Learn About Electrical Conduits, Boxes, and Grounding

Electrical wire conduit

 

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An electrical conduit is a metal or plastic pipe through which electrical wires are run. Available in either rigid or flexible forms, a conduit protects the wires and is used in exposed locations (such as along the exterior surface of a wall) as well as in unfinished areas (like basements, crawlspaces, and attics), and in surface-mounted installations outdoors.

There are several instances where you may need to have a conduit installed or replaced on your property. The first (and perhaps most obvious) reason would be to fulfill code requirements. In all 50 states, National Electrical Code (NEC) is the benchmark for safe electrical design and installation. Through its documentation, you can determine recommendations and building code specific to your project and area—it's also a good idea to check with a local inspector in your area, and get any proper permits before beginning work. Another reason for conduit work would be replacing existing materials that have become worn, damaged, or out-of-date.

If your home, apartment, or building has improper or damaged conduit, you risk issues with your electrical power or worse. Your wiring can short out, cause an electrical fire, or pose a danger to anyone servicing your system.

Like with almost all electrical work, laying your own conduit (or fixing an existing system) is something best left to a professional electrician. Any time you are dealing with live wires you want to be especially cautious, so this is not the task to take on if you're just learning your way around the parts of an electrical wiring system.

Types of Conduit

Conduit used for residential wiring includes several types of metal and plastic materials designed for different applications. Here are the most common types:

  • Electrical Metallic Conduit (EMT): Electrical metallic conduit is a rigid, thin-walled metal conduit typically made of galvanized steel. EMT is technically tubing, not conduit, hence the abbreviation uses the letter "T" instead of "C." Because the tubing is thin and lightweight, it is easy to bend with a special tool called a conduit bender. EMT is best used indoors for residential and light commercial construction—if it's installed outdoors, it needs to be assembled along with watertight fittings. Outdoors, EMT will typically last a few years—indoors, it'll last indefinitely.
  • Flexible Metal Conduit (FMT) and Liquid-tight Flexible Metal Conduit (LFMC): Flexible metal conduit has a spiral construction which allows the conduit to bend easily, simplifying installation in walls or other structures. FMT tubing is commonly used for short runs in exposed locations, such as wiring for garbage disposers and water heaters. Its close cousin, liquid-tight flexible metal conduit, is a flexible metal conduit covered with plastic sheathing to make it watertight. It is used for outdoor wiring serving air conditioning units and other outdoor equipment.
  • Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC): Rigid metal conduit and intermediate metal conduit are heavy-duty conduits made of galvanized steel. They are used for the structural piping that houses the wiring for the home's connection to the utility service lines for many overhead services. IMT has largely replaced RMC in new construction, and both RMC and IMC are joined with threaded connections. This tubing is often called Smurf tubing for its blue color, though it also comes in gray and orange, depending on the manufacturer and/or use.
  • Electrical Non-Metallic Tubing (ENT): Electrical nonmetallic tubing is flexible plastic tubing designed for use inside residential walls or concrete block structures. Although the tubing is moisture-resistant and flame-retardant, it's not suitable to exposed locations and should not be installed outdoors or anywhere that may be exposed to the elements.
  • PVC: Rigid PVC is a plastic pipe similar in size to rigid metal conduit (RMC). It can be heated and bent and is joined with glued or threaded connections. PVC is typically available in Schedule 40 and Schedule 80 options: Schedule 40 is used most often, but Schedule 80 is used when there's risk of damage to the pipes because it has thicker walls.

Conduit Is Not the Same as BX

At first glance, an electrical conduit may seem similar to BX wiring (sometimes also referred to as AC wiring for "armored cable" or MC for "metal cable"). Like conduit, loose but insulated individual wires are encased in BX's metal shell. Also like a conduit, BX wiring can be used in exposed locations. When installed correctly, both types can last upwards of 50 years or more.

The difference is that, according to the electrical code (NEC 320.12), an armored cable may not be used in damp or wet locations, in places exposed to corrosive conditions, or in places where damage might occur. By contrast, certain types of conduits may be installed in such locations.

Wiring Used With Conduit

A conduit is a hollow tube that wires are pulled through during installation. This differs from cable, which is a group of wires encased inside a flexible protective sheathing. The most common type of cable used in home wiring is non-metallic (NM), or Romex, cable. While NM cable can be run inside a conduit, this is seldom done.

The types of wire most commonly installed inside of conduit are THHN and THWN. THHN/THWN wires are individual, insulated, and color-coded wires. They are similar to the wires you see when you strip the outer sheathing off of NM cable.

Grounding a Conduit With a Raceway

Conduit always terminates at some type of electrical box or at fixed equipment that also serves as a box. Special connectors are used to secure conduits to boxes and to join lengths of conduits together to form long straight runs or bends. The boxes, connectors, and conduits form a wire enclosure system called a raceway. A raceway is an enclosed conduit that forms a physical pathway for the electrical wiring to follow. In this way, the metal raceway acts in place of the grounding wire that is found in non-metallic cable. (This is also the main reason why only metal boxes may be used with metal conduit.)

This type of grounding system was more common in older construction than it is today, and many electricians today include an insulated ground wire in a metal conduit as preferred means of grounding the circuit.

For a metal raceway to serve as a grounding system, all parts of the raceway must be electrically connected, with no interruptions. If a remodeler unwittingly installs a plastic box in the raceway, or if any connection comes loose, the ground path will be broken, leaving the circuit ungrounded.