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Section 220.61 addresses feeder or service neutral loads, for which a neutral conductor smaller than the ungrounded feeder or service conductors may be permitted, based on the likelihood that some of the load is not connected to the neutral. All that must be calculated for sizing the neutral load is the amount of load that's actually connected to the neutral.
This rule considers the possibility that power could be lost to one or more but not all of the phase or line conductors, so the calculation is based on the maximum unbalanced load between any ungrounded conductor and the neutral.
Probably the most common practice in the field is to install a neutral conductor that is sized the same as the other conductors. Generally it's very unlikely for there to be a problem with that, but Section 220.61 permits the neutral conductor to be sized a little bit smaller for many feeders or services.
© Hughes Associates,Inc. 2012