National Electrical Code Articles and Information
Code Quiz: Article 360
Based on the 2014 NEC
Questions and answers written by Mark Lamendola, who has worked as a master electrician, electrical inspector, and design engineer. Mark is an IEEE Senior Member, and the Code article author for Codebookcity.com. Since 1996, he has been writing National Electrical Code articles for electrical trade magazines and has an extensive portfolio of hundreds of NEC articles..
The Chapter Three Articles on wiring methods run in multiple series. Article 350 was the last in the sets of series about metallic conduits. Article 352 starts a series on nonmetallic conduits and after this series, a series of tubing starts with Article 358.
Article 360 provides the requirements for Flexible Metallic Tubing (FMT), which is ubuiquitous in factories and other industrial settings. A major reason for using it is vibration isolation; the tubing absorbs the shakes and shivers produced by pumps and motors. FMT is also liquidtight, another reason why it's used for that final drop to a motor. But keep in mind that it's used with tubing systems, and those are not liquidtight. So in many types of applications, you will not find flexible tubing being used beacuse the raceway is conduit.
Taking the place functionally of FMT in such applications is flexible conduit, because those applications require the sealing ability of conduit and you can't seal electrical tubing. Tubing has speed and cost advantages for installation, so where it's applicable it tends to be used instead of conduit.
Tubing is light and, because it assembles with pressure-type connectors, relatively easy to install. You can bend flexible tubing by hand to get the desired shape, but you must take care with bend the radius. Too sharp of a bend will not only damage the flexible tubing, it will probably cause problems with connector reliability and wire pulling. You want the smoothest, most gradual curve possible rather than tight bends. In some cases, this can mean using a longer run of tubing that crosses back over itself. You won't need a threader, because FMT isn't threaded.
Notice the distinction just made several times between tubing and conduit. FMT is not conduit. Many people mistakenly call it conduit, but that mistake can prove costly when trying to apply the NEC.
A final note before the quiz (below). The NEC does permit using FMT as an equipment grounding conductor [360.60]. However, using it as "the" equipment grounding conductor is not advised for those who want the most reliable system. With tubing systems, always run a separate equipment grounding conductor inside the tubing and bond it to the tubing. The tubing, being of far greater diameter than the wire, will attenuate high frequency energy far better than the wire. But the wire gives you the ultimate in reliability. So use both, and bond them together.
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