Dear Mr. Electrician: What are three-and-a-half inch 91mm electrical conduit dimensions?
Answer: Three and a Half inches (3.5″) is the trade size, not the actual dimension of the electrical conduit. Ninety-one (91) is the metric designator, not the exact dimension in millimeters.
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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.
Use a 4 1/8″ (104mm) hole saw for three-and-a-half-inch conduit fittings.
Table of Contents:
- EMT
- Rigid Metal Conduit
- Intermediate Metal Conduit
- Flexible Metal Conduit
- Sealtight
- PVC 40
- Fiberglass
- Other Conduit Sizes
- Links
For electrical conduit hole saw sizes go to my chart here.
METAL CONDUIT
3.5″ Electrical Metallic Tubing (EMT or Thinwall) is a smooth, lightweight galvanized metal conduit with 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. The conduit fittings require a 4 1/8″ hole. A tap needs a 3 3/4″ hole. Threads per inch for the fittings is 8 NPT.
3.5″ Rigid Metal Conduit (RMC) is a heavy-weight conduit with a galvanized finish. It has factory threads with a 3/4″ per foot taper, eight 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 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″ 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″ taper per foot and eight 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.
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3.5″ 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.
Three and a Half Inch 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″ Liquidtight 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 require a 4 1/8″ hole. A tap needs a 3 3/4″ hole. Threads per inch for the fittings is 8 NPT.
NON-METALLIC CONDUIT
3.5″ 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.
Three-and-a-half-inch PVC 40 has an outside diameter of 4.00″ or 101.60mm and an inside diameter of 3.548″ or 90.12mm. 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″ Reinforced Thermosetting Resin Conduit (RTRC or Fiberglass) is a nonmetallic conduit that is suitable for many conditions.
Three-and-a-half-inch RTRC Fiberglass 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-a-half-inch (3.5″) (91mm) 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″) (91mm) 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 some sizes, twenty-foot (20′) lengths. RTRC mostly comes in twenty-foot (20′) lengths. The flexible conduits come in coils and reels of various lengths depending on the trade size. Some electrical supply companies will cut flexible conduits to length.
See my article describing various electrical conduit types.
OTHER ELECTRICAL CONDUIT DIMENSIONS
1 1/4″ (35) 1 1/2″ (41) 2″ (53)
For more three-and-a-half-inch electrical conduit dimensions, conduit code details, and conduit fill, see Chapter 9, Tables 1, 4, 5, and 5A.
See the tables in Informative Annex C at the back of the National Electrical Code (NFPA 70) for the allowable number of wires in each size conduit.
CONDUIT MANUFACTURERS
Aluminum and Steel Conduit Products – https://www.westerntube.com/
Fiberglass Conduit Products – https://championfiberglass.com/
PVC Conduit products – https://www.cantexinc.com/
Steel Conduit Products – https://www.alliedeg.us/
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