Three and a Half Inch (91mm) Electrical Conduit Dimensions

A three and a half inch conduit hand knockout punch with draw stud and accessories
A three and a half inch conduit hand knockout punch with draw stud and accessories

Three and a Half Inch 3.5″ is the trade size, not the actual dimension of electrical conduit.  Ninety 0ne 91 is the metric designator not the actual dimension in millimeters.  The dimensions for various types of 3.5″ electrical conduits are below.  The information was derived from several sources and should not be considered precise.  Dimensions may vary slightly between manufacturers.  NOTE: Text links below go to applicable products on Amazon.com

Three and One Half Inch Conduit Dimensions

3.5″ (91) Electrical Metallic Tubing (EMT or Thinwall) is a smooth lightweight galvanized metal conduit that has an inside diameter of 3.834″ or 97.4mm.  The outside diameter is 4.000″ or 101.6mm.  The wall thickness of 3.5″ EMT is .083″ or 2.11mm.  100 feet of 3.5″ EMT weighs approximately 349 pounds or 158.3 kilograms.  It requires a 4 1/8″ hole for the conduit fittings.  A tap needs a 3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

3.5″ (91) 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, 8 threads per inch NPT.  100′ of 3.5″ RMC weighs approximately 880 pounds or 399.1 kilograms.  It has an inside diameter of 3.548″ or 90.12mm and It has an outside diameter of 4.000″ or 101.6mm.  The wall thickness is .215″ or 5.50mm.  3.5″ Rigid requires a 4 1/8″ hole for the conduit and fittings.  A tap needs a 3 3/4″ hole.

3.5″ (91) 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, and 8 threads per inch NPT.  It also utilizes the same couplings and fittings as RMC.  100 feet of 3.5″ IMC weighs approximately 629 pounds or 285.3 kilograms.  3.5″ IMC has an inside diameter of 3.671″ or 93.2mm.  It has an outside diameter of 3.971″ or 100.7mm.  The wall thickness is .15″ or 3.81mm.

3.5″ (91) Flexible Metal Conduit (FMC or Greenfield) is a galvanized finished flexible metal electrical conduit composed of a continuously formed interlocking metal strip.  100 feet of 3.5″ Reduced Wall Steel Greenfield weighs approximately 220 pounds (99.79024 kilograms).  Reduced Wall 3.5″ Aluminum FMC weighs 68 pounds or 30.844256 kilograms.  3.5″ FMC has a minimum inside diameter of 3.500″ or 88.9mm.  It has a minimum outside diameter of 3.860″ or 98.044mm and a maximum outside diameter of 4.060″ or 103.124mm.  The minimum bending radius is 21 inches or 533.4mm.  3.5″ Flex requires a 4 1/8″ hole for the conduit fittings.  A tap needs a 3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

3.5″ (91) Rigid Polyvinyl Chloride Conduit Schedule 40 (PVC) 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 3.548″ or 90.12mm.  It has an outside diameter of 4.00″ or 101.60mm.  The wall thickness is .226″ or 5.7404mm.  3.5″ Schedule 40 weighs 196 pounds per 100 feet (88.90 kilograms).  3.5″ PVC requires a 4 1/8″ hole for the conduit fittings.  A tap needs a 3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

3.5″ (21) Rigid Polyvinyl Chloride Conduit Schedule 80 (PVC) is a nonmetallic, non-conductive electrical conduit that is good to use indoors and outdoors, or underground.  It has limited flexibility.  Schedule 80 has an inside diameter of 3.364″ or 85.4456mm.  It has an outside diameter of 4.00″ or 101.60mm.  The wall thickness is .318″ or 8.0772mm.  3.5″ Schedule 80 weighs 256 pounds per 100 feet or 116.12 kilograms.  3.5″ PVC requires a 4 1/8″ hole for the conduit fittings.  A tap needs a  3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

3.5″ (91) 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.  100 feet of 3.5″ Sealtight weighs 294 pounds or 133.356048 kilograms.  It has an inside minimum diameter of 3.500″ or 88.9mm and a maximum inside diameter of 3.540″ or 89.916mm.  It has a minimum outside diameter of 3.960″ or 100.584mm and a maximum outside diameter of 4.000″ or 101.60mm.  The minimum bending radius is 21 inches (533.4mm).  Fittings requires a 4 1/8″ hole.  A tap needs a 3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

3.5″ (91) Reinforced Thermosetting Resin Conduit (RTRC or Fiberglass) is a nonmetallic conduit that is suitable for many conditions.  It has an inside diameter of 3.860″ or 98.044mm.  Outside diameter is 4.000″ or 101.60mm.  The conduit wall thickness is .070″ or mm.  100 feet of three and a half inch SW RTRC weighs 65 pounds and 29.48348 kilograms.  It requires a 4 1/8″ hole for the conduit fittings.  A tap needs a 3 3/4″ hole.  Threads per inch for the fittings is 8 NPT.

All three and One Half Inch 3.5″ (91) conduit and fittings require a four and one eighth inch 4 1/8″ opening into an electrical box.  The hole can be made with a hole saw or a knockout punch.

A tap for three and one half inch 3.5″ (91) conduit or conduit fittings requires a 3 3/4″ hole.  The threads per inch is 8 National Pipe Thread (NPT).

The standard length for most conduits is ten feet 10′ or 3.048 meters.  Twenty foot 20′ (6.096 meters) lengths of metal conduit are available, but not always in stock.  PVC conduit is available in ten foot 10′ and in some sizes, twenty foot 20′ lengths.  RTRC mostly comes in twenty foot 20′ lengths.  The flexible conduits come in various length coils and reels depending on the trade size.  Some electrical supply companies will cut flexible conduit to length.

See my post describing various electrical conduit types.

Other Size Electrical Conduits

1/2″ (16)       3/4″ (21)        1″(27)

1 1/4″ (35)       1 1/2″ (41)        2″ (53)

2 1/2″ (63)       3″ (78)        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 (NFPA 70) for the allowable number of wires in each size conduit.