Short-Circuit Current Rating [110-10]
Electrical equipment must have a short-circuit current rating that permits the circuit overcurrent
protection device to clear short-circuit or ground-faults without extensive damage to the electrical
components of the circuit*.
Note: You can download
a spreadsheet for this purpose. See
Fault Current in the table on this page.
*There has been debate over the years as to what's considered “without extensive
damage”. To remove the confusion, a sentence was added to the 1999 NEC to specify that equipment installed
and used in accordance with their listings are understood to comply with the “without extensive damage”
Deteriorating Agents [110-11]
Electrical equipment must be suitable for the operating environment such as the presence of corrosive
gases, fumes, vapors, liquids, or chemicals. In addition, consideration must be given ultraviolet
rays from the sun, physical (mechanical) damage, excessive temperature, or other agents that could
have a detrimental effect on the electrical equipment.
Electrical equipment identified as “dry locations,” “Type 1,” or “indoor use only” must be protected
from the weather during the building construction period.
Mechanical Execution of Work [110-12]
Electrical equipment must be installed in a neat and workmanlike manner. What is a neat and workmanlike
manner? This is a judgment call by the inspector. A sloppy job indicates lack of proper training,
supervision, or an uncaring attitude by the employer or installer. Workmanship is pride.
Unused openings in all electrical equipment must be closed with an approved fitting that provides protection
equivalent to the wall of the equipment and the internal parts of electrical equipment must not be
damaged or contaminated by foreign material, such as paint, plaster, cleaners, etc.
Damaged parts of electrical equipment such items as cracked insulators; arc shields not in place, overheated
fuse clips, and damaged or missing switch or circuit breaker handles that may adversely affect the
safe operation or the mechanical strength of the equipment cannot be installed.
Mounting and Cooling Of Equipment [110-13]
Electrical equipment shall be firmly secured to the surface on which it is mounted [370-23]. Electrical
equipment that depends on natural air circulation for cooling must be installed according to the manufacturer's
instructions. For example, the airflow cooling vents on the equipment should not be blocked. Some
floor mounted equipment requires open space around and above for the dissipation of heat. Some transformers
have special requirements for cooling [450-9].
Electrical Conductor Termination [110-14]
Dissimilar conductor materials must not make contact in a terminal or splicing device, unless the device
is identified for the purpose. Dissimilar conductor materials in contact with each other cause corrosion
that degrades the conductor, increasing the resistance for the connection or splice. This in turn
may cause dangerous overheating of the termination or splice
Terminals in general are listed to accept no more than one wire per termination. Terminals that permit
more than one wire per termination must be identified, either within the equipment instructions or
on the terminal itself.
Note: For more information a on this subject, click
Manufacturer's Markings [110-21]
Electrical equipment must be marked with the manufacturer's identification and other Code sections
might require additional markings such as voltage, current, wattage. These markings are required to
withstand the environment in which the equipment is installed.
Disconnect and Circuit Identification [110-22]
Disconnecting means for appliances, electric space heating, electric duct heating, motors, air-conditioning,
services, etc., must be legibly marked to indicate their purpose and each branch circuit and feeder
must be legibly marked to indicate its purpose [384-13].
Access and Working Space [110-26]
For the purpose of safe equipment operation and maintenance, all electrical equipment such as panelboards,
motor control centers, or disconnects must have sufficient access and working space. Enclosures housing
electrical apparatus that are controlled by lock and key are considered accessible to qualified persons.
See sections 230-91 and 240-24 on the rules governing the accessibility of overcurrent protection
devices to building occupants.
Working Space. Working space is required for equipment that may need examination, adjustment, servicing,
or maintenance while energized. The phrase “while energized” is the root of many debates; a typical
argument is that electric power to almost all equipment can be turned off, somewhere. So if it is
not energized, then the working clearance requirement of this Section is not required.
This is a gray area in the NEC; always check with your inspector if you have any doubts.
No Storage. The working space must be clear at all times and there shall
be no storage in the working space area. People who service energized parts must not be subjected
to any additional dangers by working about, around, over or under bicycles, boxes, crates, appliances,
and other impediments.
Entrance. All electrical equipment must have one entrance of sufficient
area to give access to the working space. Check with your inspector for what he or she considers “sufficient
area.” This is another gray area in the Code.
Illumination. Service equipment, switchboards, panelboards, or motor control
centers located indoors must have the working space illuminated which can be provided by a lighting
source located next to the working space. The illumination must not be controlled by automatic means
Note: A switch with a motion sensor can be used, but the use of only
a motion detecting device for this purpose is not permitted.
Headroom. The minimum headroom for working spaces about service equipment,
panelboards, switchboards or motor control centers shall not be less than 6 feet 6 inches. When the
equipment exceeds 6 feet 6 inches, the minimum headroom shall not be less than the height of the equipment.
Dedicated Equipment and Foreign Systems Space. No piping, ducts, or equipment
foreign to the electrical installation shall be located in the dedicated equipment space*.
*Dedicated equipment space is identified as the space the width and depth (footprint)
of the equipment from the floor to a height of 6 feet above the equipment, or to the structural ceiling,
whichever is lower.
Foreign Systems Space Indoors. No piping, ducts, or equipment foreign to
the electrical installation shall be located in the foreign system space* unless the electrical equipment
is provided with protection from liquids from accidental spillage or leakage from piping systems.
*Foreign system space is identified as the space the width and depth (footprint)
of the equipment, from above the equipment to the structural ceiling.
Prevent Physical Damage. Electrical equipment must not be installed where
it can be exposed to physical damage.
Note: Exposure to physical damage is subject to interpretation by the
inspector; see Sections 240-24(c) and 300-4 for additional requirements.
Article 200 – Use and Identification of the Grounded (Neutral) Conductor
The above tips were extracted from the book Understanding the 1999 National Electrical Code
written by Mike Holt. This book can be ordered
over the Internet or by calling Mike Holt Enterprises, Inc., 1-888 NEC CODE (632-2633).