You must bond the Equipment Grounding Conductor of the branch circuit to the existing Grounding Electrode System. If you run quad-plex you can use either a multi wire branch circuit or if it's thirty amperes or more you can use it as a single phase feeder to supply the existing equipment.
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Tom Horne

Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison