Missing 208v 3 phase troubleshooting

Guyute

Member
I wasn't sure where to post this, but I ran into a problem today and was hoping someone had an explanation. I did a panel change out on a 3phase 120/208 system and once I completed the change I was checking my voltages and had 120v to ground and neutral from Phase A,B, and C. However I didn't have 208v between A and C phase. I did have 208v between A and B and B and C. I went back and checked the breaker the panel was fed from and it had the same issue. What would be causing me to not have 208v between these phases? Thanks
 

wirenut1980

Senior Member
What did you measure between A & C phases? zero volts? I can't think of anything that can explain the measurements you made if you measured zero volts between A & C.
 

Guyute

Member
Nothing

Nothing

I measured 0 volts between A and C. Everything else phase to phase and to ground was normal.
 

augie47

Moderator
Staff member
It sure sounds like someone has used a 3 phase panel but fed it with with only two phases with one phase feeding both A & C conductors.
Are there any 3 phase loads ?
Is this a service panel or a sub panel ?
Are you sure the service is 3 phase ?

If you know the service is 3 phase, one possibility is that someone made a mistake in connecting feeders and accidentally connected two of your phase conductors to one phase.
 

Guyute

Member
Bad breaker

Bad breaker

The issue appears to be the breaker feeding the sub panel. The main disconnect has proper voltage between all phases and to ground, plus the other 3 phase breakers in the main panel all are reading correct. What would be happening inside a breaker to make it not have 208 between certain phases. If the breaker was faulty then nothing should be coming out of one of the legs.
 

augie47

Moderator
Staff member
Possibility.....
Some panels are designed so only a portion of the panel has 3 phase bussing and the balance is bussed for only A & C phase loads.
These are designed for 3 Phase 240/120 (Hi-Leg) Loads but someone might have installed one on your service. look at the bussing.
 

Guyute

Member
Good call Augie

Good call Augie

Good call Augie! Whoever installed the service put a 3phase 4W Delta cabinet and guts in. The bussing was just as you mentioned it could be. The sad thing is the service is 3phase 4W Y and the sub panel has three phase equipment running on two phases. I am surprised no one caught this and that the equipment is running. Thanks for the input.
 

mark32

Senior Member
Possibility.....
Some panels are designed so only a portion of the panel has 3 phase bussing and the balance is bussed for only A & C phase loads.
These are designed for 3 Phase 240/120 (Hi-Leg) Loads but someone might have installed one on your service. look at the bussing.
Say wha?
Is there a buss layout I could view, I can't seem to wrap my mind around this. By the way, sir Augie, how are you feeling?
 

augie47

Moderator
Staff member
Doing a lot better, thanks.
As far as the buss layout, I don't have a diagram.
Picture a 3 phase panel with 3 main busses from the breaker.
"Fingers" are bolted to the busses and extend out for the branch breakers to connect.
The top finger is bolted to buss A, the 2nd to "B", 3rd to "C" etc.
At some point, lets say the 10th finger, the mfg quits bolting a finger to "B" so #10 buss is on A, 11 is on C, 12 is on A etc.
These were common with 240/120 3 phase hi-leg delta panels where there was limited need for 3 phase breakers and more need for single phase, 120v connections.
Hope that helps. I will look for a diagram.
 

kwired

Electron manager
You have lost either A or C phase and there is a connected load between the two bringing them to the same potential as which ever phase is still good.

Could be a blown fuse, could be a bad connection someplace.
 

mark32

Senior Member
Thank you Augie for the visualization, I see that, coupled with kwired's explanation, that some issue still exists due to the fact the op is reading 0v's across a and c and not due to the design of the panel.
 

kwired

Electron manager
Good call Augie! Whoever installed the service put a 3phase 4W Delta cabinet and guts in. The bussing was just as you mentioned it could be. The sad thing is the service is 3phase 4W Y and the sub panel has three phase equipment running on two phases. I am surprised no one caught this and that the equipment is running. Thanks for the input.
Must not be any three phase motors - they would have figured out something was wrong with them. They just don't develop any torque without the third phase.
 
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