2014 NEC 110.26
(1) indoor
(a)Dedicated Electrical Space. The space equal to the width and depth of the equipment and extending from the floor to a height of 6' above the equipment or to the structural ceiling, which ever is lower, shall be dedicated to the electrical installation. No pipes, ducts, leak protection apparatus, or other equipment foreign to the electrical installation shall be located in this zone.
Exception: Suspended ceilings with removable panels shall be permitted within the 6' zone.
(b) Foreign Systems. The area above the dedicated space by (a) shall be permitted to contain foreign systems, provided protection is installed to avoid damage to the electrical equipment from condensation, leaks, or breaks in such foreign systems.