Bus spand support spacing can be calculated using the formula from IEEE Std. 605 "Guide for Design of Substation Rigid-Bus Structure".
L=K(FbxS/Ft)1/2
WHERE:
L= Max. Bus Support spacing
K= Multipliying factor for each type of expansion connector type
Fb=Max desirable bus stress of conductor
S = Section modulus
Ft= Total bus conductor loading