Rationale for Rule 4-022.
The neutral conductor of a system is a current-carrying conductor and shall be sized to carry the unbalanced load of the system. In case of a service neutral, the conductor shall also be capable of carrying fault current, and therefore the ampacity of the service neutral is bound by the Rules for sizing the system ground as well as the General Rules for neutral conductors.
Intent for Rule 4-022.
Fundamentally, we intend that the neutral conductor have sufficient ampacity to carry the unbalanced load. A demand factor of 70% may be applied to that portion of the unbalanced load in excess of 200 A, but no easement should be allowed in the ampacity of the neutral conductor for any portion of the load that consists of electric discharge lighting.
Although the CE Code, Part I, does not specifically mention harmonics, harmonics can cause heating in the neutral conductor. Harmonics caused by certain types of non-linear loads (eg, personal computers, printers, variable speed motor drives, certain types of electric discharge lighting) should be taken into consideration when sizing a neutral conductor. The use of 2-wire rather that 3-wire branch circuits is a good example of dealing with neutral problems in branch circuits.
Furthermore, we intend that the service neutral conductor not be smaller than No. 10 AWG copper or No. 8 AWG aluminum and the ampacity be not less than the ampacity of the grounding conductor that is sized by Rule 10-812.
When a bare neutral conductor is run in a raceway with insulated conductors, we intend that the ampacity of the neutral conductor be based on the insulation temperature rating of the adjacent circuit conductors. For example, if the circuit conductors were No. 6 AWG Cu type TW that is rated at 60°C, then the ampacity of the neutral would be based on Column 2 of Table 2, which would be 55 amperes. If the circuit conductors were type RW90 XLPE, then the ampacity of the neutral would be 65 amperes, based on Column 4 of Table 2.