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).
PVC conduit is less expensive. Its use should be avoided in long horizontal runs because sagging and buckling due to thermal instability can make for an unsightly finished product. It is the raceway of choice, however, for most underground work, for wiring embedded in concrete, or in agricultural buildings where EMT would corrode.
Though not Code required, EMT is excellent for indoor runs of data cabling such as category wire. Because it is grounded, it provides excellent isolation from RF interference. If there is a future wiring upgrade to optical fiber or a higher Cat number, the existing wire can be used as pull rope to facilitate the upgrade.
All the rules for bending conduit are applicable to bending EMT. EMT is much easier to bend than RMC, where the next size larger bender is needed due to the greater outside diameter.
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.