Duct And Cover

New text in section 424.66(A) of the 2014 National Electrical Code (NEC) covers working space for electrical enclosures of resistance heating-element-type duct heaters mounted on air-duct systems in limited-access areas. New subsection 424.66(B), Limited Access, addresses examination, adjustment, servicing or maintenance while energized. This additional text surprised many in the NEC process because most electricians, contractors and inspectors considered it to already be an NEC requirement. Close examination of the text reveals a few minor changes in the 2014 NEC requirements, but more potential changes are being contemplated for the 2017 NEC.


This limited-access provision in the 2014 NEC only applies to equipment rated 600 volts (V) or less and mounted to air conditioning and heating ducts where the equipment enclosure is located in a space above a ceiling. Four requirements must be followed. 


The first requirement is that the enclosure shall be accessible through a lay-in type ceiling or an access panel in the ceiling. The 110.26 introductory text has required this for years. 


The second is the width of the working space shall be the width of the enclosure, or a minimum of 30 inches, whichever is greater. Again, this has been a requirement in 110.26(A)(2) for many years. 


The third requirement is that all doors or hinged panels must open to at least 90 degrees. This is a subtle change, and it is different from the requirement in 110.26(A)(2), which only required the workspace to permit the panel doors to open at 90 degrees. The new requirement in 424.66(B)(3) requires the doors to open to at least 90 degrees. 


The final requirement in 424.66(B)(4) is the space in front of the enclosure must comply with the depth requirements in Table 110.26(A)(1). Additional text in 424.66(B)(4) states that a horizontal ceiling T-bar shall be permitted in this space. Based on experience working in these areas, the spreader bar between the T-bars that are supported from the ceiling can be removed to permit extra space where needed. More often than not, the electrical disconnecting means and the control/power enclosure are already mounted on or adjacent to equipment before the ceiling installers begin to install the ceiling grid. Ceiling grid installes and general contractors must coordinate so minimum working space is achieved.


For the 2017 NEC first revision process (previously called the proposal process), the “limited access” provisions would be covered in a new 110.26(A)(4), not in 424.66(A) and (B). The new text would apply where equipment operates at 1,000V, nominal, or less, to ground; where it is likely to require examination, adjustment, servicing or maintenance while energized; and where it’s located in a space with limited access. In this case, all of the following shall apply: 


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


(b) The width of the working space shall be the width of the equipment enclosure or a minimum of 30 inches, 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 structural member or access panel shall be permitted in this space. 


There will be a few minor changes between 424.66 in the 2014 NEC and the 2017 NEC. They are very subtle: The limited ceiling requirement in the 2017 NEC would apply to any electrical equipment in a limited-access area; the requirement would apply to 1,000V or less equipment, rather than 600V or less; horizontal ceiling structural members would be permitted in the space; and equipment other than duct heaters must also comply with these new requirements.


Removing horizontal structural ceiling grid to provide additional safe working space could be an issue since the structural grid would need to be replaced by the person working on the equipment, or a ceiling-grid installer could be called to do the replacement. Ensuring the ceiling grid is installed with the appropriate access is the best answer; however, this may not be feasible for many new installations.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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