The National Electrical Code (NEC) on its own has no legal status. It is managed by the National Fire Protection Association (NFPA) for the benefit of jurisdictions that wish to enact it into law. In the U.S., these entities are mostly individual states, although some states such as New York and Illinois delegate this function to municipalities or to counties.
A revised edition of the NEC is published every three years, with many changes. Outside the U.S., several countries including Mexico and Venezuela have incorporated the NEC into their laws. Canada has the Canadian Electrical Code (CEC), and in large areas of Europe and elsewhere, the International Electrotechnical Commission (IEC) administers an electrical code. Australia and New Zealand share the Standard for Wiring Rules. In China there is a highly rational body of electrical standards, some mandatory and some voluntary. Their purpose, as in other parts of the world, is to promote electrical safety in the home and in the workplace.
In the U.S., the NEC doesn’t cover utility wiring where it is part of the electrical generation and distribution system. But utility wiring installed in office space, an equipment maintenance garage, and similar areas not directly part of the generation and distribution system must comply.
The NEC declines to get involved in some other areas also. It exempts underground wiring in mines. The implication is that other underground wiring such as lighting in underground traffic tunnels is covered.
One might think the NEC would not cover wiring operating below some minimum voltage, but this is not the case. Even runs of unshielded twisted pairs (UTP) used for low-power Ethernet circuits fall under NEC jurisdiction. There isn’t enough power to constitute a shock hazard, and in the event of a parallel line-to-line fault, there is not enough arcing energy to ignite combustible material. But if fire from a different source should ignite this cable, smoke generation and fire propagation become critical safety issues. Accordingly, specific NEC rules restrict types of Category, coax and communications cable including optical fiber in various locations of increasing sensitivity.
The NEC is not a treatise on electrical theory nor is it a how-to book on electrical wiring. The focus is strictly electrical safety, i.e. protection from hazards arising from the use of electricity. This definition is generally taken to mean avoidance of conditions that could give rise to electrical shock or electrical fire hazards. But additional risks can be present. For example, an inadequately secured length of conduit could fall and injure a person.
The greatest concern is danger to human life or injury, but another area is damage to electrical equipment. A lengthy NEC article lays out in great detail overload protection for motors so they will not be damaged by temperature rise.
NEC Article 517 is all about wiring in healthcare facilities. You might wonder why the Code calls out healthcare as being different from wiring in other locations. There are two principle reasons. First, a large healthcare facility at any given time is likely to have a number of patients who, with various degrees of urgency, need a totally reliable electrical supply — those on life support and in surgery are two examples.
The other reason the NEC treats healthcare facilities specifically is that a low level of electrical fault current can be lethal if it is introduced into the body through the metallic instrumentation used in surgeries. To mitigate this hazard, the Code paradoxically spells out two opposite techniques: ultra-reliable redundant grounding and isolation from ground.
To understand how to apply the many NEC mandates that apply to different types of healthcare facilities, it is useful to begin with a few definitions:
A healthcare facility is a building or portion of a building in which medical, dental, psychiatric, nursing, obstetrical or surgical care is provided. It includes but is not limited to hospitals, nursing homes, limited-care facilities, clinics, medical and dental offices, and ambulatory-care centers, whether permanent or movable.
A hospital is a building or portion thereof used for 24-hour medical, psychiatric, obstetric or surgical care for four or more inpatients.
A limited-care facility is a building or portion thereof used for 24-hour housing of four or more persons who are incapable of self-preservation because of age, physical limitation due to accident or illness, or limitations such as mental retardation/developmental disability, mental illness or chemical dependency.
A patient-bed location is the location of a patient sleeping bed or the bed or procedure table of a critical-care area.
A patient-care area is any portion of a healthcare facility wherein patients are intended to be examined or treated. Areas of a healthcare facility in which patient care is administered are classified as general-care areas or critical-care areas. Business offices, corridors, lounges, day rooms, dining rooms and similar areas are not patient-care areas.
General-care areas are patient bedrooms, examining rooms, treatment rooms, clinics and similar areas in which it is intended that the patient will come in contact with ordinary appliances such as a nurse call system, electric beds, examining lamps, telephones and entertainment devices.
From these definitions it is clear that there is a diversity in types of locations that fall within the umbrella term healthcare facilities. Each has specialized, differing electrical installation requirements. Two general categories of requirements are in regard to grounding and the provision of alternate electrical supplies, typically with elaborate automatic transfer switching.
Wiring installed in a healthcare facility calls for specialized design and installation methods. Exceptional care is needed because human lives are at stake. This is true for data wiring as well, where reliable performance can be crucial for successful patient care procedures.
Another type of facility for which the NEC spells out special treatment is what’s termed a hazardous location. Atmospheric air is ordinarily an insulator, which is to say that it will not conduct electricity. But when voltage is applied to this dielectric material, depending on the amount of voltage and the size and humidity of the air gap, it can become ionized. It then becomes much more conductive. Typically, an electric arc is established, and it can be continuous or intermittent. In a switch, relay or motor enclosure, such an arc is generally isolated from combustible material and there is not a problem.
The situation is radically different in a hazardous or classified location, where flammable gases or vapors from flammable liquids are likely to infiltrate the enclosure. Accordingly, the NEC promulgates specialized wiring designs, methods and materials for dealing with the enhanced hazards. As may be expected, there are various mandates depending on the type of hazardous material as well as the immediacy of the hazard.
There are three classes of hazardous areas, or four if you count unclassified.
Class I locations are those in which flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are or may be present in the air in quantities to produce explosive or ignitable mixtures.
Class II locations are those that are hazardous because of the presence of combustible dust.
Class III locations are those that are hazardous because of the presence of easily ignitable fibers, or where material producing combustible flyings are handled, manufactured or used.
As may be seen, these three hazardous areas are classified in terms of diminishing danger, although this is not absolute. (In some instances, Class II locations can be a greater threat than Class I locations, as in some grain elevator explosions that have resulted in numerous fatalities.)
The NEC further divides each of the classes into two divisions, 1 and 2. Generally, Division 1 locations have more immediate hazards. For example, a Class I location where flammable liquids are routinely sprayed in the air is a Division 1 location. In contrast, a Division 2 classification would apply where sealed drums of flammable liquids are handled or stored and the liquid does not become air-born except in the event of rupture of one of the drums. Accordingly, there are six class/division categories. (Actually there are many more, because the exact type of material, for example acetylene or hydrogen, is also taken into account.)
Each category calls for different wiring methods and materials. In some categories, wiring must be in rigid metal conduit, whereas in others electrical metallic tubing or even PVC will suffice.
There are many other strategies for creating hazard-free electrical installations in hazardous locations. Gasketed explosion-proof enclosures are used extensively in Class I locations. Redundant grounding, positive pressure ventilation, intrinsically-safe circuits and conduit seals are some of the mitigating means that have proven effective.
Where there is the possibility that a location should be considered hazardous from an electrical point of view, it is essential to delineate the area and use only approved wiring methods. Of course where possible the best plan is to keep all wiring outside these locations.
nexa electrical supplies says
thanks for sharing the useful information on the basics of the national electrical code and the location it covers