Section 344 of the NEC (National Electrical Code) covers materials, installation, and protection requirements for solid metal conduit in electrical systems. Here’s what you need to know, and a detailed breakdown of what the article covers:
Definition and scope (344.1)
Section 344 applies to rigid metal conduit (RMC), which is 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 the RMC must be made of 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 are no specifications for residential, industrial, or commercial applications, and it is used in all of them.
In areas subject to severe corrosion, additional corrosion protection, such as coatings, may be required.
Use and Application (344.10)
It is permitted for use in exposed, concealed and damp locations, including direct burial and in concrete.
It cannot be used in hazardous locations unless specifically permitted by other sections of the NEC.
Bends (344.24)
Covers minimum radius of conduit bends to prevent wire damage and ensure easy cable pulling. The article does not specify an exact radius in this section but indicates it Table 2, Chapter 9 NEC, which gives the minimum radius for different sizes of channel.
For example, here are some common minimum bend radii:
1/2 inch RMC: Minimum radius 4 inches.
3/4 inch RMC: Minimum radius 4.5 inches.
1 inch RMC: Minimum radius 6 inches.
The number of bends between towing points is limited to no more than 360 degrees in one round. The limit of no more than 360 degrees between pull points means that in continuous operation, the sum of all bends (for example, two 90 degree bends or four 45 degree bends) cannot exceed 360 degrees before the junction box, the pull box, or The channel body is installed.
Expansion and Threading (344.28)
Requires reaming of RMC terminals to remove any sharp edges and prevent damage to the connectors.
The conduit must be threaded to specific standards to ensure proper connections between the conduit and the fittings.
Threading parameters are determined by ANSI/ASME B1.20.1 The standard, which specifies the dimensions and tolerances for NPT (National Pipe Thread) used in electrical conduit threads. Key points:
Threads should be tapered to ensure a snug fit when attaching conduit to connections or fittings.
Each conduit must have sufficient pins (e.g., at least 5 full pins) to ensure a strong mechanical bond and maintain electrical continuity for grounding purposes.
Support and insurance (344.30)
The RMC must be securely fixed in place and supported at intervals of no more than 10 feet.
Closer support may be required near terminations or junction boxes.
For vertical cranes, appropriate support methods must be used. These are conduit belts (installed at intervals of no more than 10 feet for vertical runs, or 3 feet from a junction box or termination point), hangers, clips, or rigid support structures for high-rise buildings.
Grounding and bonding (344.60)
The RMC may serve as an equipment grounding conductor if proper fittings are used.
They must be connected to an earthing system to ensure electrical continuity and safety in the event of a malfunction.
Physical protection (344.6)
Physical protection must be provided for conductors within the conduit.
The RMC must be installed so that it is not exposed to serious physical damage.
Common preventive installation practices include:
Burial in the canal ditch: In industrial environments, RMC can be buried underground (with sufficient depth according to NEC Table 300.5).
Protected by guards: In areas where physical damage is likely (e.g., parking lots and industrial sites), RMC can be covered with metal or concrete shields.
Elevated or installed inside walls: Running RMC in walls, ceilings, or above-ground heights reduces the risk of accidental damage.
Solid supports and mounting: Securely fastening the conduit using clamps or brackets at regular intervals ensures the stability of the conduit and is not subject to sagging or movement, which may lead to mechanical damage over time.
Electrical continuity (344.40)
Electrical continuity of the RMC must be ensured throughout the entire period of operation.
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 large temperature variations, to allow thermal expansion and contraction of the duct.
Special occupancies (344.12)
Specific restrictions on the use of RMC in classified locations (hazardous environments) such as potentially explosive atmospheres, flammable gases, or vapors. It can be used in First class, division 1 and 2 sites (flammable gases or vapors), Second class, division 1 and 2 sites (combustible dust), and Tertiary sites (combustible fibers and aircraft), where RMC is permitted due to its rigid and strong structure, its resistance to corrosion, and its ability to provide a complete electrical ground path (defined in NEC Articles 501-503).
Requires approval for use in these conditions based on NEC rules specific for hazardous locations.
Fittings and connectors (344.46)
Specifies that only the listed fittings should be used with the RMC. These can be threaded couplings, lock nuts, bushings, ground bushings, or expansion fittings.
Fittings must be designed to ensure the mechanical strength and integrity of the duct system.
Fittings must be suitable for the environment and protected from corrosion where necessary.
Installation in wet locations (344.42)
When used in wet or humid environments, fittings and ducts must be rated for these locations, including appropriate sealing to prevent moisture ingress.
Size and connectors (344.20)
Specifies the available sizes of RMC and the maximum number of connectors that can be installed in different sizes, according to NEC packaging requirements.
Available sizes of RMC, as defined by the NEC, range from 1/2 inch to 6 inches. the Maximum number of connectors That can be installed in every RMC size specified by NEC Appendix C, Table C8. This table provides the maximum fill for different sizes of conduits and for different types of wire insulation.
Below is an example of the available sizes of RMC and the 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).
Labeling (344.120)
RMC must be marked with metal type, commercial size, and manufacturer or brand name at distances not exceeding 3 meters (10 feet).
Direct burial (344.10)
RMC can be buried directly in the ground if proper erosion protection is provided. These are:
24 inches For areas subject to vehicular traffic
6 inches In protected areas
4 inch Under 4 inches of concrete
0 inch Under building panels
Requires that burial depths follow the NEC table for minimum coverage requirements.
Joints (344.42)
Dangerous sites (344.10)
When used in hazardous locations, specific NEC sections should be consulted (e.g., NEC Section 501 for Class I locations).
Deterioration factors
If multiple RMCs are operated together or conductors are packed in a conduit, power reduction may be needed to avoid exceeding the permissible ampacity of the conductors.
At NNC, you can find… Solid metal channel At excellent prices.
