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Conduit System Design

Ralph Fehr, P.E.


Course Outline

This 4-hour course discusses the principles of designing a conduit raceway system. Design techniques to facilitate cable installation will be learned. All design methods discussed will be in accordance with the National Electrical Code (NEC).

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Course Introduction

Power, control, and instrumentation cables often are installed in conduit raceways to protect the cables from physical damage. Conduit is metallic or non-metallic tubing specifically designed to protect electrical cables. Metallic conduit can be magnetic (galvanized steel) or non-magnetic (aluminum or stainless steel). Since magnetic conduit allows induced eddy currents to flow, the ability to dissipate heat is different for magnetic conduit than for non-magnetic conduit. Also, the inductive reactance of the circuit in the conduit is influenced by the magnetic properties of the conduit. Non-metallic conduit, made of such materials as fiber-based composites or polyvinyl chloride, is often encased in concrete to form a duct bank.

The NEC states which kinds of cables can be installed in the same raceway, and which cables must be separated. In general, low- and medium-voltage cables must be run in their own conduits. Control and instrumentation cables also require their own conduits.

Since metallic raceways can carry ground currents, grounding and bonding of the raceways is critical. Proper grounding assures electrical continuity and reduces the risk of electric shock to those touching the conduit.

Typically, the primary goal of raceway design is to minimize the length of the raceway. While this sounds simple, it seldom is in practice. Inevitably, there will be obstructions such as structural steel, piping, and pieces of machinery between the beginning point and the ending point of the proposed raceway. When obstructions are encountered, the raceway is usually routed around the obstacle. This not only increases the length of the raceway, but also introduces additional turns in the raceway. Bends are usually designed at either 45° or 90° to keep a neat appearance. Bends in conduit systems can greatly complicate the installation of the cables by increasing the tension required to pull the cables through the conduit. It is good design practice to minimize the number of bends per pull. The NEC allows up to four 90° bends, or equivalent, per pull. The limitation of four 90° bends does not assure that design parameters of the cable such as maximum pulling tension or sidewall pressure will not be exceeded. These quantities must be calculated to assure that the maximum values are not exceeded.

Course Content

The course content is in a PDF file (250 KB) Conduit System Design. You need to open or download this document to study this course.

Course Summary

The conduit, or raceway, protects the circuits from mechanical damage, and influences the reactance and ampacity of the cables it contains.

When designing conduit and duct systems, sizing of the conduits and pullboxes are governed by the NEC. Pulling tension and sidewall pressure must be calculated to assure that the cables will not be damaged during installation. Pulling tension is often the controlling factor for control and instrumentation cable, while sidewall pressure typically controls for power cables.

Related Links

For additional technical information related to this subject, please visit the following websites or web pages:

http://www.polywater.com/2000tech.html

http://www.southwire.com/tech/library/papers/pullten.htm

http://www.ecmweb.com/ar/electric_simple_calculations_cable/

 

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDHonline.com or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.