RBX-2 — Metal Raceways

Exam checkpoints

Compare RMC, IMC, and EMT

Rigid metal conduit (RMC) under Article 344 is the heaviest common metal raceway. Ferrous RMC, stainless-steel RMC, aluminum RMC, and red-brass RMC have material-specific characteristics. Properly protected RMC can be exposed, concealed, in concrete, in wet locations, and directly buried where the product and corrosion rules permit. Threaded connections provide strong mechanical protection, but field-cut threads, cut ends, and dissimilar metals require attention.

Intermediate metal conduit (IMC) under Article 342 is a lighter listed steel raceway with many of RMC's permitted uses. It can be exposed or concealed and used in wet, concrete, and underground conditions when installed with suitable fittings and corrosion protection. IMC is not simply thin RMC; use fittings and threading practices identified for IMC.

Electrical metallic tubing (EMT) under Article 358 is thin-wall tubing, commonly steel or aluminum, joined with listed compression or set-screw fittings rather than field-cut conduit threads. Section 358.28 prohibits threading EMT. EMT can be exposed or concealed and can be used in concrete, wet locations, and corrosive areas under the article's protection and fitting conditions. It is prohibited where subject to severe physical damage. A normal exposed run is not automatically severe, but a loading dock impact zone may require RMC, IMC, a guard, or rerouting.

The phrase “metal conduit” does not answer the selection problem. Compare impact exposure, corrosion, wet-location fittings, hazardous-location rules, support, conductor pulling, equipment grounding, and local approval. Aluminum raceway in earth or concrete can face alkali and galvanic corrosion, and ferrous raceway in corrosive soil may need supplementary protection. Section 300.6 and the raceway article work together.

Make and support the run

EMT is normally securely fastened within 3 ft of each box, cabinet, conduit body, or other termination and supported at intervals not exceeding 10 ft. RMC and IMC use the same ordinary 3-ft termination fastening and permit a general 10-ft support method. For qualifying straight runs made with threaded couplings, Table 344.30(B) permits size-based support spacing: 10 ft for 1/2- and 3/4-in., 12 ft for 1-in., 14 ft for 1 1/4- and 1 1/2-in., 16 ft for 2- and 2 1/2-in., and 20 ft for 3-in. and larger conduit. An unbroken length can use the stated fastening extension to 5 ft where structural members do not readily permit fastening within 3 ft, and concealed fished-work exceptions have conditions. Horizontal runs through framing openings can count as support when spacing and end-fastening requirements are met. Read 344.30, 342.30, or 358.30 rather than assuming an exception.

Bends cannot damage the raceway or effectively reduce its internal diameter. Use Chapter 9, Table 2 bend radii and the raceway article. The total bend between pull points cannot exceed 360 degrees. Ream cut ends to remove burrs. Where conductors 4 AWG or larger enter an enclosure, provide the insulating protection required by 300.4(G), such as a listed bushing with a smooth insulating surface. A locknut alone does not protect insulation from a sharp, unreamed end.

RMC and IMC threaded joints are made up wrench tight, and running threads are not used at couplings. Threadless fittings, where used, must be listed for the raceway and environment. EMT couplings and connectors are made up tight; in wet locations they comply with the wet-location enclosure and fitting requirements, and where embedded in masonry or concrete they are identified for that condition.

Preserve grounding and bonding continuity

RMC, IMC, and EMT are recognized equipment grounding conductor types in 250.118 when properly installed. This means a separate wire EGC is not universally required merely because the raceway is metal. It does not mean any loose collection of tubing is an effective path. Couplings and connectors must remain electrically continuous, enclosures must be bonded, and corrosion or mechanical separation cannot be allowed to open the path.

At service equipment, 250.92(B) requires bonding that assures electrical continuity and fault-current capacity around service raceways and enclosures. Standard locknuts do not by themselves satisfy the service bonding rule where connection through concentric or eccentric knockouts impairs continuity. Threaded hubs, bonding-type locknuts or bushings, bonding jumpers, and other listed methods are applied as the section permits. On circuits over 250 V to ground, 250.97 triggers an additional bonding check for metal raceways and enclosures.

A wire EGC can improve reliability and may be required by another article, specification, or flexible-section rule, but it does not excuse loose metal fittings. Bond metal raceway segments and boxes even when a green conductor passes through them. At expansion fittings, use a fitting listed to maintain electrical continuity or install the required bonding jumper.

Use flexible metal raceways within their limits

Flexible metal conduit (FMC) under Article 348 is useful for vibration and alignment in generally dry locations. It is prohibited in wet locations, where subject to physical damage, and in other listed conditions. Liquidtight flexible metal conduit (LFMC) under Article 350 adds a liquidtight nonmetallic covering and can be used in wet locations with listed liquidtight fittings. LFMC can be directly buried only where identified for that use and is still prohibited where subject to physical damage.

FMC and LFMC ordinarily use support intervals not exceeding 4.5 ft and securement within 12 in. of boxes or fittings, but short equipment-connection, fishing, and flexibility exceptions are detailed in their articles. A six-foot flexible whip is not automatically exempt in every application. Avoid tight bends that damage or constrict the raceway, and prevent liquids from being trapped where deterioration could result.

Under 250.118, certain short runs of FMC or LFMC can serve as the equipment grounding path only when all trade-size, length, fitting, overcurrent-device, and use conditions are satisfied. Where flexibility is required after installation or the conditions are not met, run a wire-type EGC with the circuit conductors and bond it at both ends. For motors, HVAC equipment, and vibrating machinery, check the equipment article in addition to Articles 348 or 350.

A final metal-raceway inspection is a fault-path inspection: verify the correct raceway type, undamaged coating, compatible fittings, tight joints, legal supports and bends, wet/corrosion suitability, bonding at services and movement fittings, enclosure bonds, and a continuous low-impedance return path to the source.