208 three phase

hillbilly1 said:
Maybe completing the info would be better. OP should have said three phase with single phase load.
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Maybe the question was written the way he posted it. For an NEC reason for the question 310.15(B)(5)(b) would cover it.
 
fastline said:
Said 3 PHASE, yet no C phase load in that at all. Are we to just guess?
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Perfectly valid question because many would guess that the correct answer is 10-5=5 amps which is wrong. He said A=10 amps, B=5 amps, 208 volts, 3Ø system. That question is answerable without guessing.
 
hillbilly1 said:
He said three phase, not three phase system. There is a difference.
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On an internet discussion forum it always helps to add additional information and not make people guess. The thread title says '208 three phase' and the original post gives phase A current and phase B current. @hillhilly1 is correct: the phase C current was not specified. Additionally the phase angles of the loads were not specified. We can only guess.

With that said, my _guess_ corresponds to @roger 's in post 11. (Though to be persnickety the equation doesn't look quite right; the calculator in post 15 has it right.) My guess is that this is a test question were we can assume that the phase C current is zero and that the load current phase angles are 120 degrees apart. Perhaps this additional information is available from the context of the test or lesson, or perhaps the information was simply left out. With additional information my answer might be different.

A method that usually works on tests is to assume you can answer the question with the information given, and you are allowed to make the simplest assumption about any necessary information that isn't given. Of course this can fail, because you are making guesses. But in most test taking situations they aren't looking to trip you up, they are looking to see if you understand the basic concept. (Unless, of course, the class is about the ways these basic concepts fail.)
 
Eddie702 said:
I thought 3 phase with only a single phase load but what do I know?
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That's exactly what was asked by the question. If this were a test question it would be designed to trick you by only including the minimum amount of information to find the answer. No need to read anything else into it.
 
infinity said:
Perfectly valid question because many would guess that the correct answer is 10-5=5 amps which is wrong. He said A=10 amps, B=5 amps, 208 volts, 3Ø system. That question is answerable without guessing.
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There was no mention this was a test question. Then there is no mention of I of C, so you are not left to "guess", but rather make assumptions, and that's where things can get sideways. Would it really be excessive in a test question to simply say either C is not connected or 0A?

I would imagine the multi choice answers would be designed to ensure the student understands the concept, in that the answer WILL be between 5 and 10, not 10, not 5, not 0, etc. Now if they want to be dirty, they might throw in (10+5)/2 = 7.5A. That would prob hook some people.
 
fastline said:
Would it really be excessive in a test question to simply say either C is not connected or 0A?
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Yes it would be if the purpose was to show that you actually understand that in a 3Ø, 208Y/120 volt system that when only the A and B phase values are given the neutral current is not found by simply subtracting 5 amps from 10 amps. The correct answer can easily be found with the information given which is the point of the question.
 
infinity said:
Yes it would be if the purpose was to show that you actually understand that in a 3Ø, 208Y/120 volt system that when only the A and B phase values are given the neutral current is not found by simply subtracting 5 amps from 10 amps. The correct answer can easily be found with the information given which is the point of the question.
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Agree to disagree. I live in the real world. The information in the poorly worded question indicates THREE PHASES! I want to know what all three are doing! I mean sure, select the most practical answer on a test and move on, but in the real world, this doesn't really hold water.
 
fastline said:
Agree to disagree. I live in the real world. The information in the poorly worded question indicates THREE PHASES! I want to know what all three are doing! I mean sure, select the most practical answer on a test and move on, but in the real world, this doesn't really hold water.
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I agree we can disagree. If this were a 3-wire MWBC there would be no C phase current because there is no connection to the C phase. Someone reading the question should be able to understand that is what it says given all of the information provided. If they're unfamiliar with 208Y/120 systems then they probably won't understand the question and say that the neutral current is 5 amps.
 
So it seems it boils down to how each of us perceived the question. Some saw it as a 4 wire wye MWBC missing information, others saw it as a 3 wire wye MWBC. In either case the 3 wire version can be solved as is and for those who want it to be 4 wire just add a number for the missing value and solve it.
 
roger said:
So it seems it boils down to how each of us perceived the question. Some saw it as a 4 wire wye MWBC missing information, others saw it as a 3 wire wye MWBC. In either case the 3 wire version can be solved as is and for those who want it to be 4 wire just add a number for the missing value and solve it.
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I didn't see anything of the like. 208 3P with a neutral. Got it, a 208/120 4 wire panel. Apparently clamp meter only fits on 2 of the wires...lol But anyway, the point of the question, IF it's even a test question at all, is understanding the 120* phase relationship with neutral, as opposed to 180* in a single phase/split phase arrangement. It would not matter how you arrive at the currents on A and B. It just doesn't. All that matters is those 2 phases are 120* apart with respect to their neutral for the purposes of calculating the neutral current.
 
fastline said:
All that matters is those 2 phases are 120* apart with respect to their neutral for the purposes of calculating the neutral current.
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Bingo.

BTW, I have Flexible clamps that would fit around all the conductors. 🙂
 
Id like to say, from alot of experience, that some of the exam questions on the written part of a motorcycle drivers license test are dead wrong..
Some can be interpreted different yet remain subjective- yet others are dead wrong and objective...

Kinda like Neil degrasse tyson describing 3 different types of truth.

Kinda like this thread...
Id prefer saying "2pole 208v branch circuit in a 120/208v system"
But op did sorta describe that... c phase zero or omitted reading all together is the same. Same way panel schedule load calcs are only blocked in on apropriate phase.

Whats for dinner?
 
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