NQOD, NQO, and NQOB Panel boards - All fronts on enclosures 56 inches high or higher, excluding 53-inch
through 68-inch high vented fronts.
NQ and NF Panelboards - Fronts 56 inches or higher on 250 A maximum interior require two locks and Front 74 inches or higher on 600 A maximum interior require two locks with the exception of those for use on panels
using LC or LI main circuit breakers
I-Line Panelboards - HCN: Series E1 (Panelboards that meet 1984 NEC® Wire Bending Space are Series E1) fronts on enclosures up to and including 74-inches