% unbalanced

[APNaps]

What is the allowed % unbalanced when balancing loads in 3phase panel, 4wire with neutral, 400/230v? Do NEC states specific requirement?

 

[rc/retired]

To my knowledge, the NEC is silent on this.

Ron

 

[Hv&Lv]

One of the great things about a four wire is the ability to tolerate imbalance.
You can try to balance as well as possible, but what’s balanced now may not be in a hour or tomorrow depending on what your turning on or off…

 

[garbo]

Years ago we would take current measurements on all the old but close to tripping main breakers feeding panels. Would write down ampere draw several times during 2 or 3 hotest days of the year then rearrange a few 120 volt loads to get the 120/208 3 phase panels closed to having all 3 legs balanced. Did the same in oldest winter days. Left all readings in a book and was surprised that some 20 amp breakers might only draw an average of 5 Amps in the summer but 15 Amps in the winter and vice verses. Ended up installing some adfitina panels to lessen total load on any one buss bar.

 

[David Castor]

I don't think it's addressed in the NEC. Three-phase induction motors (non-VFD) are not very tolerant of voltage imbalance. 3% is the recommended maximum imbalance think for NEMA motors. This is calculated based on the maximum deviation from the average of the three voltages.

 

[Seven-Delta-FortyOne]

I don't think it's addressed in the NEC. Three-phase induction motors (non-VFD) are not very tolerant of voltage imbalance. 3% is the recommended maximum imbalance think for NEMA motors. This is calculated based on the maximum deviation from the average of the three voltages.

Voltage imbalance is very different than current imbalance in a panel.

 

[David Castor]

Voltage imbalance is often caused by current imbalance.

 

[NewtonLaw]

The voltage unbalance in percent is defined by the National Electrical Manufacturers Association (NEMA) in Standards Publication no. MG 1-1993 as:

% Unbalance = (Maximum Deviation from Average Phase to Phase Voltage) / (Average of Three Phase-to-Phase Voltages)

Note that the line voltages are used in this NEMA standard as opposed to the phase voltages. When phase voltages are used, the phase angle unbalance is not reflected in the % Unbalance and therefore phase voltages are seldom used to calculate voltage unbalance.

There is also the IEC Standards to indicate the degree of unbalance is the voltage unbalance factor (VUF) which is the ratio of the negative sequence voltage to the positive sequence voltage represented as:

%Unbalance = [V2 (the Negative Sequence Voltage) / V1 (the Positive Sequence Voltage)] x 100

and are the positive and negative sequence voltages, respectively, and can be obtained using symmetrical components. If you are not familiar with Sequence Components calculations the following will allow you to calculate the ratio of V2/V1



The effect unbalance has and to what limit you should use is somewhat subjective. A NEMA curve for the derating of a squirrel cage induction motor is give in the graph below:
Derating factor for squirrel cage induction motors due to unbalanced voltage. Standard ANSI/NEMA MG1


Hope this helps.