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FAQ (Duct & Innerduct)

  • What is Innerduct?
    • Innerduct is smaller conduit (or tube) used to subdivide large ducts for the placement of optical fiber cables in the underground conduit system. 

      The typical underground conduit is 4” in diameter and was designed for large copper telecommunications cables.  With the advent of small diameter fiber optic cables innerduct was developed to better utilize underground conduit space.

  • Is innerduct different than duct or conduit?
    • Conduit and duct usually refer to a stand alone tube vs. innerduct which is used inside of another duct or conduit.  Larger diameter products are usually called duct or conduit.  Depending on the context they may be different or the same thing.

  • What is HDPE?
    • HDPE is the abbreviation for the preferred and most commonly used material to make fiber optic innerduct, High Density Polyethylene.  HDPE is flexible in all weather and never gets brittle making the ideal choice for duct and innerduct products.

  • Is HDPE the same as PVC innerduct?
    • No.  PVC is a very different plastic than HDPE.  PVC is usually used to produce rigid duct whereas HDPE is used to produce a flexible duct.  PVC also becomes brittle when cold.  PVC is naturally fire retardant.  PVC is also a term used to identify UL Listed Riser Raceway.  Endot makes Riser Raceway using Nylon instead of PVC.

  • What is the SDR (SODR, SIDR)?
    • SDR is the Size to Diameter Ratio (or Standard Dimension Ratio), that is, the ratio of the inside diameter (SIDR) or outside diameter (SODR) of the duct or innerduct to the wall thickness of the duct or innerduct.  The smaller the SDR number the thicker the wall of the duct.

      Smaller SDR numbers also mean that the duct or innerduct has higher tensile strength and greater crush resistance than a comparable product with a higher number.

  • What is Corrugated Innerduct (vs. smoothwall or ribbed smoothwall)?
    • Corrugated Innerduct is an HDPE tube with alternating large and small annular rings that allows the innerduct to have a thinner wall yet retain a significant crush strength.   Corrugated innerduct also has superior all weather flexibility.  Corrugations also eliminate memory making the innerduct easier to handle in tight conditions and eliminates spiraling of the innerduct when pulled into conduit.   Lack of memory is the primary reason corrugated duct has significantly less friction than any smoothwall product.

      Smoothwall innerduct depends on a continuous heavy wall for strength.  Ribbed innerduct is smoothwall innerduct with a series of ridges extruded into the inside wall.

      Link to Endocor Corrugated Product Info

  • Is innerduct different than duct or conduit?
    • Innerduct has become a generic term used to describe most any type of duct, conduit or premise wiring raceway. Conduit and duct usually refer to a stand alone tube vs. innerduct which refers to a duct installed inside of another duct to subdivide that duct into multiple pathways for cables.

      Larger diameter products are usually called duct. Depending on the context they may be different or the same thing. Conduit is most often used to describe pathways for electrical cables vs. innerduct and duct referring to pathways for fiber optic and telecommunications cables.

  • What are ASTM’s?
    • ASTM’s are the standards produced by the American Society for Testing and Materials.  ASTM is a not for profit organization that writes standards for materials, products, systems and services.  Standards are developed through the consensus process by over 33,000 technically qualified ASTM members worldwide.

      For smoothwall duct and innerduct the ASTM standard is F2160. 
      There is no ASTM standard for corrugated innerduct. 
      For plenum and riser products there is no ASTM Standard, but there is a UL Listing based on testing to UL Standard 2024.


  • ASTM F2160 vs. D3035, D2239 or D2447?
    • ASTM F2160 is an ASTM specifically written for conduit products. ASTM D3035, ASTM D2447, ASTM F714 and ASTM D2239 are water pipe specifications often referred to in conduit specifications. Dimensionally F2160 and the water pipe ASTM's are the same, but the material specification for water pipe is much more stringent because it is for pressure pipe and not for conduit. Using water pipe ASTM's for conduit can raise the price significantly because pressure rated resin is more expensive than conduit non-pressure rated resins.


  • Why are there so many different types of innerduct?
    • There are many opinions as to which type and size is better and we make products to suit as many people as we can.  Larger cable needs larger innerducts.  In some environments such as trenching a large duct is preferable to a small one because it is very hard to keep small ducts straight in the trench.

  • Which is the better product, corrugated or smoothwall?
    • This depends on your application and environment that the innerduct will be installed in.  Corrugated provides significantly lower pulling friction vs. smoothwall or ribbed smoothwall.  Smoothwall type products have much higher tensile strength, but also weigh as much as two times corrugated making them harder to handle because of stiffness and reel memory and much more expensive than corrugated.

      For direct burial and for cable blowing applications smoothwall duct is the best choice.  Corrugated duct is not suited for direct burial or for cable blowing.

      Give us a call to discuss you needs in more detail.

  • What advantage does ribbed innerduct have?
    • Supposedly the longitudinal ribs reduce surface area and therefore reduce friction.   Ribbed innerduct is still basically a smoothwall product with the major cause of friction being the reel memory of the plastic (or helical set) which causes spiraling of the duct when pulled through a conduit system.  Reducing the surface area does little to reduce pulling friction.  Corrugated innerduct has no memory and therefore does not spiral resulting in much lower friction than smoothwall products.  When blowing cables into conduit the smoother the interior of the conduit the better it is for blowing the cable through it.

  • How many cables can be placed in an innerduct?
    • In the underground cable plant the general rule is one cable to one innerduct.  This allows future access to each cable individually if the need should arise to remove or replace it.  If multiple cables are placed in a innerduct all cables will have to be removed to remove one due to spiraling of the cables when placed.

      In the premise plant when plenum and riser products are being installed the general rule is a fill of no more than 40% of the ID.

  • What size innerduct do I need?
    • The general rule for Outside Plant duct depends on the method of cable placement.   If the cable is being pulled into place then the Inside Diameter of the innerduct should be twice the Outside Diameter of the cable.   If the cable is being blown into the conduit the cable should be just slightly smaller than the primary conduit..

      See “A Primer on the Installation of Corrugated Innerduct”.

  • How many innerducts can fit in one conduit.
    • The rule of thumb is a maximum fill factor of 67% of innerduct mass to conduit size.  For example three 1-1/4” innerducts will fit comfortably in a 4” conduit.   See “A Primer on the Installation of Corrugated Innerduct”.

  • Can a pull line be installed in the innerduct at the factory?
    • Yes.  Endot can install a wide range of pull lines, from inexpensive polypropylene rope (PP Rope) to expensive Kevlar pull tapes that are pre-lubricated and measured and marked.  Depending on your project we can suggest the pull line that is best for your application.  See “Types of Pull Line.”

  • How does the pull line get installed in the innerduct?
    • The innerduct is extruded over the pull line.  The extruder has an opening in the back side of the extrusion die that forms the HDPE tube.  The pull line is inserted into this opening at the same rate as the HDPE tube is extruded providing a continuous pull line inside the innerduct. 

  • Can cable be installed in the duct or innerduct at the factory?
    • Yes.  This is called Cable In Conduit (CIC).  Just about any type of cable can be installed in duct or innerduct.  It is installed just like the pull line.  Endot does not make CIC products at this time.

  • What colors are available?
    • Almost any color you might want. The National Public Works Association recommends Orange for Telecommunication conduit in the underground. Endot produces most duct and innerduct in black and orange, but can make many other colors. Black is recommended for any application where the duct is exposed to sunlight. Colors, even those with UL Stabilizers (other than black) cannot tolerate sunlight for extended period of time without deterioration. Endot strongly recommends that Yellow NOT be used in the underground because of the chance of confusion with gas mains that have the same color.


  • What is the longest length that can be put on a reel?
    • That depends on the Outside Diameter of the innerduct.  The larger the innerduct the less material will fit on a reel.  Endot stocks reels as large as 114” to allow the maximum amount of material to be shipped to our customers on a single reel.  Endot also carries small reels designed to hold as little as 500 feet of material.  See “Reel Sizing/Capacity Chart”.

      Link to Reel Capacity charts Chart 1 & Chart 2

  • Can the innerduct be supplied in coils without a reel?
    • Yes.  This type of packaging is usually for quantities of less than 1000 feet per coil.   Coils are generally taped at multiple points to keep them together and palletized for shipping.

  • Can Innerduct be ordered with more than one length or color per reel?
    • Yes.  Endot provides special packaging called paralleling.  Innerduct that is parallel packaged can have two, three or four different lengths and/or colors per reel.   Endot also has available segmented reels that keep the colors totally separated on the reel.

  • What is Plenum Innerduct or Plenum Raceway?
    • Plenum Innerduct or Raceway is a specialized innerduct produced from PVDF, Polyvinylidene Fluoride, a Flouropolymer.  ENDOCOR Plenum Raceway is a UL Listed Corrugated Innerduct that is suitable for use in buildings where electrical codes will not allow the use of HDPE innerduct.  ENDOCOR Plenum Raceway is available in 1/2”, 3/4”, 1”, 1-1/4”, 1-1/2” and 2” ID sizes.

      Link to Plenum,  Riser and Zero Halogen  product info.

  • What is Riser Raceway or Riser Innerduct?
    • Riser Raceway is a conduit tested according to UL Standard 2024, Section 9. Riser Raceway is designed for use in vertical riser shafts of buildings for the installation of fiber optic cables within the raceway. ENDOCOR Riser is UL Listed and available in sizes of 1/2", 3/4", 1", 1 1/4", 1 1/2" and 2" ID. All sizes are equipped with a UL Listed pull line and available in a wide variety of put ups. Riser raceway does not eliminate the need for riser cables.

      Link to Plenum,  Riser or Zero Halogen products.

  • If I want to install an innerduct or raceway in other than a Plenum or Riser area of a building what do I use?
    • Inside a building only UL Listed products should be used. Areas not rated as plenum or riser spaces are classified as “general purpose”.  In these areas the NEC suggests that a flame retardant material be used.  Endot recommends that a UL Listed raceway such as ENDOCOR Riser be used.  Riser is flame retardant and lower cost than plenum raceway.

      Selecting the Correct Indoor Raceway product

  • What color does Plenum or Riser Raceway (innerduct) have to be?
    • There is no color code standard for Plenum or Riser Raceway in the NEC or UL standards. Endot makes their Plenum and Riser raceways in both orange & white.  Special colors can be produced on request.

      Special Color Information

  • Why can I not order Plenum or Riser with inexpensive PP rope installed?
    • Underwriters Laboratories requires that any product being installed in a Plenum or Riser area of a building meet all testing requirements, this includes any type of pull line that is shipped with the innerduct or raceway. UL is concerned that the PP rope which burns easily and smokes a great deal may not be removed and thereby increase the risk of a dangerous fire/smoke condition if a fire should develop.

  • What if I want to install a fiber optic cable in a building area that is not considered a Plenum or a Riser?
    • In areas other than Plenum or Riser, generally known as General Purpose areas Riser is the product of choice. The NEC does not provide for a general purpose raceway, only for Plenum or Riser.

      Selecting the Correct Indoor Raceway

  • Where is Endot Innerduct Made?
    • ALL Endot products are 100% USA made, both raw materials and the final products.

      Endot has three production facilities, New Jersey, Oklahoma and Tennessee.  Water and gas pipe is made at all three plants  Innerduct products are made in New Jersey and Oklahoma.

      Made in USA Letter

  • Is Endot Innerduct Made in America?
    • Not only is Endot innerduct Made in The USA, all of the materials used to produce all Endot products are Made in The USA also.

      Made in USA Letter