Electrical Rigid Metal Conduit: Steel (RMC) for Directional Boring

Directional Boring (DB), also called Horizontal Directional Drilling (HDD) is a trenchless method for installing electrical conduit for the protection of cable and wire circuits. It is used to install conduit beneath streets, highways, and railroad railbeds, as well as for other applications to accommodate aesthetic or environmental considerations.

DB trenchless technology is gaining popularity as an efficient and environmentally friendly alternative to installing communication and power raceways. When considering the cost of surface restoration required using the open-trenching method on highways, streets and parking areas; the need for bore and receiving pits; and damage to the landscape and impact on the environment, DB might be the cost-effective solution. In downtown areas, an often-overlooked but equally important consideration is the public goodwill gained and safety issues avoided in not disrupting the flow of traffic. Rigid Steel Conduit is manufactured from mild steel tube and will withstand the normal stresses generated by the directional boring equipment. The minimum yield strength of the conduit is 30,000 psi. This is your assurance that the conduit will not “neck-down” and cause damage to the cable or conductors during installation.

The interior and exterior surfaces of the conduit are thoroughly and evenly coated with zinc using the hot-dip galvanizing process, so that metal-to-metal contact and galvanic protection against corrosion are provided. A clear coating of a white rust preventative solution is applied over the zinc coating. Welding of each joint is neither required nor permitted by the NEC because of the stress it places on the interior coating or possible creation of a weld-bead that could damage the cable or conductors upon insertion. Section 300.18 (B) Welding, of the 2014 National Electrical Code® (NEC®) says, “Metal raceways shall not be supported, terminated, or connected by welding to the raceway unless specifically designed to be or otherwise specifically permitted to be in this Code.” No such permission or exception exists in the NEC

Rigid Metal Conduit produced in standard lengths of 10 feet (3.05 m) and 20 feet (6.10 m), including a coupling, is threaded on both ends, with a coupling applied to one end and a thread protector to the other. The pitch of threads conforms to the American National Standard for Pipe Threads, General Purpose (Inch), ANSI / ASME B1.20.1. The taper of threads is 3/4 inch per foot (1 in 16). Threads are protected after cutting by an application of molten zinc.

Applications

Galvanized Rigid Steel Conduit can be installed indoors or outdoors, in dry or wet locations, exposed or concealed, in all kinds of atmospheric conditions, and in hazardous locations when in compliance with National Electrical Code (NEC) 2014 Article 344. Galvanized Rigid Metal Conduit to be used with DB equipment shall:

1) Be approved for use with directional boring equipment (NEC 1014 section 300.5(K))

2) Withstand the forces involved in the directional boring process and have a minimum yield strength of 30,000 psi

Because of the varied environments in which electrical equipment is installed, local amendments are often added. Always consult local codes prior to any installation. Each straight length of conduit is labeled and states, “Consult manufacturer for proper installation” or equivalent language, so if you have questions regarding a specific product, contact the manufacturer

Specifications

Galvanized Rigid Metal Conduit is manufactured in accordance with the latest edition of the following:

American National Standards Institute — American National Standard for Electrical Rigid Steel Conduit (ERSC), ANSI® C80.1

Underwriters Laboratories Standard for Electrical Rigid Metal Conduit — Steel (ERMC-S), UL 6.


2 Responses

  1. You mention a minimum yield strength of 30,000 psi for steel conduit. Is there a source or reference that confirms this value? I am trying to confirm what the minimum yield strength of rigid steel conduit would be manufactured to C80.1 industry standards.

    Any help would be appreciated.

    • We do not publish this information as product standards for electrical metallic tubing (EMT), galvanized rigid conduit (GRC) and intermediate metal conduit (IMC) do not require tensile and yield testing, such tests are not routinely performed., Manufacturers do occasionally perform these tests. The following information are typical” properties are not certified, as testing results will vary. These products are not designed and manufactured nor intended to be used for structural purposes.
      A typical test result for EMT would be approximately 45k to 50k psi yield strength and 40k to 50k psi ultimate tensile strength.
      GRC would be approximately 50k to 60k psi yield strength and 30k to 40k psi ultimate tensile strength.
      IMC would be approximately 60k to 65k psi yield strength and 50k to 60k psi ultimate tensile strength.

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