According to NEC 2014 250.130 :
Article 250 - Grounding and Bonding
VII. Methods of Equipment Grounding
250.130 Equipment Grounding Conductor Connections. Equipment grounding conductor connections at the source of separately derived systems shall be made in accordance with 250.30(A)(1). Equipment grounding conductor connections at service equipment shall be made as indicated in 250.130(A) or (B). For replacement of non–grounding-type receptacles with grounding-type receptacles and for branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be permitted as indicated in 250.130(C).
(C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following:
- (1) Any accessible point on the grounding electrode system as described in 250.50
- (2) Any accessible point on the grounding electrode conductor
- (3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates
- (4) To an equipment grounding conductor that is part of another branch circuit that originates from the enclosure where the branch circuit for the receptacle or branch circuit originates
- (5) For grounded systems, the grounded service conductor within the service equipment enclosure.
- (6) For ungrounded systems, the grounding terminal bar within the service equipment enclosure
This allows for a non grounded branch circuit to borrow a ground from another branch circuit. (related stackexchange question Is it OK to borrow a ground wire from a different circuit?)
My question would be what would be the best way to bond it? Run THHN inside a conduit? Is a conduit necessary? Is bare copper bad? Should I use a bonding lug on the outside of a box? Should you wrap the conduit in green tape? Can you use green THHN without a conduit? Is it bad to use romex and just cut off the non-grounding wires?
I'm trying to think of the best way to do it without confusing future electricians too much.