Balancing 3 phase loads

[trickett]

It is my understanding that three phase circuits involving a neutral must be balanced. It is also my understanding that for example if you put two "A" phases and one "B" or "C" phase on a neutral that it can burn through the neutral because the unbalanced load is only between the total two "A" phases and the "B" or "C" phase.

Your coments are welcome.

 

[rattus]

Re: Balancing 3 phase loads

It is desirable to balance 3-phase loads with or without a neutral. I never heard that it is required.

 

[Ed MacLaren]

Re: Balancing 3 phase loads

The answer depends on what you mean by the statement "circuits involving a neutral must be balanced."

Although it is desirable, it is not a must that the loads connected to the three phases be balanced. The neutral wire is able to carry the unbalanced load current.

If you mean that a single neutral conductor must not serve more than one circuit conductor supplied from the same phase, that is true.

Ed

 

[charlie]

Re: Balancing 3 phase loads

Trickett, you may be referring to the requirements of the serving electric utility. In our Goldbook (Page 11, last line), we have a statement, "Load shall be balanced as nearly equal as possible on all 3 phases." The reason for this is so the transformer will be loaded evenly and we will not be required to replace it because of uneven loading.

There are no Code requirements to balance loads.

 

[iwire]

Re: Balancing 3 phase loads

Originally posted by Ed MacLaren:
If you mean that a single neutral conductor must not serve more than one circuit conductor supplied from the same phase, that is true.

It is true if the neutral conductor is rated the same as the overcurrent device.

It can be down with a larger neutral.

A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).

[ December 26, 2004, 09:09 AM: Message edited by: iwire ]

 

[frankbrown]

Re: Balancing 3 phase loads

It is not a requirement that loads be balanced, but rather a good design practice. Balancing a load allows for maximum utilization & protection of equipment. If a 3 phase load becomes extremely unbalanced, one or more of the circuit protecting devices (fuses or Oil Circuit Recloser)will likely trip that phase of the circuit. In Distribution Design, the 3 phase electronic VWE is the way to go. The VWE senses large phase imbalances and is designed to trip all three phases if an extremity of this situation occurs, unless the settings are changed after sent from the manufacturer. In Plant Industrial Applications such as large 3 phase motors, if starter overloads do not trip and a fuse is blown on the circuit that is unbalanced, a "Single Phase" situation will occur. If this happens, the windings of the motor will likely be damaged and the motor will have to be removed from service until properly rewound or disposed of if completely non-repairable.