Buried Electrical Conduit Specifications

Conduit Size

• The size of the conduit in relation to the space filled by the electrical cables inside is a key element of planning the installation of an underground electrical conduit. The NEC bases conduit size on the amount of space inside the conduit taken up by the cables. For conduits housing one cable, the cable should take up no more than 53 percent of the interior capacity of the conduit. Two cables should not exceed 31 percent, while more than two cables should take up no more than 40 percent of the interior of the conduit. The fill ratio applies to both low and high voltage cables. As with other standards established in the NEC, there are exceptions, with the code noting that circumstances may require more or less of a conduit fill ratio.

Number of Conduit Bends

• Conduit bends are an essential part of underground electrical conduit installation, as it allows for the running of cables between the different components of the system -- such as from a panel board to an outlet box. The NEC limits the number of conduit bends to no more than four 90-degree bends in the length of conduit. This ensures cables can be pulled through the conduit with little trouble. These rules apply to both high and low voltage cables, up to 50,000 volts.

Conduit Bend Radius

• While the NEC established requirements on bend radius for both lead-sheathed and non-lead-sheathed cables, the lead-sheathed models have become increasingly obsolete as new forms of cable insulation are introduced. Compliance with NEC rules on non-lead-sheathed cables is easily accomplished with the use of a conduit bending tool, or using pre-bent cables manufactured to NEC specifications. Non-lead sheathed cables with a smaller diameter require a smaller conduit bend radius. For example, a cable one-half inch in diameter will have a bend radius of four inches, while a cable measuring three inches in diameter will have a bend radius of thirteen inches.

Location

• Safety is an important part of underground electrical conduit placement. In an attempt to avoid disasters, the NEC classifies and develops standards for locations deemed hazardous. Hazardous location designations are based on three location types called Class I, Class II, and Class III.

  Class I locations are those where combustible gases or fumes may be present; such as oil refineries, dry cleaning location, and fuel storage or service areas.

  Class II locations are those with the risk of flammable dust; such as aluminum or magnesium powder plants, grain mills and elevators, and plastic and pharmaceutical manufacturers.

  Class III locations are those locations where flammable textiles are produced or stored. In addition to the class designation, conditions play a major factor with areas evaluated on whether conditions are normal or abnormal.

  Lastly, the NEC considers the type of hazardous substance at the location, based on ignition temperature, conductivity, and explosion pressure.