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Balancing Incoming Main Feed Lines

Balancing Incoming Main Feed Lines

Balancing Incoming Main Feed Lines

How can you balance out the main incoming feeder lines?
I've seen 3-phase voltage swings many times when taking readings.  Two always seem close but the third one is always different - why is that?

Just how critical is the voltage swings between these three leads?  


Wayne E. Lovison

RE: Balancing Incoming Main Feed Lines

Normally this is caused by unequal loading of the three phases.  Sometimes this can be due to single-phase transformers on the utility's incoming feeder, but typically the imbalance is within the facility.

The only cure is to balance out the load as much as possible.  

Voltage imbalance can be a problem for three-phase motors.  The voltage unbalance creates much larger current imbalance in the motor, with subsequent overheating.  

The imbalance is defined as the maximum deviation from the **average** of the three voltages.

RE: Balancing Incoming Main Feed Lines

i had some good info on this i will try to locate.

RE: Balancing Incoming Main Feed Lines

An oft-debated matter is for how long a period and to what degree voltage and current are allowed to remain unbalanced before expensive insulation damage occurs in components like motors and generator windings.  Another example is a significant increase in AC-line harmonics for rectifier-based power supplies and drives.  

In addition to establishing and maintaining balanced conditions, careful selection of protective devices for damage minimization and production loss should be high on the list.  

Search electrical forums at this site for keywords 'unbalance' and 'imbalance' for reference material and applicable links.  One basic example is:

RE: Balancing Incoming Main Feed Lines


Thanks for the data it really shed light on the issue.
A great source for information too.  I've run a copy
for three other's here who work with me on such issues.


I tried to pull up your ftp: site but it wouldn't open.

Thanks Guys appreciate the feedback.

Wayne E. Lovison

RE: Balancing Incoming Main Feed Lines

no problem, have a good weekend

RE: Balancing Incoming Main Feed Lines

Suggestion: Original posting indicates that there were seen some swings on the meter scale. There are transient voltage imbalances caused by starting single phase loads and their transients. This kind of imbalance does not have continuous nature, therefore, it is less harmful to motors, and other loads.

RE: Balancing Incoming Main Feed Lines

If you have an open delta 3 phase supply it is inherently unbalanced, although some (especially power vendors) disagree with that the fact remains that open delta systems have a balance problem, and by my experience usually are.

The power vendor can close the delta I have been told by a genious engineer working for FPL (Florida Power and Light).

If you do not have open delta, but something decent then you can still have problems coming from the source.  To check the source, if possible turn off everything and see if the unbalance is due to the load you place on the grid, or if the grid is unbalanced.

If the grid is unbalanced then make the vendor fix it.

If your load unbalances the voltage then locate every single phase power source in your distribution panels and reconnect them in a more balanced manner.

Richard Neff

Richard Neff
Irrigation Craft

RE: Balancing Incoming Main Feed Lines

Remember the old technique of determining if the unbalance is due to the line voltage or the load side? This involves taking current readings on all three legs, rolling the leads one way or the other, taking current readings again, and doing it a third time. If the high leg of current stays with the same utility phase then it is likely a problem upstream from where the measurements were taken. If the high leg follows the same load leg, then it is likely a problem down stream from there.
Unbalance voltage is a big deal for long motor life. NEMA recommends that a motor not operate above 1% voltage unbalance without derating the motor. The max unbalance is supposed to be 5%. The current unbalance can be as high as 10times the voltage unbalance.
VUB(%) (NEMA method)= max deviation from average of any one leg/avg *100.

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