Shared neutrals in branch circuits meets the NEC, but not recommended when sensitive equipment is used. It is also not recommended when the circuit is powering non-linear loads. Non linear loads are "switch mode power supplies" as found in computers and other microprocessor products.
The theory behind using shared neutrals is valid only with linear loads. Linear loads have a unity power factor while switch mode loads do not. In theory a three-phase system is balanced as each phase voltage is 120 degrees behind (lagging) the phase before it. Phase currents are separated by 120 degrees as well. If each phase is carrying equal current (10 amps as an example), the equivalent currents will cancel each other as they combine at the neutral for return to the source. The result can be mathematically and algebraically shown to result in no (0 amps) of neutral current.
In reality the previous example assumes the electrical system is powering linear loads that the system is resistive in nature, that it is operation at unity power factor, and further, that the system operates in a state of equilibrium. In the real world, three phase systems are never in this state even though electricians do their best to balance loads. Elevators, compressors, and air handlers cycle in their operation. Computers, lights, copy machines, etc. are constantly being turned on or off. These changing conditions create natural imbalances in the three-phase distribution system. As soon as currents become unbalanced cancellation of neutral currents cease. As neutral current begins to flow, physical laws take over and the flow through the impedance of the neutral conductor creates a voltage drop that can be measured with reference to ground. The amplitude of the voltage will be directly proportional to the amount of neutral current flow and the impedance of the neutral conductor. Result, Neutral to ground voltage often called common mode voltage.
Branch circuit length, induced and conducted voltages all impact neutral to ground voltages, but the most common cause is outlined above. Sharing neutrals where switch mode power supplies are involved is not recommended because they are such a contributor to the imbalance. Neutral to ground (common mode) events can cause significant disruption to the operation of microprocessor based equipment. These devices constantly measure logic voltage against the "zero voltage reference" of life safety ground. The microprocessor expects to see less than 0.5 volts between neutral and ground.
It is common practice and meets the NEC to have shared ground and neutral conductors in 120-volt branch circuits (most cases). It is not good practice to have shared conductors for several reasons. Those that relate to the neutral and ground conductor will be explained briefly.
The ground conductor (not neutral) is the life safety and equipment chassis ground reference for the standard (120-volt) branch circuit. Other equipment uses ground as the "neutral" or drain wire is for the 120-volt branch circuit. Single-phase (208 & 240 volt) equipment is often wired; phase (hot), phase (hot) & ground. The efficiency of the equipment will determine how much of the energy is not used by that equipment. The unused energy uses the ground wire as the drain. This resulting energy dumped on the ground wire can have a very negative effect on sensitive equipment relying on the same ground. Noise, stray voltages and other anomalies are not good for sensitive networked equipment.
The below standards should be a guide to the proper installation of branch circuits. The wire size, outlet type, etc., should be selected to meet the NEC and the equipment requirements. The below standards are for 120VAC 15 ampere and 20-ampere branch circuits. All low voltage circuits should meet the grounding requirements below.