According to NEC 200.4(A), a neutral conductor shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless:

• The circuits are all on the same phase.
• The neutral is sized at 200% of the largest ungrounded conductor.
• All circuits protected at 20 amperes or less.
• The enclosure is marked to indicate shared neutral usage.
• Specifically permitted elsewhere in the Code.