Original Version

Rule 59.3-B

59.3-B.    Size

In common neutral systems the common neutral line conductor shall have a cross-sectional area approximately 50 percent or more of the area of the largest related primary phase conductor, as set forth in Table No. 14 and in no case shall have less conductivity or mechanical strength than No. 6 AWG medium-hard-drawn copper wire.

This Table is based on the requirement that the common neutral line conductor shall have a minimum area of approximately 50 per cent of the area of the largest related primary phase conductor and that the phase and neutral conductors are of the same material.  Where these are not of the same material, the copper conductance equivalents of the table will be considered as meeting the requirements.

Strikeout and Underline Version

Rule 59.3-B

59.3-B.    Size

In common neutral systems, the common neutral line conductor shall have a minimum ampacity at each location of not less than one-quarter (25%) that of the cross-sectional area approximately 50 percent or more of the area of the largest related primary phase line conductor, as set forth in Table No. 14 and, provided that the short time ampacity of the neutral conductor is maintained.  The short time ampacity of a common 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 under the applied tensions; or the current that an insulated conductor can carry for the time that the current flows without damaging the insulation.

No neutral line conductor need have greater ampacity than either:  the phase line conductor which would supply the ground current, or the maximum current which can flow through the neutral line conductor to the ground or electrode of the common neutral grid system.  The neutral line conductor shall have a short time ampacity adequate for the fault current which can flow in the neutral line conductor for the operating time of the system protective device.  If this value cannot be readily determined, the continuous ampacity of the neutral line 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 No. 6 AWG medium–hard–drawn copper wire.

This Table is based on the requirement that the common neutral line conductor shall have a minimum area of approximately 50 per cent of the area of the largest related primary phase conductor and that the phase and neutral conductors are of the same material.  Where these are not of the same material, the copper conductance equivalents of the table will be considered as meeting the requirements.

Final Version

Rule 59.3-B

59.3-B.    Size

In common neutral systems, the neutral line conductor shall have a minimum ampacity at each location of not less than one-quarter (25%) that of the largest related primary phase line conductor, provided that the short time ampacity of the neutral conductor is maintained.  The short time ampacity of a common 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 under the applied tensions; or the current that an insulated conductor can carry for the time that the current flows without damaging the insulation.

No neutral line conductor need have greater ampacity than either:  the phase line conductor which would supply the ground current, or the maximum current which can flow through the neutral line conductor to the ground or electrode of the common neutral grid system.  The neutral line conductor shall have a short time ampacity adequate for the fault current which can flow in the neutral line conductor for the operating time of the system protective device.  If this value cannot be readily determined, the continuous ampacity of the neutral line 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 No. 6 AWG medium–hard–drawn copper wire.