It has to do with the Dedicated Space associated with the electrical equipment. The NEC Article in reference here is Art. 110.26(F)(1)(a)-(d)
**[FONT=Times-Bold]size=2 Dedicated Equipment Space.
All switchboards, panelboards,
distribution boards, and motor control centers
shall be located in dedicated spaces and protected from
*Exception: Control equipment that by its very nature or
because of other rules of the Code must be adjacent to or
within sight of its operating machinery shall be permitted
in those locations.
Indoor installations shall comply with
110.26(F)(1)(a) through (d).
(a) Dedicated Electrical Space. The space equal to the
width and depth of the equipment and extending from the
floor to a height of 1.8 m (6 ft) above the equipment or to
the structural ceiling, whichever is lower, shall be dedicated
to the electrical installation. No piping, 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 1.8-m (6-ft) zone.
(b) Foreign Systems. The area above the dedicated
space required by 110.26(F)(1)(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.
© Sprinkler Protection. Sprinkler protection shall be
permitted for the dedicated space where the piping complies
with this section.
(d) Suspended Ceilings. A dropped, suspended, or
similar ceiling that does not add strength to the building
structure shall not be considered a structural ceiling.