If a 2/0 Aluminum conductor in a switch is being replaced with a 250 KCML Aluminum because of voltage drop, what is the minimum space required between the conductor and the top of the enclosure?

If a 2/0 Aluminum conductor in a switch is being replaced with a 250 KCML Aluminum because of voltage drop, what is the minimum space required between the conductor and the top of the enclosure?