NEC (National Electrical Code) Article 344 covers materials, installation, and protection requirements for rigid metal conduit in electrical systems. Here is what you need to know, the detailed breakdown of what the article covers:

Definition and Scope (344.1)

• Article 344 applies to rigid metal conduit (RMC), which is a thick-walled threaded tubing used to protect electrical wires and cables.
• RMC can be made of steel, stainless steel, or aluminum.

Allowed Materials (344.2)

• It is specified that RMC must be made from steel, stainless steel, or aluminum and must be listed and labeled for its intended use.

Corrosion Protection (344.10)

• RMC must be adequately protected against corrosion depending on the environment in which it is installed. There is no specification for residential, industrial, or commercial applications, and it is used in all of them.
• In areas subject to severe corrosion, supplementary corrosion protection, such as coatings, may be required.

Use and Application (344.10)

• Permitted for use in exposed, concealed, and wet locations, including direct burial and in concrete.
• Cannot be used in hazardous locations unless specifically permitted by other sections of the NEC.

Bends (344.24)

• Covers the minimum radius of conduit bends to prevent damage to wires and ensure ease of pulling cables. The article does not specify an exact radius in this section but refers to Table 2, Chapter 9 of the NEC, which gives the minimum radius for different sizes of conduit.

For example, here are some common minimum bend radii:

• 1/2 inch RMC: Minimum radius of 4 inches.
• 3/4 inch RMC: Minimum radius of 4.5 inches.
• 1 inch RMC: Minimum radius of 6 inches.

• Limits the number of bends between pull points to no more than 360 degrees in a single run. The limit of no more than 360 degrees between pull points means that in a continuous run, the sum of all bends (e.g., two 90-degree bends or four 45-degree bends) cannot exceed 360 degrees before a junction box, pull box, or conduit body is installed.

Reaming and Threading (344.28)

• Requires the ends of RMC to be reamed to remove any sharp edges and prevent damage to the conductors.
• Conduit must be threaded according to specific standards to ensure proper connections between conduit and fittings.

The threading standards are defined by the ANSI/ASME B1.20.1 standard, which specifies the dimensions and tolerances for NPT (National Pipe Thread) used in electrical conduit threading. Key points:

• The threads must be tapered to ensure a tight fit when the conduit is joined with couplings or fittings.
• Each conduit must have sufficient threads (e.g., at least 5 full threads engaged) to ensure a strong mechanical bond and maintain electrical continuity for grounding purposes.

Supports and Securing (344.30)

• RMC must be securely fastened in place and supported at intervals no greater than 10 feet.
• Closer supports may be required near terminations or junction boxes.
• For vertical risers, appropriate supporting methods must be used. These are conduit straps (installed at intervals no greater than 10 feet for vertical runs, or 3 feet from a junction box or termination point), hangers, clamps, or rigid support structures for tall buildings.

Grounding and Bonding (344.60)

• RMC may serve as an equipment grounding conductor if the proper fittings are used.
• Must be bonded to the grounding system to ensure electrical continuity and safety in case of a fault.

Physical Protection (344.6)

• Must provide physical protection to conductors inside the conduit.
• RMC must be installed so that it is not subject to severe physical damage.

Common protective installation practices include:

• Burial in conduit trench: In industrial environments, RMC can be buried below ground (with sufficient depth as per NEC Table 300.5).
• Protected by guards: In areas where physical damage is likely (e.g., parking lots, industrial sites), RMC can be covered by metal or concrete guards.
• Elevated or installed inside walls: Running RMC in walls, ceilings, or elevated above ground minimizes the risk of accidental damage.
• Rigid supports and fastening: Securely fastening the conduit using clamps or brackets at regular intervals ensures the conduit is stable and not subject to sagging or movement, which could lead to mechanical damage over time.

Electrical Continuity (344.40)

• Electrical continuity of RMC must be ensured throughout the entire run.
• Fittings must be tight to maintain a continuous ground path.

Expansion Joints (344.42)

• Expansion joints may be required in certain environments, such as areas with significant temperature changes, to allow for thermal expansion and contraction of the conduit.

Special Occupancies (344.12)

• Specific restrictions on using RMC in classified locations (hazardous environments) such as explosive atmospheres, flammable gases, or vapors. It can be used in Class I, Division 1 & 2 locations (flammable gases or vapors), Class II, Division 1 & 2 locations (combustible dust), and Class III locations (ignitable fibers and flyings), where RMC is permitted because of its rigid, robust structure, resistance to corrosion, and ability to provide a complete electrical ground path (specified in NEC Articles 501-503).
• Requires approval for use in these conditions based on specific NEC rules for hazardous locations.

Fittings and Connectors (344.46)

• Specifies that only listed fittings should be used with RMC. These can be threaded couplings, locknuts, bushings, grounding bushings, or expansion fittings.
• Fittings must be designed to ensure the mechanical strength and integrity of the conduit system.
• Fittings should be appropriate for the environment and protected from corrosion when necessary.

Installation in Wet Locations (344.42)

• When used in wet or damp environments, the fittings and conduit must be rated for these locations, including proper sealing to prevent moisture ingress.

Size and Conductors (344.20)

• Specifies the available sizes of RMC and the maximum number of conductors that can be installed in different sizes, following NEC fill requirements.

The available sizes for RMC, as specified in the NEC, range from 1/2 inch to 6 inches. The maximum number of conductors that can be installed in each size of RMC is determined by NEC Annex C, Table C8. This table provides the maximum fill for different sizes of conduit and for various types of wire insulation.

Here's an example of the available sizes of RMC and corresponding maximum conductor fill (for THHN wire):

• 1/2 inch RMC: Maximum 9 conductors (14 AWG).
• 3/4 inch RMC: Maximum 16 conductors (14 AWG).
• 1 inch RMC: Maximum 26 conductors (14 AWG).
• 1-1/4 inch RMC: Maximum 35 conductors (14 AWG).
• 1-1/2 inch RMC: Maximum 49 conductors (14 AWG).
• 2 inch RMC: Maximum 84 conductors (14 AWG).

Marking (344.120)

• RMC must be marked with the type of metal, trade size, and manufacturer’s name or trademark at intervals not exceeding 3 meters (10 feet).

Direct Burial (344.10)

• RMC can be buried directly in the ground if the appropriate corrosion protection is provided. These are:
• 24 inches for areas subject to vehicular traffic
• 6 inches in protected areas
• 4 inches beneath 4 inches of concrete
• 0 inches beneath building slabs
• Requires that burial depths follow the NEC table for minimum cover requirements.

Joints (344.42)

• All RMC joints must be made by threaded connections or other approved means.

• Setscrew or compression-type fittings must be listed and approved for use.

Hazardous Locations (344.10)

• When used in hazardous locations, specific sections of the NEC must be referenced (e.g., NEC Article 501 for Class I locations).

Derating Factors

• If multiple RMCs are run together or conductors are packed into a conduit, derating may be required to avoid exceeding the allowable ampacity of the conductors.