2.5 Innerduct and Listed Optical Fiber Raceway (ENT Rules)

Innerduct and Listed Optical Fiber Raceway (ENT Rules)

Innerduct is flexible or semi-rigid plastic raceway placed inside larger conduit, laid in cable tray, or run on its own to give optical fiber cable a protected and identifiable path. The INSTF blueprint calls this task installing inner duct for fiber (ENT) because the NEC requires listed raceway used for optical fiber inside buildings to be installed under the rules written for electrical nonmetallic tubing (ENT).

Types of Innerduct

What the NEC Requires (2023 NEC)

• The listing must match the space. Inside buildings, use listed plenum, riser, or general-purpose communications raceway selected from NEC Table 800.154(b), the same hierarchy used for cable: plenum raceway in plenum spaces, riser raceway in risers, and general-purpose raceway elsewhere (NEC 770.110(A)(2)).
• ENT installation rules apply. Listed communications raceway is installed according to NEC 362.24 through 362.56:
  • bends no tighter than the radius in Chapter 9, Table 2 (362.24);
  • no more than 360 degrees of total bends between pull points (362.26);
  • cut ends trimmed of rough edges (362.28);
  • secured within 3 ft (900 mm) of each box or termination and at intervals of no more than 3 ft (362.30), except where it is fished through concealed spaces.
• Innerduct inside conduit. Listed communications raceway may be installed as innerduct inside any Chapter 3 raceway (NEC 770.111).
• Fill. Chapter 3 and Chapter 9 fill rules apply to optical fiber cable only when it shares a raceway with electric light or power conductors (770.110(B)); otherwise follow TIA-569 and the manufacturer.
• Outside plant innerduct stops at the building. Older NEC editions said so explicitly: unlisted underground or outside plant plastic innerduct entering from outside is terminated at the point of entrance. Current editions reach the same result by requiring listed raceway indoors.

Installing Innerduct

1. Plan the count and size. In a typical 4-inch conduit, installers place three 1-inch or 1.25-inch innerducts, or a multi-cell fabric innerduct, so future cables can be added without disturbing existing ones. Check each innerduct's inside diameter against the cable's outside diameter and the expected pulling tension.
2. Pull innerduct with a swivel. Innerduct is pulled into conduit like cable: attach it to the pull line through a swivel, lubricate if the manufacturer recommends it, and keep several ducts from twisting around each other.
3. Leave a pull line in every innerduct. Many innerducts come with factory-installed pull tape; if not, blow or fish one in before the ducts are secured.
4. Respect the radius. The innerduct's bend radius must also protect the cable inside it. Use sweeps rather than tight hand bends, and never kink corrugated duct over a tray edge.
5. Secure and support. Follow the 3-ft securing rule for listed raceway, use straps that do not crush the corrugations, and support innerduct in tray just as you would cable.
6. Plug, seal, and label. Plug unused innerducts and seal around cables in occupied ones at building entrances to block water and gas. Label each innerduct at both ends and at pull points, and use color or numbering to tell parallel ducts apart.
7. Firestop rated barriers. Plastic innerduct burns away in a fire, so penetrations need a listed system for plastic penetrants, typically an intumescent wrap and steel collar (Section 3.2).

Pulling Fiber into Innerduct

• Use flat pull tape rather than thin string; string can saw through plastic duct at bends (friction burn-through).
• Use lubricant compatible with both the cable jacket and the innerduct.
• Stay within the cable's rated tension and minimum bend radius. Long innerduct runs with many bends behave like conduit, and the 360-degree limit per pull section helps keep tension down.
• After pulling, check that the cable is not twisted with the innerduct at the ends and that slack is left at each termination point.