Page "National Electrical Code" Paragraph 17

Many NEC requirements refer to " listed " or " labeled " devices and appliances, and this means that the item has been designed, manufactured, tested or inspected, and marked in accordance with requirements of the listing agency.

To be listed, the device has to meet the testing and other requirements set by a listing agency such as Underwriters Laboratories ( UL ), MET Laboratories, Inc. ( MET ), Intertek Group ( ETL ), Canadian Standards Association ( CSA ), and FM Approvals ( FM ).

These are examples of " national recognized testing laboratories " ( NRTLs ) approved by the United States Department of Labor, Occupational Safety and Health Administration ( OSHA ) under the requirements of 29CFR1910. 7.

Only a listed device can carry the listing brand ( or " mark ") of the listing agency.

Upon payment of an investigation fee to determine suitability, an investigation is started.

To be labeled as fit for a particular purpose ( for example " wet locations ", " domestic range ") a device must be tested for that specific use by the listing agency and then the appropriate label applied to the device.

A fee is paid to the listing agency for each item so labeled, that is, for each label.

Most NRTLs will also require that the manufacturer's facilities and processes be inspected as evidence that a product will be manufactured reliably and with the same qualities as the sample or samples submitted for evaluation.

An NRTL may also conduct periodic sample testing of off-the-shelf products to confirm that safety design criteria are being upheld during production.

Because of the reputation of these listing agencies, the " authority having jurisdiction " ( or " AHJ " – as they are commonly known ) usually will quickly accept any device, appliance, or piece of equipment having such a label, provided that an end user or installer uses the product in accordance with manufacturer instructions and the limitations of the listing standard.

However, an AHJ, under the National Electrical Code provisions, has the authority to deny approval for even listed and labeled products.

Likewise, an AHJ may make a written approval of an installation or product that does not meet either NEC or listing requirements, although this is normally done only after an appropriate review of the specific conditions of a particular case or location.