A residential service brings in two hot conductors and a neutral. Each hot measures 120 volts to neutral, and the two hots measure 240 volts to each other because they are opposite in phase. Inside the panel those two conductors feed two bus bars, and the breaker positions alternate between them going down the panel.

How circuits are distributed across those two bars is called balancing, and it is one of the details that separates a tidy panel from one that merely works.

The neutral conductor is the reason it matters. Current returning from loads on one leg partially cancels against current returning from loads on the other, and the neutral only carries the difference. A panel where both legs are loaded roughly equally puts very little current on the neutral. A panel where most of the heavy circuits sit on one bar puts the imbalance through it continuously.

Uneven loading also produces uneven voltage. The leg carrying more current sees slightly more drop, which means lights and equipment on that side run marginally lower and the other side marginally higher. In extreme cases, and particularly where a neutral connection is deteriorating, the imbalance is what makes lights on one half of the house brighten while the other half dims.

Balancing is done by moving breakers between positions so that the large and frequently used loads are spread. An electric range, a dryer or an air conditioner already spans both legs because they are 240 volt loads, so the work concerns the 120 volt circuits around them.

It is easiest to do at installation and awkward afterwards, because conductors are cut to length for the position they were landed in. That said, it is worth checking whenever a panel is open for other reasons, especially in houses where circuits have been added one at a time into whatever slot happened to be free.

Anyone measuring this can do it without disturbing anything, using a clamp meter around each incoming hot conductor with the usual loads running. Two readings that sit within a reasonable margin of each other indicate a panel that is distributed sensibly. A wide gap points at where the next set of circuits should go.

Worth a read: http://forum.vellmart.com/profile.php?id=5007