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).
Many jobs involving a single 90° bend are quite simple and require no advanced knowledge or expertise. A fundamental principle in all EMT Electrical Conduit installations is that the pipe should conform closely to the wall or ceiling finish surface. In other words, do not allow the raceway to take a shortcut through open space in a building interior. Nor, generally, should a diagonal route be taken even if that would reduce the amount of raceway and wire required.
To make an interior corner, use a bender, forming a uniform 90° sweep. To make an exterior corner, use a conduit fitting such as a 90° conduit body with removable cover, to facilitate conductor installation.
The NEC specifies that each individual raceway run is to be installed as a complete system including end terminations prior to pulling in conductors. The NEC also states that conductors are to be pulled through the equivalent of no more than four 90° bends between terminations and/or open pull points. This figure is true for all sizes of conduit. There is no limit to the distance of the run.
Quite often a conduit run consists of a straight stub between boxes. To simplify the installation, you can postpone tying down one of the boxes until after the pipe is terminated. Another common, simple job involves running pipe from a box to an interior corner, known in the trade as a brick wall, where a conduit bend is needed. You can leave one or both legs long, then mark and cut them to fit after the bend is made. Alternately, and more elegantly, use the standard deduction for a given size conduit. For half-inch conduit, the deduction is five inches. For 0.75-in. conduit, the deduction is six inches. For IMC Electrical Conduit the deduction is eight inches. Usually the deduction is stamped on the bender.
Mark the conduit, minus the deduction for the conduit size. Put the conduit in the bender with the mark on the Rigid Electrical Conduit aligned with a mark on the bender, usually an arrow called the B mark.