30 CFR §75.804

1. (a)Underground high-voltage cables used in resistance grounded systems shall be equipped with metallic shields around each power conductor with one or more ground conductors having a total cross sectional area of not less than one-half the power conductor, and with an insulated external conductor not smaller than No. 8 (A.W.G.) or an insulated internal ground check conductor not smaller than No. 10 (A.W.G.) for the ground continuity check circuit.
2. (b)All such cables shall be adequate for the intended current and voltage. Splices made in such cables shall provide continuity of all components.