Select duct and innerduct from project specifications

What you should be able to do

Read an approved duct or innerduct specification, compare it with product information, and identify mismatches before material is installed. Selection means matching the designed system; it does not authorize an apprentice to redesign it.

Start With The Project

Record the specification section, drawing/detail, current revision and approved product submittal. Identify where the pathway will be used and the intended installation method. Duct is a pathway for cable; innerduct is used within another duct to organize or subdivide that pathway. A product suitable for one location or installation process is not automatically suitable everywhere on the route.

Build a comparison with these fields: manufacturer and part number; material; wall designation; actual dimensions and tolerances; required ratings or markings; compatible couplings, seals and accessories; installation method; and document revision. Follow the project's material approval process when a supplier proposes a substitute.

Read The Cross-Section

The poster uses invented round dimensions. Duct outside diameter is 25 mm, inside diameter is 20 mm and cable outside diameter is 10 mm. The duct wall is drawn proportionally: (25 − 20) ÷ 2 = 2.5 mm. The drawing centers the cable for clarity, not to predict its installed position.

The usable bore is related to ID, not simply the name on a purchase order. Product wall systems and dimension conventions vary. Use the actual product table, including tolerances. A nominal or trade size must not be silently treated as a measured inside diameter.

Two Different Ratios

For this single round cable and round bore, diameter ratio is 10/20 = 0.50, or 50%. Cross-sectional area fill is the cable area divided by the bore area. Because circle area scales with diameter squared, this is (10/20)² = 0.25, or 25%. The same geometry therefore produces two different percentages.

Both calculations are mathematically correct. Neither is an installation approval or a universal allowable fill. Name the measure required by the applicable guidance. For more than one round cable, an area calculation sums cable areas; it does not simply square the sum of diameters. Geometric fit, installation performance and compliance are separate checks.

Method And Condition Matter

Manufacturer guidance for air-assisted placement uses dimensions differently from area-based fill checks. Confirm the actual cable, duct, equipment and method-specific documentation. Do not transplant one product's target ratio, pressure or installation-distance claim into another system.

Duct condition also matters. Ovality, deformation, restrictions and unsuitable fittings can affect the usable pathway. Pulling limits and bend limits come from the relevant cable and pathway specifications. Resolve compatibility questions with the responsible designer and manufacturers; a successful arithmetic calculation cannot replace that process.

Original Exercise: A Proposed Substitute

A fictional approved schedule calls for product A with a documented minimum bore of 20 mm. A proposed product B has the same nominal trade designation but documents only an 18 mm minimum bore. The selected cable has a 10 mm OD. Those facts do not prove B fails every possible application, but they establish that it is not dimensionally identical.

For the simplified round-bore example: A: diameter ratio = 10/20 × 100 = 50%; area fill = 25%. B: diameter ratio = 10/18 × 100 ≈ 55.6%; area fill ≈ 30.9%.

Record the difference and request the prescribed substitution review. Also compare wall type, fittings, ratings and installation requirements. Do not mark B approved merely because the cable fits through a short sample. Preserve the designer's disposition and revised documents if the substitution is accepted.

Receiving Check

On a training delivery record, enter product identity, reel/lot reference, visible markings, quantity and condition. Compare the delivered item with the approved submittal. A matching color is not a substitute for identity or rating verification. Record unreadable markings, damage or specification discrepancies and follow the project's hold-and-resolution process.

Practice with documentation and samples under supervision. This lesson is not a pressure-testing, cable-blowing or confined-space procedure.

Knowledge Check

Worked through

Use the fictional 10 mm round cable and 20 mm round bore. First identify the requested measure: diameter ratio is 10 / 20 = 0.50, or 50%. For area fill, divide circular areas: (pi x 10 squared / 4) / (pi x 20 squared / 4) = 100 / 400 = 0.25, or 25%. Record both names beside their values. If a document asks for area fill, entering 50% answers a different question. Neither result supplies an allowable limit; compare the correct measure with the actual approved system and installation-method requirements.

Where beginners go wrong

Sources