There are whole sections of code requiring balancing of load...

Section 220.22 describes the basis for calculating the neutral load of feeders or services as the maximum unbalanced load that can occur between the neutral and any other ungrounded conductor.
For a household electric range or clothes dryer, the maximum unbalanced load may be assumed to be 70 percent, so the neutral may be sized on that basis. Although 220.22 permits the reduction of the feeder neutral conductor size under specific conditions of use, the last two sentences, revised for the 2002 Code, cite two specific cases that would prohibit reducing a neutral or grounded conductor of a feeder.
If the system also supplies nonlinear loads such as electric-discharge lighting, including fluorescent and HID, or data-processing or similar equipment, the neutral is considered a current-carrying conductor if the load of the electric-discharge lighting, data-processing, or similar equipment on the feeder neutral consists of more than half the total load. Electric-discharge lighting and data-processing equipment may have harmonic currents in the neutral that may exceed the load current in the ungrounded conductors. It would be appropriate to require a full-size or larger feeder neutral conductor, depending on the total harmonic distortion contributed by the equipment to be supplied (see 220.22, FPN No. 2).
In some instances, the neutral current may exceed the current in the phase conductors. See the commentary following 310.15(B)(4)(c) regarding neutral conductor ampacity.

The maximum unbalanced load determines the size of the grounded service conductor. Section 220.22 allows this value to be reduced, based on the maximum unbalanced load that can occur between the ungrounded conductors and the grounded (neutral) conductor. However, the minimum size of the grounded service conductor cannot be smaller than that required by 250.24(B)(1) and (B)(2).
The additional heating effect of harmonic currents, due to nonlinear loads should be considered when sizing the neutral conductor of a 3-phase, 4-wire wye system. If the service to a building is a single-phase, 3-wire, 120/240-volt system with no 240-volt loads, the maximum current in the neutral would be the same as the maximum current in the ungrounded conductor. If all loads connected to one leg are “on” and all the other loads on the other leg are “off,” maximum current will flow in the neutral. In such cases, the service neutral size cannot be reduced but would be sized the same as the ungrounded conductors. See 310.15(B)(4) for more information on sizing the ungrounded conductor.

During the 1996 NEC cycle, a task group composed of interested parties was created to recommend to the National Electrical Code Committee the direction its standards should take to improve the safeguarding of persons and property from conditions that can be introduced by nonlinear loads. This group was designated the NEC Correlating Committee Ad Hoc Subcommittee on Nonlinear Loads. The scope of this subcommittee was as follows:
(1) To study the effects of electrical loads producing substantial current distortion upon electrical system distribution components including, but not limited to
a. Distribution transformers, current transformers, and others
b. Switchboards and panelboards
c. Phase and neutral feeder conductors
d. Phase and neutral branch-circuit conductors
e. Proximate data and communications conductors
(2) To study harmful effects, if any, to the system components from overheating resulting from these load characteristics
(3) To make recommendations for methods to minimize the harmful effects of nonlinear loads considering all means, including compensating methods at load sources
(4) To prepare proposals, if necessary, to amend the 1996 National Electrical Code, where amelioration to fire safety may be achieved
The subcommittee reviewed technical literature and electrical theory on the fundamental nature of harmonic distortion, as well as the requirements in and proposals for the 1993 NEC regarding nonlinear loads. The subcommittee concluded that, while nonlinear loads can cause undesirable operational effects, including additional heating, no significant threat to persons and property has been adequately substantiated.
The subcommittee agreed with the existing Code text regarding nonlinear loads. However, the subcommittee submitted many proposals for the 1996 NEC, including a definition of nonlinear load, revised text reflecting that definition, fine print notes calling attention to the effects of nonlinear loads, and proposals permitting the paralleling of neutral conductors in existing installations under engineering supervision.
As part of the subcommittee's final report, nine proposals for changes to the 1993 NEC were submitted. All were accepted without modification as changes to the 1996 NEC. Also included in this report and now pertinent to the 2002 NEC 310.15(B)(4)(c) is the following discussion.
SHOULD NEUTRAL CONDUCTORS BE OVERSIZED?
There is concern that, because the theoretical maximum neutral current is 1.73 times the balanced phase conductor current, a potential exists for neutral conductor overheating in 3-phase, 4-wire, wye-connected power systems. The subcommittee acknowledged this theoretical basis, although a review of documented information could not identify fires attributed to the use of nonlinear loads.
The subcommittee reviewed all available data regarding measurements of circuits that contain nonlinear loads. The data were obtained from consultants, equipment manufacturers, and testing laboratories, and included hundreds of feeder and branch circuits involving 3-phase, 4-wire, wye-connected systems with nonlinear loads. The data revealed that many circuits had neutral conductor current greater than the phase conductor current, and approximately 5 percent of all circuits reported had neutral conductor current exceeding 125 percent of the highest phase conductor current. One documented survey with data collected in 1988 from 146 three-phase computer power system sites determined that 3.4 percent of the sites had neutral current in excess of the rated system full-load current.

when phase A pulls more current than phase B or C", or vise versa. As a consiquence, you'll have higher than normal "current measured on a nuetral,

This current, in extreme situations can result in over load of the neutral. Some people consider it good practice to over-size neutrals in 3 phase loads where there is an imbalance known.

When balance is achieved, near equal loading, neutral current is reduced.

Unbalanced loads can also damage generator sets, and transformers.