Your inspector is correct if you have a grounded surface on the opposite side of the workspace.
NEC 110-16 deals with working clearances.
Table 110-16(a) gives the working clearances as they apply do different situations.
At 151-600V to ground, the clearances are:
1)No live or grounded parts on other side=3'
2)Grounded parts on the other side=3-1/2'
3)Live parts on both sides=4'
In any case, the 30" in the UMC is not sufficient by NEC, as the requirement at 0-150V is 3' in any of the conditions mentioned above.