30 CFR § 75.701-4
Grounding wires; capacity of wires.
Where grounding wires are used to ground metallic sheaths, armors, conduits,
frames, casings, and other metallic enclosures, such grounding wires will be
approved if:
(a) The cross-sectional area (size) of the grounding wire is at least
one-half the cross-sectional area (size) of the power conductor where the
power conductor used is No. 6 A.W.G., or larger.
(b) Where the power conductor used is less than No. 6 A.W.G., the
cross-sectional area (size) of the grounding wire is equal to the
cross-sectional area (size) of the power conductor.