3.9 Optical Fiber Innerduct & Electrical Nonmetallic Tubing (ENT)

Optical Fiber Innerduct & Electrical Nonmetallic Tubing (ENT)

The Installer 1 Exam Content Outline lists "install inner duct for fiber (ENT)" as a discrete task. It sits in the Establish Pathways and Space area, and it is the one pathway task written specifically around optical fiber.

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1. What Innerduct Is and Why Fiber Gets Its Own Pathway

Innerduct (also called subduct) is a smaller flexible duct installed inside a larger conduit, or run as a standalone raceway in a cable tray or ceiling pathway, to create a dedicated channel for one optical fiber cable.

+-----------------------------------------------------------------------------+
|                  4-INCH CONDUIT WITH THREE 1-INCH INNERDUCTS                |
|                                                                             |
|                      .-------------------------.                            |
|                   .-'                           '-.                         |
|                 /        (  ID-1  )                 \                       |
|                |     (  ID-2  )    (  ID-3  )        |                      |
|                 \                                   /                       |
|                   '-.                           .-'                         |
|                      '-------------------------'                            |
|                                                                             |
|  ONE unusable 4 in duct  --->  THREE independently pullable fiber pathways  |
|  Each innerduct carries one fiber cable and its own pre-installed pull tape |
+-----------------------------------------------------------------------------+

Why not just pull the fiber into the 4-inch conduit? Four reasons:

1. A future pull would destroy the first cable. Pulling a second cable into an occupied conduit drags it across the installed fiber under tension, abrading the jacket and risking a fiber break. Innerduct isolates each pull completely.
2. Sidewall pressure and lay. A small-diameter fiber cable in a large duct snakes and coils, multiplying friction. Confined in a 1-inch innerduct it runs straight.
3. Identification and segregation. Colored, printed innerduct makes it obvious which pathway carries fiber and which carries copper, all the way through the building.
4. Reserved capacity. An empty innerduct is a guaranteed future pathway, unlike an empty corner of a shared conduit that fills with whatever gets installed next.

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2. Innerduct Types

Common Sizes and Fill Practice

• Rigid innerduct is commonly 1 in, 1-1/4 in, and 1-1/2 in trade size.
• Three 1-inch innerducts is the classic fit inside a 4-inch conduit. Four is a common fit in larger duct.
• The traditional design rule is one fiber cable per innerduct. Where a second cable must share, the conduit fill limits from Section 3.2 apply to the innerduct exactly as they apply to conduit — 53% for one cable, 31% for two, 40% for three or more.

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3. Electrical Nonmetallic Tubing (ENT)

ENT is a listed, corrugated, flexible, nonmetallic raceway that is flame-retardant and moisture-resistant. It is covered by NEC Article 362. In the field it is instantly recognizable: corrugated, usually blue, and bendable by hand without tools.

• Permitted and prohibited uses are stated in NEC 362.10 and NEC 362.12 and include provisions on concealment, building construction type and height, ambient temperature, physical damage, and direct-burial. Those provisions have changed between NEC editions — check the edition your jurisdiction has adopted and confirm with the AHJ rather than relying on memory.
• ENT is not a substitute for a rated firestop. Where ENT crosses a fire-rated assembly the penetration is firestopped with a listed system, and where it runs in a plenum it must carry the appropriate listing.
• Bending: ENT bends by hand, which is exactly why installers over-bend it. The bend radius of the fiber inside still governs — 10x the cable outer diameter at rest and 20x during the pull, per Section 5.2. A hand-crushed ENT elbow is a fiber kink waiting to happen.
• Support and termination: support ENT at the intervals its listing requires, and terminate it into enclosures and boxes with the listed fittings and bushings made for it.

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4. Installation Practice

1. Pre-install the pull tape. Order innerduct with a factory-installed, pre-lubricated pull tape with sequential footage markings, or install one during the innerduct pull. Fishing a line into an installed innerduct later is slow and often impossible around bends.
2. Pull the innerducts together, once. Pull all three innerducts into the host conduit in a single coordinated pull with lubricant. Pulling a second innerduct later drags it across the first.
3. Keep them from twisting. Innerducts that spiral around each other inside the host duct create a corkscrew that dramatically raises the friction of every future cable pull. Feed them from separate reels, level, in a fixed clock orientation, and maintain light back-tension.
4. Respect the host conduit's limits. The 100 ft (30.5 m) between pull points and 180 degrees cumulative bend rules from Section 3.2 apply to the innerduct run exactly as they apply to the conduit.
5. Cut clean and bush the ends. Square-cut the innerduct, deburr it, and fit a bushing or a listed end fitting. A ragged, unbushed innerduct end will score a fiber jacket during the pull.
6. Seal and firestop. Plug or cap unused innerducts to keep out debris, moisture, and rodents. Where innerduct passes through a rated wall or slab, install a listed firestop system that covers the innerduct, the host sleeve, and the annular space, and tag it with the UL system number per Section 4.1.
7. Label both ends. Machine-print an identifier on each innerduct at every access point per ANSI/TIA-606-D, including at pull boxes and both terminations. Three identical orange ducts in a pull box with no labels is a fault-isolation problem you have created for someone else.
8. Terminate at the enclosure. Land the innerduct at the fiber enclosure with a listed fitting, transition to the radius-limited slack storage inside the enclosure, and preserve at least the 1 m (3.3 ft) of slack that Section 2.2 specifies at the fiber outlet and 3 m (10 ft) in the TR.

[!TIP] The retrofit answer is fabric innerduct. When a 4-inch conduit already holds two fiber cables and the customer needs a third pathway, rigid innerduct will not go in. A two- or three-cell fabric innerduct compresses around the installed cables during the pull, then holds its cells open, converting a fully occupied duct back into a multi-pathway asset without disturbing service.