Conduit and cable protection
In home construction, wiring is commonly allowed to be installed directly in walls without further protection. However in commercial and industrial buildings, wire needs to be better protected from damage, and so it is more commonly installed inside conduit or ductwork made of metal, plastic, or passageways cast in concrete.
While some types of wiring are available already inside a protective flexible spiraled metal shell, it is more common for conduit and ductwork to be installed empty and the wire added later by threading it through the finished passageways. The NEC spends considerable time documenting safe methods of installing cable in conduit, with the primary concerns being the friction and abrading of insulation due to pulling, damage to the wires or insulation due to sharp bending and kinking, and damage due to too much pulling strain on the cable.
A wire pulled with excessive force may break inside the conduit, requiring costly removal and replacement of the damaged wire. However, a wire pulled with just enough force to stretch the wire but not break it creates a fire and future-failure hazard. The stretched wire section will have a thinner conductive cross-section than other parts of the cable, causing the stretched wire to heat more rapidly due to lower current-carrying capacity. The stretched wire insulation also becomes thinner, reducing the voltage needed to penetrate the insulation. Breaks may also form in the stretched insulation, which may not immediately be discovered until the circuit is powered and damage from arcing or shorting has occurred.
Because most conduit and cabinets are made from metal, it is common for these components to have sharp metal edges due to the manufacturing processes. The NEC specifies a number of protective measures to help protect wire insulation from being cut or damaged by these edges both during installation and later when in actual use. Insulated cables may not be inserted directly through knockouts, for example, due to the sharp edge around nearly all knockout holes. Clamping and other wire protection is often not required for plastic conduit parts, since the plastic is not likely to damage insulation in contact with it.
In potentially hazardous locations, more robust cable protection may be necessary. Common conduit and ductwork protects against direct physical abuse, but is neither airtight nor watertight. In wet locations, conduit may resemble standard threaded pipe in appearance, with sealed gasketed box openings to keep moisture out. Areas with potentially explosive gases need further protection to prevent electrical sparks from igniting the gases, and internal conduit gas-tight barriers to prevent potentially ignited gases from traveling inside the conduit to other parts of the building.
Posts
