Below I drew a simplistic view of a distribution transformer which supplies a particular public area for example some households and shops ect.:

enter image description here

Va, Vb and Vc are three phases(line voltages) of the transformer.

In my previous question I've asked about why balancing the line currents i.e making Ia, Ib and Ib close is important.

Now imagine Z1, Z2 and Z3 are initially same. Let's say they represent same type of electric owens. And we can say Ia, Ib and Ic are initially equal.

And now imagine one of the electric oven Z3 is plugged of from the mains.

In this case, will the system automatically equalize Ia, Ib and Ic again for the sake of a balanced system? Where and how is that done practically? Is there a control mechanism unit for that purpose?


2 Answers 2


In this case, will the system automatically equalize Ia, Ib and Ic again for the sake of a balanced system?

No it won't automatically balance and, as a result there will be some current passing down the neutral wire to the 3 phase power source. Previously, with all three loads balanced, neutral current would have been zero.

How are load currents balanced in a distribution system?

Statistically, with many households and factories, the currents are reasonably balanced in full load scenarios but nothing is perfect and current imbalances do take place.

  • \$\begingroup\$ But I dont get it. How can they know that maybe all of a sudden one of the line's all loads will be switched off at the same time. That would result huge neutral current. So they plan it before based on assumptions? \$\endgroup\$
    – floppy380
    Feb 27, 2018 at 10:29
  • \$\begingroup\$ The neutral wire has to be rated accordingly and yes, it can mean large currents up to 1.732 times the line current (1.732 = \$\sqrt3\$). \$\endgroup\$
    – Andy aka
    Feb 27, 2018 at 10:32
  • \$\begingroup\$ I also would be glad if you have comment on this: electronics.stackexchange.com/questions/358604/… The answer was not satisfactory. \$\endgroup\$
    – floppy380
    Feb 27, 2018 at 10:40
  • \$\begingroup\$ Your misconception is local distribution currents had to be balanced. They hadn't. A en.wikipedia.org/wiki/Zigzag_transformer is used to distribute uneven local currents evenly on the high voltage side. \$\endgroup\$
    – Janka
    Feb 27, 2018 at 10:45

Nothing is done to balance the loads actively. Balance relies on statistically having equal current draw.

While line balance is good, it's not essential. The neutral conductor is sized according to the expected degree of worst case balance.

For public single phase distribution, where it's possible to lose one line, the neutral is as big as the line conductors, so it can cope with 100% imbalance.

For power to a single well balanced 3 phase load, the neutral may be omitted altogether.


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