General Order 128
Section III
Requirements for Supply Systems
36.3
Neutral Conductors
A. Material
Neutral conductors shall be of suitable corrosion–resisting
material.
B. Size
Neutral conductors shall be of the size required to
carry the estimated current to which they may be subjected. As a minimum they
shall have an ampacity of not less than one–quarter (25%) that of the
largest related phase conductor, provided that the short time ampacity of
the neutral conductor is maintained. The short time ampacity of a neutral
conductor is either: the current that a non–insulated conductor can
carry for the time which the current flows without annealing, melting, or
separating; or the current that an insulated conductor can carry for the time
that the current flows without damaging the insulation. In no case shall neutral
conductors be smaller than the equivalent of #6 AWG copper.
No neutral conductor need have a greater ampacity than either the phase
conductor which would supply the ground current, or the maximum current which
can flow through the neutral conductor to the ground or electrode of the
neutral grid system. The neutral conductor shall have a short time ampacity
adequate for the fault current which can flow in the neutral conductor for
the operating time of the system protective device. If this value cannot
be readily determined, the continuous ampacity of the neutral conductor shall
not be less than the full load continuous current of the system supply transformers
or other source of supply. In no case shall the neutral conductor have less
ampacity than #6 AWG copper.