EMT vs rigid conduit comparison
The comparison between Electrical Metallic Tubing (EMT) and Rigid Metal Conduit (RMC) is essential for selecting the appropriate raceway system in commercial and industrial electrical installations. EMT is thin-wall and non-threadable, while RMC is thick-wall and threaded, resulting in significant differences in weight, mechanical strength, joining method, and cost. Below is a detailed analysis of both systems according to NEC regulations and standardized dimensional characteristics.
General Comparison Table
Section titled “General Comparison Table”| Feature | EMT (Electrical Metallic Tubing) | RMC (Rigid Metal Conduit) |
|---|---|---|
| Wall thickness (size 1/2“) | 1,07 mm / 0.042 in (thin wall) | 2,72 mm / 0.107 in (thick wall) |
| Thread | None; uses compression or set-screw fittings | NPT threaded on both ends |
| Material and coating | Galvanized steel exterior, anticorrosive interior coating | Galvanized steel, stainless steel, or aluminum |
| Relative weight (1/2“ x 3 m) | Approximately 0,79 kg / 1.74 lb | Approximately 1,56 kg / 3.44 lb |
| Maximum support spacing (straight runs, ≤2“) | 3 m / 10 ft | 3 m / 10 ft (up to 2“); 4,9 m / 16 ft (2½“ – 3“) with threaded couplings |
| Typical joining method | Compression couplings, push-fit connectors | Threaded couplings, set-screw unions |
| Physical damage protection | Moderate; suitable for exposed indoor installations or embedded in concrete | High; withstands severe impacts, exposed installations in industrial areas |
| Relative cost (material) | Lower material cost | Higher material cost |
Dimensions and Tolerances
Section titled “Dimensions and Tolerances”EMT dimensions are standardized for each trade size. The outer diameters of RMC are the same as those of EMT for the same trade size, but the inner diameter is smaller due to the greater wall thickness. The typical dimensions of EMT are presented below.
| Trade size (in) | Metric size (mm) | Outer diameter | Inner diameter |
|---|---|---|---|
| 1/2 | 16 | 17,9 mm / 0.706 in | 15,8 mm / 0.622 in |
| 3/4 | 21 | 23,4 mm / 0.922 in | 20,9 mm / 0.824 in |
| 1 | 27 | 29,5 mm / 1.163 in | 26,6 mm / 1.050 in |
| 1 1/4 | 35 | 38,4 mm / 1.510 in | 35,1 mm / 1.380 in |
| 1 1/2 | 41 | 44,2 mm / 1.740 in | 40,9 mm / 1.610 in |
| 2 | 53 | 55,8 mm / 2.197 in | 52,5 mm / 2.070 in |
| 2 1/2 | 63 | 73,0 mm / 2.875 in | 69,3 mm / 2.730 in |
| 3 | 78 | 88,9 mm / 3.500 in | 85,2 mm / 3.360 in |
| 3 1/2 | 91 | 101,6 mm / 4.000 in | 97,4 mm / 3.830 in |
| 4 | 103 | 114,3 mm / 4.500 in | 110,0 mm / 4.330 in |
For 1/2“ RMC, with a wall thickness of 2.72 mm / 0.107 in, the inner diameter is reduced to approximately 12.5 mm / 0.492 in. EMT provides more internal space for conductors, approximately 30 % more usable area in the same trade size.
Material and Corrosion Protection
Section titled “Material and Corrosion Protection”EMT is made of galvanized steel on the exterior and has an anticorrosive interior coating to protect conductors during pulling. RMC can be galvanized steel (GRC), stainless steel, or aluminum, with integral corrosion protection. Both systems are suitable for indoor and outdoor environments, but RMC directly allows installation in classified hazardous locations with proper seals, while EMT is not approved for those locations unless supplemented with approved fittings.
Mechanical Strength
Section titled “Mechanical Strength”RMC offers considerably higher impact resistance than EMT. Due to its thick wall (2.72 mm / 0.107 in vs. 1.07 mm / 0.042 in for 1/2“), it withstands severe blows and mechanical stresses typical of heavy industrial environments. EMT, although strong, is more susceptible to dents and deformation, so it is recommended for areas with moderate traffic or in concealed installations. Both metal conduits can be used as an equipment grounding conductor when installed with proper couplings and connectors, but the maximum fault circuit length is limited by the path resistance, especially in long runs of thin-wall EMT.
Joining Method and Threading
Section titled “Joining Method and Threading”EMT is not threadable; joints are made using compression couplings, push-fit connectors, or set-screw clamps that tighten onto the smooth surface of the tube. These fittings must be tightened firmly to ensure an effective ground-fault current path, as required in NEC 250.4(A)(5) and 300.10. RMC, on the other hand, has standard NPT tapered threads and is joined with threaded couplings; this threaded joint provides a robust electrical and mechanical connection and allows the conduit to be used as an equipment grounding conductor without additional length limitations.
Installation and Support
Section titled “Installation and Support”Both types of conduit must be secured within a maximum of 0.9 m / 3 ft from any box, fitting, or termination. For straight runs, the maximum support spacings are:
| Trade size | EMT / RMC ≤2” support every | RMC >2” support every (threaded couplings) |
|---|---|---|
| 1/2“ – 2“ | 3 m / 10 ft | 3 m / 10 ft |
| 2 1/2“ – 3“ | 3 m / 10 ft | 4,9 m / 16 ft |
| 3 1/2“ – 4“ | 3 m / 10 ft | 6,1 m / 20 ft |
| 5“ – 6“ (RMC only) | 3 m / 10 ft (RMC) | 6,1 m / 20 ft |
Exposed vertical runs supplying fixed equipment may be supported up to 6.1 m / 20 ft provided the conduit consists of threaded couplings and is securely fastened at both ends.
Applications and Restrictions
Section titled “Applications and Restrictions”EMT is widely used in commercial buildings, light industrial facilities, and embedded in concrete, when the installation does not require threading and the expected physical damage is low. RMC is mandatory or preferred in areas exposed to severe impacts (vehicle traffic areas, chemical plants, offshore platforms), in classified hazardous locations (Class I, Division 1 and 2), and in installations where the mechanical integrity of the conduit is critical.
Both types comply with the conductor fill limits established in NEC Chapter 9, Table 1; the complete system must allow pulling and removal of conductors without damaging the insulation. The total of bends between pull points shall not exceed 360° in any of the conduits. EMT is not approved for direct burial or for use in underground ducts without an additional coating, while RMC, with proper coatings and thread seals, can be installed underground.
Cost and Weight Considerations
Section titled “Cost and Weight Considerations”EMT is more economical than RMC in both material and labor costs, due to its lower weight and ease of installation with push-fit fittings. A 3 m / 10 ft length of 1/2“ EMT weighs approximately 0.79 kg / 1.74 lb, while the same length in RMC weighs about 1.56 kg / 3.44 lb, almost double. For large-scale installations, this weight difference significantly reduces support and labor costs. However, in projects where extreme mechanical protection is a code or safety requirement, the higher cost of RMC is justified.
Regulatory Compliance
Section titled “Regulatory Compliance”The installation of EMT is regulated by NEC Article 358, and RMC by Article 344. Both require the use of nonmetallic or metallic bushings at terminations to protect conductors from abrasion, unless the box or fitting provides such protection. Couplings and connectors for EMT in wet locations must be listed for that use, while for RMC embedded in concrete or masonry, concrete-tight fittings are required. When dissimilar metals are installed, precautions against galvanic corrosion must be taken.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum support spacing for EMT in straight runs?
Section titled “What is the maximum support spacing for EMT in straight runs?”The maximum support spacing for EMT in straight runs is 3 m / 10 ft, regardless of trade size, per NEC Article 358.30.
How much does a 3 m length of 1/2“ RMC weigh compared to an EMT?
Section titled “How much does a 3 m length of 1/2“ RMC weigh compared to an EMT?”A 3 m / 10 ft length of 1/2“ RMC weighs approximately 1.56 kg / 3.44 lb, while the equivalent in EMT weighs 0.79 kg / 1.74 lb, meaning RMC weighs double.
What is the inner diameter of a 3/4“ EMT?
Section titled “What is the inner diameter of a 3/4“ EMT?”The inner diameter of a 3/4“ EMT is 20,9 mm / 0.824 in, providing approximately 30 % more usable area than RMC of the same trade size.
At what distance from a junction box must EMT or RMC conduit be secured?
Section titled “At what distance from a junction box must EMT or RMC conduit be secured?”Both conduits must be secured within a distance not exceeding 0.9 m / 3 ft from each box, fitting, or enclosure, per NEC 358.30 and 344.30.
Can EMT be bent manually without special tools?
Section titled “Can EMT be bent manually without special tools?”Yes, EMT can be bent with manual benders for sizes up to 1 1/4“, maintaining a minimum radius per NEC Chapter 9, Table 2; RMC requires hydraulic benders or pre-threading due to its rigidity.
Can EMT directly conduct fault current as an equipment grounding conductor?
Section titled “Can EMT directly conduct fault current as an equipment grounding conductor?”Yes, EMT can serve as an equipment grounding conductor if all joints are tightened properly and requirements of NEC 250.4(A)(5) and 300.10 are met, although its resistance is higher than that of threaded RMC.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/conduit-size-d_1738.html
- mikeholt.com: https://www.mikeholt.com/newsletters.php?action=display&letterID=2626
- electrical4u.com: https://www.electrical4u.com/classification-of-electrical-conducting-material/