46 C.F.R. § 35.30-35
(a) Where Grades A, B, C, and D liquid cargoes are involved, power driven or manually operated spark producing devices shall not be used in bulk cargo tanks, fuel oil tanks, cargo pumprooms, or enclosed spaces immediately above or adjacent to bulk cargo tanks unless all the following conditions are met:
(2) The compartments adjacent and the compartments diagonally adjacent are either:
[CGFR 65-50, 30 FR 16704, Dec. 30, 1965, as amended by USCG-2004-18884, 69 FR 58345, Sept. 30, 2004]