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In Chapter 3 of the NEC, the articles addressing specific wiring methods list “Uses permitted” (e.g., 338.10) and “Uses Not Permitted” (e.g. 338.12). Most of these Articles say you can’t use the wiring method where exposed to damage.
You won’t see this restriction for a few of the wiring methods, such as Rigid Metal Conduit (RMC) (Article 344) and Intermediate Metal Conduit (IMC) (Article 342). These aren’t the only wiring methods the NEC permits for “exposed to damage” applications, so consider the others also.
RMC has a thicker wall than IMC. It’s heavier and costs more per stick. You need heavier hardware, and (usually) must drill larger holes than if using IMC. A crew can run 4-inch IMC in less time, with less fatigue, and at lower cost than running RMC. So why would you use RMC?
Because RMC has a thicker wall than IMC. The decision on which wiring method to run does not rest on barely meeting Code at the lowest compliance cost. It rests on protecting the conductors, and then looking at costs.
RMC provides the most protection. You might not need that much protection. Or it might not be enough protection.
No matter how thick the conduit wall is, a direct hit with the counterweight of a big lift truck will crush it. Simply running the heaviest conduit isn’t enough.
Other measures include:
Source: Mark Lamendola | Mindconnection