30 CFR §75.1103-10

For each conveyor belt flight exceeding 2,000 feet in length, where the average air velocity along the belt haulage entry exceeds 100 feet per minute, an additional cache of the materials specified in § 75.1103-9(a)(1), (2), and (3) shall be provided. The additional cache may be stored at the locations specified in § 75.1103-9(a), or at some other strategic location readily accessible to the conveyor belt flight.