3/4″ is the trade size, not the actual dimension of electrical conduit. 21 is the metric designator not the actual dimension in millimeters. The actual dimensions for various types of 3/4″ electrical conduit are below. The information below was derived from several sources and should not be considered precise. Dimensions may very slightly between manufacturers.

3/4″ (21) **Electrical Metallic Tubing** (**EMT** or Thinwall) is a smooth light weight galvanized metal conduit that has an inside diameter of .824″ or 20.93mm. It has an outside diameter of 0.922″ or 23.4mm. The wall thickness of 3/4″ EMT is .049″ or 1.25mm. Standard length is 10′ or 3.048 meters. Tables C.1 and C.1(A) in NFPA 70 tell you how many conductors are permitted in EMT conduit. Article 358 tells you the installation requirements. 3/4″ EMT is manufactured in several colors to distinguish different electrical systems such as fire alarm circuits from power circuits.

3/4″ (21) **Rigid Metal Conduit** (RMC) is a heavy weight conduit with a galvanized finish throughout. It has factory threads with a 3/4″ per foot taper. 100′ of 3/4″ RMC weighs approximately 109 pounds or 49.4 kilograms. It has an inside diameter of 0.824″ or 20.93mm. It has an outside diameter of 1.050″ or 26.7mm. The wall thickness is .107″ or 2.70mm. Standard length is 10′ or 3.048 meters. Tables C.9 and C.9(A) in NFPA 70 tell you how many conductors are permitted in RMC conduit. See Article 344 for code installation requirements.

3/4″ (21) **Intermediate Metal Conduit** (IMC) is a lighter weight version of RMC. It has a galvanized finish inside and out and can be used in many of the same environments as RMC. It has factory threads with a 3/4″ per foot taper. It also utilizes the same couplings and **fittings** as RMC. 3/4″ IMC has an inside diameter of 0.864″ or 21.9mm. It has an outside diameter of 1.029″ or 26.1mm. The wall thickness is .083″ or 2.1mm. Standard length is 10′ or 3.048 meters. Tables C.4 and C.4(A) in NFPA 70 tell you how many conductors are permitted in IMC conduit. Article 342 explains the requirements for installation.

3/4″ (21) **Flexible Metal Conduit** (FMC or Greenfield) is a galvanized finished flexible metal electrical conduit composed of a continuously formed interlocking metal strip. 3/4″ FMC has a minimum inside diameter of 0.812″ or 20.6248mm and a maximum inside diameter of 0.835″ or 21.209mm. It has a minimum outside diameter of 1.045″ or 26.543mm and a maximum outside diameter of 1.105″ or 28.067mm. FMC is available in steel and aluminum. Dimensions may vary slightly between the two types. The minimum bending radius is 5″ or 101.6mm. Standard coil length is 100 feet or 30.48 meters. Also available on reels. Tables C.3 and C.3(A) in NFPA 70 tell you how many conductors are permitted in FMC conduit. Article 348 explains the installation guidelines.

3/4″ (21) **Rigid Polyvinyl Chloride Conduit** (PVC) schedule 40 is a nonmetallic, non-conductive electrical conduit that is good to use indoors and outdoors, or underground. It has some flexibility with an inside diameter of 0.824″ or 20.9296mm. It can also be heated and bent. It has an outside diameter of 1.050″ or 26.67mm. The wall thickness is .113″ or 2.8702mm. This is available in 10′ (3.048 meters) and 20′ (6.096 meters) lengths. Tables C.11 and C.11(A) in NFPA 70 tell you how many conductors are permitted in PVC schedule 40 conduit. Read about its expansion characteristics and how to compensate for them in Article 352.

3/4″ (21) **Rigid Polyvinyl Chloride Conduit** (PVC) schedule 80 is a nonmetallic, non-conductive electrical conduit that is good to use indoors and outdoors, or underground. It has limited flexibility, but can be heated and bent to any angle. Schedule 80 has an inside diameter of 0.742″ or 18.8468mm. It has an outside diameter of 1.050″ or 26.67mm. The wall thickness is .154″ or 3.9116mm. This is available in 10′ (3.048 meters) and 20′ (6.096 meters) lengths. Tables C.10 and C.10(A) in NFPA 70 tell you how many conductors are permitted in PVC schedule 80 conduit. Its expansion characteristics and how to compensate for them are in Article 352.

3/4″ (21) **Electrical Nonmetallic Tubing** (**ENT** or Smurf) is a nonmetallic, non-conductive flexible conduit with an inside diameter of .76″ or 19.304mm. 3/4″ ENT has an outside diameter of 1.05″ or 26.67mm. The minimum bend radius is 6″. Available in 10′ (3.048 meters) lengths, or coils and reels. There are couplings, fittings, and electrical boxes made specifically for ENT. Tables C.2 and C.2(A) in NFPA 70 tell you how many conductors are permitted in ENT conduit. Article 362 describes how it can be used. ENT is available in colors, yellow for communication circuits, red for fire alarms, and blue for power. It is important to use the correct transition adapters when terminating ENT into an electrical box.

3/4″ (21) **Liguidtight Flexible Metal Conduit** (LFMC or Sealtight) is a flexible metal conduit with a non-metallic, non-conductive outer jacket that makes it suitable for use in wet locations. It has an inside minimum diameter of .820″ or 20.828mm and a maximum inside diameter of .840″ or 21.336mm. It has a minimum outside diameter of 1.030″ or 26.162mm and a maximum outside diameter of 1.050″ or 26.67mm. The minimum bending radius is 5″. It is available in 100′ coils or on reels. Many electrical supply companies will cut it to length. Tables C.8 and C.8(A) in NFPA 70 tell you how many conductors are permitted in LFMC conduit. Read about it’s approved uses in Article 350.

3/4″ (21) **Liguidtight Flexible Nonmetallic Conduit** (LFNC) is a nonmetallic conduit that is suitable for wet conditions. It has an inside minimum diameter of .820″ or 20.828mm and a maximum inside diameter of .840″ or 21.336mm. It has a minimum outside diameter of 1.030″ or 26.162mm and a maximum outside diameter of 1.050″ or 26.67mm. The minimum bending radius is 5″. It is available in 100′ coils or on reels. Many electrical supply companies will cut it to length. Tables C.5, C.6, C.7 and C.5(A) C.6(A), and C.7(A) in NFPA 70 tell you how many conductors are permitted in LFNC conduit. See article 356.2(1), (2), and (3) to understand the different types of LFNC.

All 3/4″ (21) conduit and fittings require a 1 1/8″ opening into an electrical box. The hole can be made with a hole saw or a knockout punch.

A tap for 3/4″ (21) conduit or conduit fittings requires a **59/64″** hole. The threads per inch is 14 National Pipe Thread (NPT).

Other size electrical conduits: **1/2″ (16)**,

1″ (27), 1 1/4″ (35), 1 1/2″ (41),

2″ (53), 2 1/2″ (63), 3″ (78),

3 1/2″ (91), 4″ (103), 5″ (129),

6″ (155).

For more conduit code details and conduit fill see Chapter 9, Tables 1, 4, 5 and 5A.

See the tables in Informative Annex C in the back of the “National Electrical Code” for the allowable number of wires in each size conduit.