Note: This article is based on the 2023 NEC.

Nonmetallic raceway does have some advantages over metal raceways, primarily in the area of corrosion resistance. There are also disadvantages, for example you can’t use them as Equipment Grounding Conductors (EGC)s.

Six types
The six types of metal raceway are:

1. Intermediate Metal Conduit (IMC) [342]. IMC is a circular metal raceway that can be threaded. It has the same outside diameter as rigid metal conduit (RMC) but is made of a stronger metal which allows a thinner wall, making it lighter and providing a larger interior cross-sectional area for holding conductors. It’s typically threaded, but threadless connectors can also be used (same for RMC).
2. Rigid Metal Conduit (RMC) [344]. RMC is also a circular metal raceway that can be threaded. It costs less per foot than IMC, but is heavier and isn’t as strong.
3. Flexible Metal Conduit (FMC) [348]. FMC is a circular raceway made of a helically wound, formed, interlocked metal strip.
4. Liquidtight Flexible Metal Conduit [350]. LFMC is essentially FMC with an outer liquidtight, nonmetallic, sunlight-resistant jacket.
5. Electrical metallic tubing (EMT) [358]. EMT is a thin-wall alternative to conduit. Like conduit, it is of circular cross section. It is used with light compression fittings that cost less than their counterparts for IMC and RMC. EMT isn’t threaded.
6. Flexible metallic tubing (FMT) [360]. FMT is a thin-walled version of FMC. Like EMT, it is used with compression fittings and is not threaded.
   

Common requirements
All metal raceways have these requirements:

• Each type of metal raceway and its associated fittings must be listed. None of them are interchangeable among types [e.g., 342.6].
• When the raceway is cut in the field, reaming (nonflexible) or trimming (flexible) is required to remove the rough edges [e.g., 342.28].
• Raceway must be securely fastened and supported. The exact dimensions vary by raceway type. For example, IMC within 3 ft of each box but FMC within 12 inches of each. But IMC every three feet of run and FMC every 4.5 feet of run. If you look at the relative wall strength and material weight, you can see why this is.
• The number of conductors cannot exceed the percentage fill specified in Chapter 9, Table 1.
• Avoid using with dissimilar metals.
• Make bends in a manner that does not damage the raceway or significantly change the internal diameter. For the nonflexible raceways, this means using the correct shoe type in your bender (it will be marked for EMT or whatever) and using the shoe or shoe groove that matches the raceway size. For the flexible raceways, it means bending to the desired angle rather than bending past it and coming back. All bends must follow a smooth radius.
• The total of bends (including offsets) between pull points cannot exceed 360 degrees.
• All fittings must be made up tight [e.g., 344.42]. But this does not mean crank down as hard as you can. For example, you tighten the coupling for EMT only enough to make a slight dimple in the tubing. EMT fittings have small screws for that reason. Going beyond this could distort the tubing wall, adding excess friction for a conductor pull.
  

Differences
We already noted that the support distances vary by raceway type. What are some other differences? We can start with “Uses Permitted”, which is section xxx.10 of each article. This is a major factor in deciding which raceway to use.

IMC and RMC can be used in all atmospheric conditions and occupancies [342.10(A), 344.10(A)]. They can also be used where subjected to physical damage, while no other metal raceway can be used under this condition. FMC, LFMC, EMT, and FMT have this condition under “Uses not permitted” in section xxx.12 of each article.

IMC and RMC can be used in concrete or direct burial (or in areas subject to severe corrosive influences) where provided with corrosion protection approved for the condition. They can be used in wet locations if you follow the requirements of 342.10(D) or 344.10(D). No other metal raceway can be used in wet conditions.

Consider the “Uses Not Permitted” for FMC. It cannot be installed:
(1) In wet locations.
(2) In hoistways, other than as permitted in 620.21(A)(1).
(3) In storage battery rooms.
(4) In any hazardous location, except as permitted by 501.10(B).
(5) Exposed to material having a deteriorating effect on the installed conductors.
(6) Underground or embedded in poured concrete.
(7) Where subject to physical damage.

Compare this to IMC and RMC, which do not even have a “Uses Not Permitted” section.

For EMT, the “Uses Not Permitted” are:

• Where subject to physical damage.
• For the support of luminaires or other equipment except as conduit bodies no larger than the largest trade size of the tubing [358.12].
  

IMC, RMC, and LFMC can be used in Hazardous Locations. FMC, EMT, and FMT cannot be used in Hazardous Locations.

IMC and RMC can be threaded or they can use threadless connectors. If threaded, a bushing must be used to protect the conductors where they exit. No bushing is required with the other metal raceways, but those others are not allowed to be threaded.

If the end of a metal raceway is threaded, you must cut off the threaded part if you wish to use a threadless connector or the contact area will be greatly reduced. Not only will the connector have less grip, this mix will reduce the efficacy of the raceway as an EGC.

Threadless connectors for IMC and RMC have some limitations. For example, if buried in concrete, these connectors must be of the concrete-tight type.

Size limits vary. FMC and LFMC cannot be smaller than ½” or larger than 4 inches [348.20]. IMC, RMC, and EMT cannot be smaller than ½” or larger than 6 inches. And FMT is available only in 3/8, ½, and ¾.

Flexible versus non-flexible
If flexibility is necessary to minimize the transmission of vibration from equipment, or to provide flexibility for equipment that requires movement after installation, choose a flexible raceway for the last few feet. Running a non-flexible raceway to a big pump, for example, guarantees something will break. Maybe a raceway connector will loosen (and start a corrosion process if threaded) or maybe the equipment will be damaged.

When using a flexible raceway, an EGC of the wire-type must be installed with the circuit conductors [348.60 and 350.60] or the length must not exceed 6 ft [250.118.5.d, 6.d, 7.b].

If an EGC is installed outside the flexible metal raceway, it can be no longer than 6ft and must be routed with the flexible metal raceway per 250.102(E)(2). So to go beyond 6ft, install the wire-type EGC inside the metal raceway.

Raceway choices
Each type of raceway has its pros and cons. The correct choice of raceway depends upon the conditions of the application. If the raceway would be exposed to damage, it seems you should choose RMC or IMC. But what if you choose Rigid Polyvinyl Conduit (PVC) [352] and run the raceway underground? Or what if you set bollards in front of the raceway because you identified there’s a danger of trucks backing into it? Perhaps you could run the raceway along the top of the wall, out of the way of trucks? Maybe you can’t do any of these protective measures so you opt for IMC. You still want to mitigate the danger of damage as much as you can. While IMC is tough, it isn’t invincible.

The primary advantage of a metal raceway over nonmetallic is you can use it as an EGC. Before choosing the type of metal raceway, consider its “Uses Permitted” [e.g., 348.10] and “Uses Not Permitted” [e.g., 348.12]. Conforming to the installation requirements for that particular raceway is a matter of observing the EGC rules and providing support at the required distances.