If you have an undersized service supply neutral or load neutral, then the unbalance load current must not be exceded the the size of the rating of the neutral wire and neutral bus. That is why it is a good idea to use 100 percent rated neutral buses and 100 percent ground buses and cable, because you don't know when these loads will be extremely unloaded. Also check the utility ground wire they usually run undersized ground service cables.
Also,I found that with small generators the voltage on the unloaded size will rise and the loaded side will drop with extreme load unbalances, which is bad.
Check with your utility co and source transformer mfg., they may have some unbalance current requirements.
There’s probably not much in US consensus standards on current imbalance. There is a curve in ANSI C84.1-1995 that compares motor-horsepower derating to voltage imbalance. Current imbalance is more likely limited as a condition of service.
The [customer] must balance his demand load as nearly as practicable between the two sides of a three-wire single-phase service and between all phases of a three-phase service. The difference in amperes between any two phases at the customer's peak load should not be greater than 10 percent or 50 amperes (at the service delivery voltage), whichever is greater… It will be the responsibility of the customer to keep his demand load balanced within these limits.
The reason I ask is our company invested in power quality metering (RPM) and in the metering parameters it has a maximum of 5% current unbalance but dosent say where it gets that number. If its a standard or made up. The ANSI sounds better with the 10%.
I misunderstood your post. I thought the ANSI standard specified the 10%. So the question remains is there a set standard on unbalance current. Example, if I performed a 30-day load study on a main service of a building and the result show an average of 80% loading on a 1000KVA wye transformer and an average current unbalance of 15%. Is the current unbalance an acceptable value.
The comment I made about ANSI and 10%. I only said it because the current unbalance parameter on the metering equipment is set a at 5%. In some cases this value would make sence and other it would not.
I'm a bit lost here - why is the unbalance an issue? If you have many single phase loads, but no three-phase motors, it doesn't matter a *** to you as a consumer. The distributor might think otherwise, though. On the other hand, if all your loads are three-phase motors, then the supply voltages are unbalanced, and you have a lot of unwanted heating going on in your motors. The 5% or 10% figures really only matter when you are trying to fix a problem - if you have no problem, and you've only noticed the unbalance 'cos a nice new meter told you about it, maybe you don't need to do anything. On the other hand, maybe you should check that the numbers you have don't mean a problem for you. bBt only you can work that out by analysis of your particular circumstances.
The (italicized) verbiage in my 30-Jul entry is not from C84.1—it is an example of service conditions described by an electric utility. C84.1 discusses voltage imbalance.
I have seen unbalance as much as 100 percent on service panels without effecting the loads, it depends on what is feeding the service panel. If the transformer and generator are balance by other loads on the system then your service panel unbalance will have no impact to yor service loads. The Key is to find out what the load imbalances are to the transformers and generators serving your panel. Go check other panels phases with a clamp on meter you will see what I am talking about.
correction-before you run off checking panels make sure they are low voltage type such as 120/208v 3 phase. I would not use clamp on cuurrent meters on anything higher than 480 volts.
