Do retrofit ground wires need to be run in conduit?

Question

I'm moving my water heater to another corner of the room it is in, to allow for more usable space in the room. However, the grounding wire is only 14 ga instead of the required 10, and it's not in the same cable as the power conductors. I need to upgrade the ground wire or run a new 10/2 w/ground cable, but I'm not sure which will be simpler.

This answer about grounding an oven receptacle states that you can use "any achievable route" to retrofit a ground. Does that mean you can run THHN outside of conduit, even though you'd normally never be allowed to?

Answer 1

Conduit or armor is only needed in certain cases

Retrofitted equipment grounding conductors (i.e. grounding conductors not run alongside the circuit conductors) only need the protection of conduit or cable armor if they are smaller than 6AWG and run exposed or otherwise subject to physical damage. This is laid out in NEC 250.120(C):

(C) Equipment Grounding Conductors Smaller Than 6 AWG. Where not routed with circuit conductors as permitted in 250.130(C) and 250.134(B) Exception No. 2, equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of buildings or structures and where not subject to physical damage.

Normally, unless you're dealing with particularly egregrious physical damage exposures, the armor on a Bare Armored Ground cable should suffice; upsizing the wire to 6AWG is also a suitable approach in many cases.