Back to Basics: Commercial Building Wiring Methods

Electrical engineers and designers must have a solid understanding of the different wiring and raceway methods used in commercial construction to ensure the proper application of raceways, wiring, cabling, and busways.

Learning Objectives:

• Understand the different types of raceways and conductor methods.

• Learn the differences between manufactured cabling assemblies.

• Gain insight into the application of busways and bus ducts.
  

Electrical engineers and designers have various methods to supply power within a building, from wiring, cabling, to the use of busways. The choice of a particular method depends on multiple factors such as jurisdictional requirements, the nature of the building (new construction vs. remodel), project cost, the building type, and future requirements.

When selecting wiring methods, cabling, or busway systems, engineers must balance functionality, flexibility, and installation costs. These decisions should be guided by the specifications set forth in the National Electrical Code (NEC), 2017 Edition, Chapter 3.

Raceway and Conductor Methods

The NEC Chapter 3 outlines various wiring methods that are particularly relevant to commercial construction with voltage applications of 600V or less. The NEC 300 provides general requirements for wiring methods and materials, addressing considerations such as physical protection, securing, and supporting. Similarly, NEC 310 applies to conductor specifications, including insulation, ampacity, and allowable uses.

Common types of raceways in commercial buildings include:

• Intermediate Metal Conduit (IMC): A thin-wall version of Rigid Metal Conduit (RMC), typically specified for cost-effectiveness.

• Rigid Metal Conduit (RMC): A thick-walled conduit designed for maximum protection against physical damage.

• Flexible Metal Conduit (FMC): Used for final connections to equipment prone to vibration.

• Rigid Polyvinyl Chloride Conduit (PVC): A nonmetallic option often used for underground applications.

• Electrical Metallic Tubing (EMT): The most common raceway in commercial construction, suitable for less exposed installations.

Advantages and Disadvantages of Raceway and Conductor Methods

Raceways offer several advantages:

• Flexibility in circuit configuration changes, depending on the type and size of the raceway.

• Enhanced protection in areas subject to severe physical damage when using IMC or RMC.

• Greater ease of use during renovations or for multi-tenant spaces.
  

However, there are challenges:

• Higher installed costs due to labor for initial setup.

• Limited number of bends before junction boxes or pull boxes are required.

• Some conduits, like PVC, cannot be used in certain high-damage areas and have limitations regarding temperature and exposure to sunlight.

Manufactured Cable Assemblies

Manufactured cable assemblies consist of conductors enclosed in either a metallic or nonmetallic sheath. These are factory-assembled, making them a convenient and efficient option for many commercial projects. Common types include Armored Cable (AC), Metal Clad Cable (MC), and Nonmetallic Sheathed Cable (NM).

Advantages of these assemblies include:

• Faster installation due to fewer labor requirements.

• Lower costs due to the elimination of the need for separate conduit installation.

• Greater flexibility in cable routing, especially for AC and MC types.

However, there are limitations:

• They require more frequent securing and support.

• Once installed, changes in circuit configurations are not as easy to make.

• Some types, such as NM, may not be approved for certain building types.

Busway Methods

Busways, or bus ducts, are another method of wiring commonly used for larger commercial applications. They provide more compact solutions for high-amperage applications.

Advantages:

• Compact size compared to multiple conduits with the same ampacity.

• Plug-in busway options offer flexibility for adding or changing load take-offs.

Disadvantages:

• Higher installed costs, particularly when many elbows and offsets are needed.

• Susceptibility to damage from water leaks in some installations.

• Limited physical damage tolerance.

  
  

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Credit to Original Article

This content is based on the article “Back to Basics: Commercial Building Wiring Methods,” originally published on Consulting-Specifying Engineer.

Written by:

Clinton R. Gordon is a project executive with Bear Label Consulting Engineers, specializing in electrical systems design and project management for large commercial projects.Matthew Steinmetz is the CEO and principal mechanical/electrical engineer with Bear Label Consulting Engineers, with over 16 years of experience in multidiscipline engineering projects.