110.26(A)(4) Limited Access.  Where equipment operating at 1000 volts, nominal, or less to ground and likely to require examination, adjustment, servicing, or maintenance while energized is required by installation instructions or function to be located in a space with limited access, all of the following shall apply:

(a) Where equipment is installed above a lay-in ceiling, there shall be an opening not smaller than 559 mm x 559 mm (22 in. x 22 in.), or in a crawl space, there shall be an accessible opening not smaller than 559 mm x 762 mm (22 in. x 30 in.).

(b) The width of the working space shall be the width of the equipment enclosure or a minimum of 762 mm (30 in.), whichever is greater.

(c) All enclosure doors or hinged panels shall be capable of opening a minimum of 90 degrees.

(d) The space in front of the enclosure shall comply with the depth requirements of Table 110.26(A)(1).  The maximum height of the working space shall be the height necessary to install the equipment in the limited space.  A horizontal ceiling structure member or access panel shall be permitted in this space.

Comment:  The requirements for limited access working space for duct heaters contained in 424.66(B) of the 2014 NEC have been relocated to 110.26(A)(4) and expanded to include all equipment operating at 1000 volts, nominal, or less to ground that are likely to require examination, adjustment, servicing, or maintenance while energized.  This could include equipment such as motors, air-handling equipment, or transformers.  The access opening in a grid ceiling is based on a standard 2 ft. x 2 ft. lay-in ceiling grid opening.  The dimensions for a crawl space opening is based on similar requirements for signs at 600.21(E) with a minimum width of 30 in. or the width of the enclosure, whichever is greater.  Enclosure doors or hinged panels must be capable of opening a minimum of 90 degrees.  The height requirement is based on the minimum height required to install the equipment in the limited space, instead of the 6 ½ ft. or height of the equipment required by 110.26(A)(3).