National Electrical Code Articles and Information

1. OK, we just about gave that away in the prelude to the questions. It's flexible metal conduit. It's made by helically (think spiral) winding metal strips in a way that causes them to interlock. These are special, pre-shaped strips formed specifically for this process.
    

2. Article 348 lists only one permitted use [348.10]. It can be used "in exposed and concealed locations." That's quite vague. Does this mean you can use it underwater? No. You can use it except as prohibited under "Uses Not Permitted." So you can generate a list of 100 permitted uses if you have the time to sit down and write it out.
   
   
   
   
   
   From an engineering standpoint, you want to avoid using FMC except where you need the flexibility. It's necessarily weaker than RMC or IMC and is more easily broached due to its continuous joints. Don't use it for the reason of trying to get around raceway support issues. It still must be supported, and it's going to be more difficult to install than RMC or IMC simply because it is flexible.
   
    

3. Article 348 lists seven prohibited uses [348.12]. The last one "where subject to physical damage" is the standard "prohibited use" for raceway; think of it as a disclaimer for violations of common sense. The others are also derived from common sense, and are based on the fact the joints of the flexible conduit are not the same as the walls; things can get in, rather easily.
    

4. If you use angle connectors, they can't be concealed [348.42]. As a general practice, use FMC as short-length means of connecting IMC or FMC to the final load. For example, you need to connect a motor but you don't want to vibrate your conduit off the wall. Use FMC between them.
   
   Your thought process in deciding where to use is essentially that of extending an existing RMC or IMC run, but with flexibility added to accomplish a specific goal. Such as connecting to a motor. Note that it's no accident that FMC comes in sequence after RMC and IMC.
    

5. Use only bushings, couplings, connectors, brackets, etc., listed for use with FMC. Article 348 doesn't explicitly state this, but the listing requirement is a general requirement provided elsewhere in the Code.
   
   You can't, for example, use connectors designed for use with plumbing systems. Nor can you run a length of PVC in an FMC system. You can connect a PVC system to an FMC system, but that would be an atypical setup. That setup would also require running a bonding jumper through the PVC to eliminate differences of potential along the FMC. Generally, don't mix metallic and nonmetallic raceway.