do the watts used in the Phase B element, and so too do the watts in the Phase C element. In that sense, it is easy to see that the total watts used in the three phases will be equal to the sum of the watts used in each of the three elements.

However, the current that leaves the source via Phase A does not disappear from the electrical system. Rather, some of it returns to the source via Phase B, and the rest returns to the source via Phase C. The 200 amps you measure in Phase A is not in addition to the amps you measure in the other two phases. Rather, the amps you measure in Phase A will become the amps you measure in the other two phases. You don?t add them because they are the same amps.

It is like saying that you can take a dollar bill out of your left pocket with your left hand, transfer it to your right hand, and place it into your right pocket, and by doing so you now have a ?total of 2 dollars.? And why not? After all, your left hand was moving a dollar, and your right hand was moving a dollar, so weren?t you moving a total of two dollars? I could wish that getting rich would be so easy, but in the end you still have only one dollar, whichever pocket it resides within.
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. :grin:

Roger
 

infinity

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Charlie what about this example:

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?
 

peter d

Senior Member
Location
New England
I agree with Roger but suggest you think in terms of watts not amps.

A 200 amp 120/240 service can provide 48,000 watts of power.

240 * 200 = 48,000



A 200 amp 208Y/120 service can provide 71,968 watts of power.

208 * 200 * 1.73 = 71,968
I would go a step further and not use the term "watts" at all, but rather use volt-amperes.
 

Besoeker

Senior Member
Location
UK
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. :grin:Roger
Maybe you and Charlie are talking at cross purposes.
If I recall correctly, the original service mentioned a 200A, 208V service.
Wouldn't that imply 3-phase 3-wire?
If it is, you wouldn't be able to load the phases independently as Charlie says in a rather convoluted way.
That said, I don't quite agree with his point about Amps. I do think you need consider ratings in terms of Amps. That, after all, is how you size cables switchgear, the windings in motors, transformers, etc.
To that extent, VA and kVA are more relevant measures that Watts.
 

roger

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Maybe you and Charlie are talking at cross purposes.
No maybe about it. :D
If I recall correctly, the original service mentioned a 200A, 208V service.
Wouldn't that imply 3-phase 3-wire? If it is, you wouldn't be able to load the phases independently as Charlie says in a rather convoluted way.
If the OP actually meant a 208 service it would be but, if my assumption that the OP was talking about a 208y/120v service is correct it would not. It would be a four wire and the phases could be loaded independently of the others.

Roger
 
Thank you

Thank you

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.
 

iwire

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Location
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I would go a step further and not use the term "watts" at all, but rather use volt-amperes.
That is a more accurate way of putting it, you are without a doubt correct.

But I am going to tell you an old electricians secret, usually watts will get us close enough. If your engineering a job from the ground up I agree VA would be critical. Figuring a few branch circuits watts will get you there. :)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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.
I would say you would have 20a @ 120v times three, times two, and times one, respectively.

To put it in perspective, if a 1-phase system, I'd say 20a @ 240v as opposed to 40a @ 120v.
 
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