I want to install a ceiling fan where a ceiling light currently exists in my garage. The box for the current light has only two wires coming in however: a black & a white (no ground - looks like original wiring from 1950's). My question is how can I ground the ceiling fan if I install it in this 2-wire box location? Can I run the ground wire from the ceiling fan to a separate nearby ceiling light box which does have a ground wire coming into it? This nearby ceiling box is on the same circuit (i.e. both the ceiling fan and the nearby electrical box are controlled by the same circuit breaker). I realize I would of course need to first install a ceiling fan rated electrical box to attach the ceiling fan to, but can I ground the ceiling fan to this nearby ceiling light box which is properly grounded back to the circuit breaker panel?
Yes you can
Recent NEC editions allow you to run a retrofit ground wire (equipment grounding conductor) from an ungrounded box/circuit to a grounded branch circuit's ground wire, provided the donor circuit has a large enough ground wire to be a suitable ground for the circuit being retrofitted. (For instance, if you are grounding a 20A circuit this way, the donor circuit would need to have a 12AWG or larger ground wire.) The permission for this is given in NEC 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) 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
Informational Note: See 406.4(D) for the use of a ground-fault circuit-interrupting type of receptacle.
Furthermore, since ground wires do not need to run with the circuit they are grounding as per NEC 300.4(B)(2):
(2) Grounding and Bonding Conductors. Equipment ground‐ ing conductors shall be permitted to be installed outside a race‐ way or cable assembly where in accordance with the provisions of 250.130(C) for certain existing installations or in accordance with 250.134(B), Exception No. 2, for dc circuits. Equipment bonding conductors shall be permitted to be installed on the outside of raceways in accordance with 250.102(E).
, you can simply run a bare 14AWG wire from the new ceiling fan box, through the ceiling, to the "donor" box, bring the wire into the box, and connect it to the existing ground wire bundle there.
Why are you using the existing wiring to install a new fixture? This isn't code compliant and it's saving $10 to do it in a manner that's unsafe... Run new wire that's grounded and stop trying to cut corners.