I was looking at some meter data that was given to me on a panelbaord to verify loading. The readings came back as (A=22A; B=4A; C=15A) after converting to kw adding the load to get the total panelboard feeder load but my concern was the imbalance seems pretty significant from A-B and B-C. At what point do the imbalance become a factor? What negative affect will this have on a "system"? thanks.
Load Imbalance ?
- Thread starter Mike01
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ptonsparky
Tom
- Occupation
- EC - retired
What is your neutral current? What is the size of your feeders? What is the resulting voltage taken phase to neutral and phase to phase? These currents could very well be normal and not a problem at all.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I would look at the difference in current as a percentage of the panelboard's rating (or its feeder/main breaker rating, if that is smaller). The highest to lowest reading you have is a difference if 18 amps. If this is a 30 amp panel, you may have an issue. If it is a 225 amp panel, there is no issue.
Panel Ampacity
Panel Ampacity
Thanks for the responses the panelbaord in questions is a 100A feeder, I will be getting voltage and neutral current information today, also. One thing that got me thinking on this panelbaord / feeder the feeder installed is a 1/0 conductor. the panel is rated 100A the load on the panel is the sum of the three phases. Why do we size conductors per phase for the total current of the panel when the individual "phase" will never see the full total load?
Panel Ampacity
Thanks for the responses the panelbaord in questions is a 100A feeder, I will be getting voltage and neutral current information today, also. One thing that got me thinking on this panelbaord / feeder the feeder installed is a 1/0 conductor. the panel is rated 100A the load on the panel is the sum of the three phases. Why do we size conductors per phase for the total current of the panel when the individual "phase" will never see the full total load?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
The individual phases will be able to see up to 100 amps for a 100 amp panelboard. If you have a panel with all line to neutral loads it is up to the installer to make sure the loads are balanced between the three phases.
You could have 300 amps of 120 volt loads in this case if balanced between phases, if you put it all on one phase you are loaded 300% of the capacity of that phase.
Load on a panel is not (or should not be) expressed in amps if it is sum of all three phases it should be expressed as volt-amps. If load is expressed in amps it should be the highest amperage of the phases.
As far as the readings in the OP is all possible loading running at time of reading? Is it possible to change anything to get more balance? Are there a lot of continuous loads or is there mostly non continuous loads.
A lot of information is needed to know if there is any problem here. These were just a few of the possible questions that are needed to know what you have.
Agreed the ampacity per phase is converted to VA totaled and then the overall panel ampacity can be determined calculated.
The schedules are accurate and the majority of the load on this particular panel is accounted for, there is a logging meter set up this was just an instantaneous value taken during peak occupancy to start working with but the full data is a week out.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
As has been mentioned if this is a 30 amp panel you may have an issue. It is a 100 amp panel and you do not have a problem with the panel or feeder.
Now if you have several feeders with all imbalance like that on same phases they will start to add up and could start to become a problem for transformers, service, other upstream feeders, etc.
Once heard a story about a new school where they had transformer problems and it took a while to find out the problem. What they found was all the classrooms had three banks of lights. Each bank was on separate phase and all had same number of lights. Each room had something like this:
Front row - phase A
Middle row - phase B
Back row - phase C
If all lights were on the system was fairly balanced. The problem that arose was during hours just before and after school many of the classrooms only had the front row turned on and all the front rows were on same phase so they were having high load on one phase while others were lightly loaded.
I was not involved with this problem just heard about it from another source so I can't give much more details about exactly what went on there.
Now if you have several feeders with all imbalance like that on same phases they will start to add up and could start to become a problem for transformers, service, other upstream feeders, etc.
Once heard a story about a new school where they had transformer problems and it took a while to find out the problem. What they found was all the classrooms had three banks of lights. Each bank was on separate phase and all had same number of lights. Each room had something like this:
Front row - phase A
Middle row - phase B
Back row - phase C
If all lights were on the system was fairly balanced. The problem that arose was during hours just before and after school many of the classrooms only had the front row turned on and all the front rows were on same phase so they were having high load on one phase while others were lightly loaded.
I was not involved with this problem just heard about it from another source so I can't give much more details about exactly what went on there.
robbietan
Senior Member
- Location
- Antipolo City
using the formula (max deviation from average amps/ average amps) *100 and I get something like 70% imbalance. using something I heard of, 8% current imbalance ~ 1% voltage unbalance, I get more than 8% unbalance.
why not check the voltage unbalance at the feeder? if the unbalance is higher than the NEMA MG-1 recommended limit of 3%, the three phase motors connected to the load could be heating up and wasting energy
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