Although many types of raceway are similar, each exists for particular reasons. Weigh the advantages and disadvantages of each before making a final selection.
In addition to the cable Articles, Chapter 3 of the NEC contains a sequence of Articles pertaining to raceways. The Art. 100 definition of raceway indicates that it is an enclosed channel designed expressly for holding wires, cables, or busbars. When viewing the types of raceways included in this definition, it can be broadly divided into circular raceways and non-circular raceways, such as wireways. Two specific types of circular raceway are conduit and tubing (see Sidebar: Conduit vs. Tubing below).
Intermediate Metal Conduit
Intermediate metal conduit (IMC) is a circular metal raceway with an outside diameter equal to that of rigid metal conduit (RMC). The wall thickness of IMC is less than that of RMC, so it has more interior room for conductors.
IMC is lighter and less expensive than rigid metal conduit, but you can use it in all of the same locations as RMC. Though IMC has thinner walls than RMC, IMC is actually the stronger of the two — because of the steel alloy used to make it.
The NEC has long required that raceways be secured within 36 in. of enclosures. Although it was generally accepted that this allowed raceways less than 36 in. to be unsupported, some inspectors required these short raceways to be supported, because there was no allowance for an unsupported raceway.
A revision to 342.30(C) in the 2008 Code required support for any length of raceway that’s coupled or terminates in a ringed knockout. It also decreased the 36 in. to 18 in., without substantiation. Subsequently, the 2011 NEC deleted this subsection. Now the language matches that used in the 2005 NEC.
IMC requires a bushing wherever the conduit enters a box, fitting, or other enclosure, unless the design of the box, fitting, or enclosure provides some sort of protection. The protection afforded has long been required to be “equivalent” to that of a bushing, but some Code users had a hard time understanding what “equivalent” meant.
Section 342.46 in the 2011 NEC clarifies how this protection should be provided: To protect conductors from abrasion, a metal or plastic bushing must be installed on conduit termination threads, regardless of conductor size, unless the box, fitting, or enclosure is designed to provide this protection [342.46] (click here to see Fig. 1).
Rigid Metal Conduit
RMC, commonly called “rigid,” has long been the standard raceway for providing protection from physical impact and from difficult environments. The changes we discussed for IMC in Art. 342 were also made for RMC in Art. 344.
Flexible Metal Conduit
Flexible metal conduit (FMC), commonly called “Greenfield” or “flex,” is a raceway of an interlocked spiral metal strip. Although the length of an FMC installation is not limited, it’s primarily used for the final 6 ft or less of raceways between a more rigid raceway system and equipment that moves, shakes, or vibrates. Examples of such equipment include pump motors and industrial machinery.
Over the last few Code cycles, Art. 348 has been revised to allow for greater lengths of unsupported FMC in installations where flexibility is a concern. While these changes have gone a long way toward uniform understanding and enforcement, not all NEC users agree on how to measure these raceways. With the revisions introduced in 2011, it becomes clear that measurement begins at the last point of support for these systems.
The NEC requires equipment to have an equipment grounding conductor (EGC). It’s been common to use the metal strip in FMC as the EGC. But what about vibration? It can be argued that an unusually high amount of vibration can compromise the integrity of FMC’s ability to act as an EGC, so now 348.60 requires an EGC of the wire type in those instances (click here to see Fig. 2). This change provides consistency with the requirements contained in 250.118(5).
Liquidtight Flexible Metal Conduit
Liquidtight flexible metal conduit (LFMC), with its associated connectors and fittings, is a flexible raceway. Where vibration or frequent relocation is an issue, LFMC is often the solution for connecting the equipment. LFMC is of similar construction to FMC, but also has an outer liquidtight thermoplastic covering to provide protection from moisture and some corrosives.
The changes we discussed for FMC in Art. 348 were also made for LFMC in Art. 350.
Polyvinyl Chloride Conduit
Rigid polyvinyl chloride conduit (PVC) is a rigid nonmetallic conduit that provides many of the advantages of RMC, while allowing installation in areas that are wet or corrosive. PVC conduit is an inexpensive, easily installed raceway. It’s lightweight, easily cut, and relatively strong.
On the downside, conduits manufactured from PVC are brittle when cold, and they sag when hot. PVC is commonly used as an underground raceway because of its low cost, ease of installation, and resistance to corrosion and decay.
The changes to support requirements we discussed for IMC in 342.30 were also made for PVC in 352.30. Another change was to take the exception from 352.12(E) — it addressed conductors rated with higher temperatures than the raceway — and put it in 352.10(I), as shown in Fig. 3 (click here to see Fig. 3). So rather than state it as an exception to uses not permitted, it’s restated as a permitted use. This change is consistent with ongoing efforts by the Code-making panels to convert exceptions into positive text.
Electrical Metallic Tubing
Electrical metallic tubing (EMT) is a lightweight raceway that’s relatively easy to bend, cut, and ream. Because it isn’t threaded, all connectors and couplings are of the threadless type and provide quick, easy, and inexpensive installation compared to threaded metallic conduit systems. Consequently, EMT is very popular. EMT is manufactured in galvanized steel or aluminum; the steel type is more commonly used.
While 358.10(B) permits you to install EMT in corrosive areas (if you use corrosion protection approved as suitable for the location), the time seeking such approval — and the cost of implementing it — would certainly add significantly to the project cost. Therefore, it’s usually much more effective to choose an alternative raceway, such as PVC.
The changes to support requirements we discussed for IMC in 342.30 were also made for EMT in 358.30.
Metal wireways are commonly used where access to the conductors within the raceway is required to make terminations, splices, or taps to several devices at a single location. High cost precludes their use for other than short distances, except in some commercial or industrial occupancies where the wiring is frequently revised. Both metal wireways and nonmetallic wireways are often called “troughs” or “gutters” in the field.
With the 2011 revision, the “Uses Permitted” section was changed to recognize wet location use [376.10]. You can use metal wireways:
- For exposed work.
- In any hazardous (classified) location, as permitted by other articles in the NEC.
- Wet locations, where the wireway is listed for the purpose.
- Unbroken through walls, partitions, and floors. The catch: Access to the conductors must be maintained on both sides of the wall.
This Code section was in desperate need of cleaning up. Previously, 376.10(3) included both hazardous (classified) locations and wet locations, causing confusion as to the relationship between these locations. This 2011 NEC change eliminates the wet location confusion and enhances the readability of the NEC.
If you’re planning to use metal wireways in hazardous locations, see 501.10(B), 502.10(B), and 504.20 for the requirements.
Preventing Raceway Overload
Chapter 3 addresses many other types of raceway in addition to the ones we discussed here. We started with Art. 342 and stopped with Art. 376. But we skipped quite a few Articles along the way.
With so many types of raceway, how can you keep all the requirements straight? Well, there’s a pattern that’s usually consistent from Article to Article. In fact, many of the requirements are the same, and the Articles are laid out in a similar manner.
For example, say you are trying to decide between IMC and EMT for a particular application. Turn to Sec. 10 of Art. 342 and Art. 358, and read the Uses Permitted. Then, read the Uses Not Permitted in Section 12. This will tell you if you can use either wiring method for a particular application. If both pass this test, then you can work through each Article and compare the requirements of the two raceways.
Sidebar: Conduit Vs. Tubing
Many people erroneously refer to electrical metallic tubing (EMT) as “conduit.” EMT is actually a tubing rather than a conduit. Conduit and tubing are two different wiring methods that differ in application and installation requirements, although neither is defined by the Code.
In the NEC, the phrase “conduit or tubing” appears multiple times, indicating a choice between two different things. To see an example of this, look at Table 4 of Chapter 9.
The term “raceway” can be used to refer to an enclosed wiring method, including both conduit and tubing. Just remember that there are other types of raceway as well, such as wireways.