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Basics of electrical conduit
The first electrical distribution system provided direct current to residential and commercial customers in lower Manhattan. It was built by Edison Illuminating Co., beginning in 1882. Earlier in the century, telegraphy networks had become widespread, and ampacity, over-current protection and similar principles had been worked out. Fuses and even circuit breakers were available early in the nineteenth century, and Edison’s crews were able to create relatively safe installations. This is not to say that electric shock fatalities and electrical fires did not occur.
For homeowners and small commercial operations, it was a new technology, not without risks. But the advantages of incandescent over gas lighting (which also had hazards), was immense, so the demand for electrical service was great.
Doing premises installations, electricians quickly realized that they could make use of existing gas piping to route wiring throughout buildings. They could install numerous concealed wire runs without tearing up paneled walls and cutting into stamped-tin or plastered ceilings. Moreover, the repurposed metal piping provided excellent protection for the conductors and would contain thermal energy that might result from line-to-line arc faults.
This wiring method had drawbacks. The existing gas piping did not have pull boxes or large-radius bends, so when installers encountered an elbow, the pull stopped abruptly. Additionally, because there was no general consensus on grounding (some early codes prohibited it altogether) there was the possibility that abraded electrical insulation would permit an entire run of metal pipe to become energized.
Today’s electricians have the benefit of well-developed raceway and conduit products of metal, plastic and fiberglass. In addition to the pipe, numerous types of compatible fittings facilitate routing and wire-pulling tasks. Various types of flexible conduit, metal as well as PVC, some liquid-tight, permit installation in difficult settings or to control vibration as in the electrical supply to a motor.
Rigid metal conduit (RMC) resembles galvanized water pipe. Sizes, threads and so on are compatible. But it is an NEC violation to use water pipe where conduit is required. RMC has a smooth interior finish, making for easier pulls and less chance of insulation abrasion. RMC can be cut and threaded in the field, but the material is expensive and heavy, making for a labor-intensive installation. It is used in special applications such as hazardous areas, underground where bedrock prevents conventional burial depth, and in some high-voltage installations.
Electrical metallic tubing (EMT) is technically not a conduit. The proper term is metal raceway. However, when using it, electricians usually speak of putting wiring in conduit. It has a much thinner wall than RMC, but it is plenty rugged for most commercial and industrial applications, and it is widely used. Because of its light weight, it is quick and easy to deploy, and it is relatively inexpensive. It is not threaded in the field, but instead goes together easily with set-screw fittings (compression for outdoor work).
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