Verifying system voltage

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zcanyonboltz

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Quiz question I need some help understanding. You have a 3-pole 200 amp panel with 3 hot conductors and another that is connected to the neutral bar. None of these four conductors have phase tape. You check with your voltmeter and get the following readings.

A to neutral =120 volts
B to neutral =120 volts
C to neutral =120 volts

You now know the system is:
a. 120/208-volt, 3ph, 4-W b. 120/240-volt ,3ph, 4-W

I'm not sure how to rule out one or the other. Any teachings are appreciated. Thanks
 
zcanyonboltz said:
Quiz question I need some help understanding. You have a 3-pole 200 amp panel with 3 hot conductors and another that is connected to the neutral bar. None of these four conductors have phase tape. You check with your voltmeter and get the following readings.

A to neutral =120 volts
B to neutral =120 volts
C to neutral =120 volts

You now know the system is:
a. 120/208-volt, 3ph, 4-W b. 120/240-volt ,3ph, 4-W

I'm not sure how to rule out one or the other. Any teachings are appreciated. Thanks
Click to expand...
If you look at the way 120/240 3ph, 4-W is wired, you will see that the A-N and C-N voltages are both 120V, which matches the problem statement, but the B-N voltage would be 208V. So answer b cannot be correct.

By the way, the most common description (and the one used in the NEC) for situation #a is 208Y/120. That by itself includes the 3ph and 4-W.
 
Thank you very much, what was throwing me off was the fact that the delta b phase has 208 which tells you the answer to this question is the 208 wye system. Makes sense now thanks.

So in a delta system,
circuits powered only by B phase are 208v circuits or is this not how its done? I guess im saying why is delta 120/240 or 240/480 but 208 is not included even though its there on B phase?
 
zcanyonboltz said:
Thank you very much, what was throwing me off was the fact that the delta b phase has 208 which tells you the answer to this question is the 208 wye system. Makes sense now thanks.

So in a delta system,
circuits powered only by B phase are 208v circuits or is this not how its done? I guess im saying why is delta 120/240 or 240/480 but 208 is not included even though its there on B phase?
Click to expand...
You don't have any line to neutral loads tied to the high leg. It is possible to do it, but it's just not done.....transformer sizing, finding appropriate breakers, good manners are all reasons not to.
 
zcanyonboltz said:
Thank you very much, what was throwing me off was the fact that the delta b phase has 208 which tells you the answer to this question is the 208 wye system. Makes sense now thanks.

So in a delta system,
circuits powered only by B phase are 208v circuits or is this not how its done? I guess im saying why is delta 120/240 or 240/480 but 208 is not included even though its there on B phase?
Click to expand...

It is not included because we do not hook up loads on B phase L-N which is 208V, we use B phase for L-L and 3 phase loads that results in 240V.
 
zcanyonboltz said:
Thank you very much, what was throwing me off was the fact that the delta b phase has 208 which tells you the answer to this question is the 208 wye system. Makes sense now thanks.

So in a delta system,
circuits powered only by B phase are 208v circuits or is this not how its done? I guess im saying why is delta 120/240 or 240/480 but 208 is not included even though its there on B phase?
Click to expand...
The physics of how a delta system works and how a wye system works dictates that the delta will never have all three output leads at an equal voltage to a common point, as there is no natural common point that is equal potential to the other points of the system. A wye system does have a common center point that is equal number of turns to each "phase" conductor and therefore it will be same voltage from this neutral to each phase conductor.

It also is the physics of how it works out that in order for a wye system to have 120 volts to the neutral the phase to phase voltage will be 208 volts. We primarily use this system because of the need to supply several 120 volt loads. If a lot of three phase motor load is the bulk of the load, then the delta @ 240 volts is more desirable - yet you do have some 120 volt ability if you ground the midpoint of one phase, which is what a 240/120 delta system is.
 
jumper said:
She thinks I am that nutty guy who hooked up one of those multi voltage 120V-277V fluorescent ballasts to a high leg Delta, 208V L-N, just to see if it would work.

It does.:)
Click to expand...
You gotta make a trip out and see the shop where I work. We have a high leg service......even better, there are old ballasts, transformers, rectifiers, old motors, capacitors, and a test bench to throw down anything you want try putting power to.
 
ActionDave said:
You gotta make a trip out and see the shop where I work. We have a high leg service......even better, there are old ballasts, transformers, rectifiers, old motors, capacitors, and a test bench to throw down anything you want try putting power to.
Click to expand...

I think we need a "Show your work bench / Shop" thread :happyyes:
 
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