Transformer Readings

Chris3585

Member
Location
Smyrna, Georgia
We are looking to connect a new water heater to an existing circuit. The existing circuit serves a 480/208 volt, 45 kva transformer. We placed a recording amp probe on the circuit breaker feeding the transformer and recieved A-24 amps, B-46 amps, C-26 amps. This transformer feeds three panels that serve a small cafe. I was suprised to see the high load on the B phase. Most of the loads are 120 volts (small coolers, sandwich presses, etc). Could this be an indication that the transformer is going bad or is this simply loads way out of ballance?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I would guess the loads are simply out of balance. I would connect the water heater to the 2 lightly loaded legs.
 

Chris3585

Member
Location
Smyrna, Georgia
Unfortunately, the water heater is large at 24KW, 480 volt, 3 phase. It is possible that it is out of ballance, but I think it is unlikely with the type of loads connected. We are planning to measure the secondary voltages on the transformer. I suspect a problem with the transformer or improper grounding.
 

kwired

Electron manager
Location
NE Nebraska
May want to monitor the load over time, it may be balanced at times or unbalanced in a different way at times, depending on load usage patterns.

I don't see how improper grounding will effect the current in the primary winding. It is going to draw whatever the load requires from it. If transformer is developing an insulation problem it likely will not last very long at all and you were just lucky to measure it at right time. If primary windings are a WYE configuration and neutral is connected to the common point this could be a problem, but will be a problem right away not some time later. Most transformers like you are describing are delta primary and this will not even be possible.
 

Chris3585

Member
Location
Smyrna, Georgia
I agree it could be unbalanced load, but the 100% increase in the B phase looks strange to me. The readings are on the primary side of the transformer, which makes the amperage readings on the 208 volt side look even more strange. The transformer should only be serving a small cafe (about 900 square feet) that contains almost all 120 volt plug in equipment, with the exception of a residential range. The readings are the max readings over a 3 day period.

Is it possible the transformer could have a bad coil? It is at least 30 years old and has been sitting in the back of a kitchen.
 

jghrist

Senior Member
Can you get secondary current readings? If the secondary currents are unbalanced, can the cafe balance their single phase loads?
 

Chris3585

Member
Location
Smyrna, Georgia
I am working on getting that. The transformer is tapped twice on the secondary and the secondary conductors are not as easily accessible as the primary conductors were.
 

Crionics

Member
May want to monitor the load over time, it may be balanced at times or unbalanced in a different way at times, depending on load usage patterns.

I don't see how improper grounding will effect the current in the primary winding. It is going to draw whatever the load requires from it. If transformer is developing an insulation problem it likely will not last very long at all and you were just lucky to measure it at right time. If primary windings are a WYE configuration and neutral is connected to the common point this could be a problem, but will be a problem right away not some time later. Most transformers like you are describing are delta primary and this will not even be possible.
I don't follow your comment on the delta primary. If you have imbalances on your Wye secondary, this will translate into imbalances in your primary, regardless of connection.

Rob Wolf, PE
 

Chris3585

Member
Location
Smyrna, Georgia
what size primary OCP is present for the transformer ? Is the transformer secondary protected at 125% ?
The primary circuit breaker is 80 amps. The secondary has two taps. One uses a 125 amp, 3 pole enclosed circuit breaker and the other is in a main lug only panel that appears to be 225 amps, 3phase that then feeds a 100 amp, single phase load center. It is pretty old stuff and I know it is not up to current codes, but it may have been up to the code at the time of install.
 

kingpb

Senior Member
With the arrangement you describe with the two taps and single phase sub-panel, I would think you have an issue with unbalance.

But I'm not seeing the dilemma. You are hooking up a 24kW 480V 3ph load. I assume your not tapping the 480V transformer feeder; what is the 480V panel capacity and spare breaker availability?

Don't see how the transformer fits into it:blink:
 

augie47

Moderator
Staff member
Location
Tennessee
For what it's worth, and you may already realize, it appears your transformer secondary is not protected at 125%, therefore, the 80 amp breaker on the primary is a violation. (70 amp max), also, if the transformer ever sees full load, that plus your heater will go beyond
the 80 amp breaker
 

kwired

Electron manager
Location
NE Nebraska
I don't follow your comment on the delta primary. If you have imbalances on your Wye secondary, this will translate into imbalances in your primary, regardless of connection.

Rob Wolf, PE
It is not a problem with a delta primary. I mentioned this because it has been addressed many times here before, usually when back feeding a wye with 120/208 to achieve 480 volt. When doing this you do not want to connect the 120 volt neutral to the common point on the wye. I doubt this was the case with the OP. Whenever this has come up the person asking about it usually has a transformer that did not last very long and it overheated, let the smoke out, etc.
 

Chris3585

Member
Location
Smyrna, Georgia
We finally got to the bottom of this one. I received some reading from one of the electrical panels on the secondary side of the transformer and it indicated readings on the C phase that were close to those on the B phase. I have concluded that the original readings from the C phase on the primary were incorrect. I suspect the meter was not clamped on the C conductors properly. In any case, I decided to pull a new feed from one of the buildings main switchgear for the new water heater. It will be more expensive than using the local panels in the tenant space, but I think it is the best solution.
 

Chris3585

Member
Location
Smyrna, Georgia
With the arrangement you describe with the two taps and single phase sub-panel, I would think you have an issue with unbalance.

But I'm not seeing the dilemma. You are hooking up a 24kW 480V 3ph load. I assume your not tapping the 480V transformer feeder; what is the 480V panel capacity and spare breaker availability?

Don't see how the transformer fits into it:blink:
There is one spare breaker in the main switchgear feeding this space. It is located two levels below this space in a parking deck. I was looking a tapping the primary feed for the transformer to feed the new water heater. There is a disconnect on the primary side of the transformer that could be tapped. The transformer serves two panels. One serves a small cafe (that was originally a full service restaurant). The other panel serves a restaurant that has been vacant for about 20 years. It originally appeared there may be adequate capacity in this feeder, but I have determined otherwise.
 
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