When to Use Flexible Metal Conduits
Electrical conduit protects and routes electrical wiring. It can be made of many materials, including metal, plastic, fiber, or fired clay. There are flexible and rigid conduits available. The type of conduit used is determined through wiring regulations by national and local codes.
When to use flexible metal conduits versus a rigid conduit can be determined by analyzing your wiring situation. A flexible metal conduit can typically be found in sizes of ½ to ¾ inches, although it ranges in size from 3/8 to 4 inches in diameter. It is primarily used in places where a rigid conduit would be difficult to install as the flexible conduit will bend.
Although flexible conduits are easier to work with than rigid conduits, it offers less protection for the conductors it carries. Bending a rigid conduit can be an arduous task so it is often preferable to opt for the flexible variety. Being a metal conduit, it has an advantage over PVC conduits which cannot bend. It may also serve as the grounding path although some local codes may require that you also run a green grounding wire.
The Basics of Flexible Metal Conduit
Flexible metal conduits are made from steel or aluminum by coiling a self-interlocking ribbed strip of the metal. This forms a tube. The diameter of this tube can vary and different end applications call for different diameter tubes. Wires are pulled through this tube and are thereby protected from damage, both internal and external.
The tubing is flexible and can bend to fit your installation application. Unlike rigid conduits where you either have to manually bend the conduit to a fixed position or buy a pre-bent conduit, the flexible metal conduit can change its shape as you install it and as repairs or changes need to be made. The word flexible in its name may refer to its ability to flex in a given situation, but it also can refer to the flexibility it offers in your installation.
Short segments of FMC are called whips. You often find these as circuit pigtails between junction boxes and fixtures. A suspended ceiling is an excellent application for a flexible metal conduit whip. Whip assemblies are available for sale and can save you time when you need several of these pigtails.
NEC has set forth regulations and restrictions on the length of a flexible metal conduit in a lighting fixture application. Their guidelines limit the length to 6 feet to limit the ground return path. There are exceptions to this rule and you need to view the actual code to see if you have a situation where a longer conduit is allowed. Check local codes for more NEC regulations on conductivity and grounding.
Location of Installation
Flexible metal conduits are primarily used in dry applications although FMC is available with a UV resistant polymer that makes it watertight. Appropriate liquid tight fittings are required when using this type of conduit in a wet application.
Flexible metal conduits can be installed in most of the same places that rigid conduits are installed.
All conduits offer, to some degree, passive fire protection. The conduit can be both fire stopping by impeding the spread of the fire as well as fireproofing. The conduit can shield your wires from an accidental fire. A flexible metal conduit will do a better job at protecting from fire than a PVC conduit will.