High current on the high leg

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Steve Courtney

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I am working on a three phase service that feeds a number of A/C condensers in multiple locations on the site. Our voltage to ground readings are: L1-121V, L2-220V, and L3-120V. Line to line are as follows L1- L2-245V, L1- L3-245V, and L2-L3-247.

Current flow on our 7.5 ton ACCU is very unnerving though. L1-24.5A, L2-32.7A, L3-27.2A. The stinger leg is very high compared to the other two. We don't see a linear relationship between the difference in the reference voltage and the excessive amperage draw on the stinger leg. This condition exists in two other locations on site. There have been multiple compressor failures over the last few years and our HVAC contractor asked us to look for answers.

To be honest we are a little baffled at the extreme high leg current draw and believe, for the moment , that is attributable to the high compressor failure rates.

Any ideas?


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N

NewtonLaw

Senior Member
Location
Honesdale, PA USA
The ACCU Nameplate Data Would Be Helpful

The ACCU Nameplate Data Would Be Helpful

Steve Courtney said:
I am working on a three phase service that feeds a number of A/C condensers in multiple locations on the site. Our voltage to ground readings are: L1-121V, L2-220V, and L3-120V. Line to line are as follows L1- L2-245V, L1- L3-245V, and L2-L3-247.

Current flow on our 7.5 ton ACCU is very unnerving though. L1-24.5A, L2-32.7A, L3-27.2A. The stinger leg is very high compared to the other two. We don't see a linear relationship between the difference in the reference voltage and the excessive amperage draw on the stinger leg. This condition exists in two other locations on site. There have been multiple compressor failures over the last few years and our HVAC contractor asked us to look for answers.

To be honest we are a little baffled at the extreme high leg current draw and believe, for the moment , that is attributable to the high compressor failure rates.

Any ideas?


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Is the ACCU unit a three wire three phase unit or is it a four wire unit requiring 208Y/120 volt supply?

Have your considered any single phase load that is part of the unit and could this be hooked up to to the wrong phase? Seems unusual that the high-leg is at 220 volts with the remaining phase to ground readings of only 120, 121 volts. If the high leg is 220 volts, I expected the phase to ground readings to be higher unless there is an unbalance on the primary supply to your transformers. Unbalance does not seem to be the problem based on your phase to phase readings.
 
ActionDave

ActionDave

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Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
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You can't have 245V, 247V phase to phase and get 220V phase to ground on the high leg. It should be around 213V, 215V. You either have bad information or an ungrounded something somewhere. Is there a transformer other than the POCOs anywhere?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
ActionDave said:
You can't have 245V, 247V phase to phase and get 220V phase to ground on the high leg. It should be around 213V, 215V. You either have bad information or an ungrounded something somewhere. Is there a transformer other than the POCOs anywhere?
Click to expand...
Actually, he can. It's a 240/120V system... 240V nominal line to line; 208V "nominal" B-line to neutral. And he said he's measuring to ground... not a neutral. The 220V may be a little high (6%) compared to the 3% line to line, but given his L1-L3 and L1-N-L3 voltages don't add up exact either, it doesn't appear to be disproportionate. Certainly not enough to explain the disparity in line currents.
 
kwired

kwired

Electron manager
Location
NE Nebraska
If there is limited load on the high leg (likely just motor loads) and the other two lines are loaded pretty moderately, I can see that possibly throwing the overall balance of things off some.

Something else that impacts this is whether there is a full delta with three equal sized units as the source, different size units as the source, or if it is open delta with same or different sized units - all that with consideration of the actual connected load.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
kwired said:
If there is limited load on the high leg (likely just motor loads) and the other two lines are loaded pretty moderately, I can see that possibly throwing the overall balance of things off some.

Something else that impacts this is whether there is a full delta with three equal sized units as the source, different size units as the source, or if it is open delta with same or different sized units - all that with consideration of the actual connected load.
Click to expand...
If it's an open delta source, "B" line should show voltage sag when the load current is high.
 
S

Steve Courtney

Member
Phil Corso said:
Steve...

Is this a new or existing system?

Phil Corso
Click to expand...

This was once a single phase service supplying a 400 amp main distribution panel. It has since been converted to a three phase Delta with a high leg The building,a nursing home is probably 30 years old. The three-phase loads consist up for air-conditioning units that are fed from two different sub panels. The ownership is only three years old, the HVAC Group has been involved for seven years.


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S

Steve Courtney

Member
NewtonLaw said:
Is the ACCU unit a three wire three phase unit or is it a four wire unit requiring 208Y/120 volt supply?

Have your considered any single phase load that is part of the unit and could this be hooked up to to the wrong phase? Seems unusual that the high-leg is at 220 volts with the remaining phase to ground readings of only 120, 121 volts. If the high leg is 220 volts, I expected the phase to ground readings to be higher unless there is an unbalance on the primary supply to your transformers. Unbalance does not seem to be the problem based on your phase to phase readings.
Click to expand...

We have had a wide range of voltage readings... I will return today and record readings again. The HVAC Group has recorded the voltages for some time , I will gather that info and post it.


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Smart $

Smart $

Esteemed Member
Location
Ohio
Steve Courtney said:
This was once a single phase service supplying a 400 amp main distribution panel. It has since been converted to a three phase Delta with a high leg The building,a nursing home is probably 30 years old. The three-phase loads consist up for air-conditioning units that are fed from two different sub panels. The ownership is only three years old, the HVAC Group has been involved for seven years.
Click to expand...
A prime candidate for an open delta source.

Steve Courtney said:
We have had a wide range of voltage readings... I will return today and record readings again. The HVAC Group has recorded the voltages for some time , I will gather that info and post it.
Click to expand...
If possible, a distinction between running and not running voltages would be nice.
 
S

Steve Courtney

Member
kwired said:
If there is limited load on the high leg (likely just motor loads) and the other two lines are loaded pretty moderately, I can see that possibly throwing the overall balance of things off some.

Something else that impacts this is whether there is a full delta with three equal sized units as the source, different size units as the source, or if it is open delta with same or different sized units - all that with consideration of the actual connected load.
Click to expand...

Open Delta... 2 primary lines, 2 pole mounted transformers


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J

JFletcher

Senior Member
Location
Williamsburg, VA
Nameplate FLA? Where were your voltage readings taken, the panels? What are the voltages at the unit(s) when it is running, and off?

istm if you have multiple units with the same problem, it isnt the units, but the supply, or the feeder(s) to the subpanels.

Your current imbalance is ~25%; way too high.
 
S

Steve Courtney

Member
L1-ground 118
L2-ground 220
L3-ground 119

L1-L2 - 249V
L1-L3 - 238V
L2-L3- 250V

Current draw
L1 23.9
L2 32.5
L3 25.4



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S

Steve Courtney

Member
Readings taken at the MDP and at the unit disconnect were within 1% of each other. The AC units are within 20 feet of the MDP AND SUB PANEL


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S

Steve Courtney

Member
The following is list of readings made by the HVAC team;


Since our readings vary under different conditions, here are 2 readings;



October 24th 2016

74 degrees outside

AM light load;

L1-L2- 245 Volts

L1-L3- 242

L2-L3- 243

Amps

L1- 21.6

L2- 29.2

L3- 21.9

*FULL LOAD AMP RATING OF THE MOTOR IS 25



May 17th 2017

84 degrees outside

PM heavy load

L1-L2- 245

L1-L3- 240

L2-L3- 234

Amps

L1- 21.2

L2- 31.1

L3- 21.3

*FULL LOAD AMP RATING OF THE MOTOR IS 25









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Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Did you happen to notice that your unit has a single phase motor in there too? It's drawing 3.1A at full load, SINGLE phase 208-240V. No indication of which phases it is connected to, but my guess is L2 is one of them, and that is making your imbalance worse.

"Roll" your conductors feeding this and see if the imbalance follows the roll. Rolling means move L1 to L2, L2 to L3, L3 to L1; rotation remains the same, but you shift which line goes to where. If the unbanned stays exactly the same, it's an internal issue in the A/C unit. If the issue changes, in in the source. Rolling is also a first strategy for mitigating it, you may end up taking advantage of slight differences in internal natural resistances/ impedance in the motors.
 
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