Cable Trays and Cable Ladders

1. Definition

Cable trays and cable ladders are open or partially enclosed support systems that route power, control, and instrumentation cables through industrial plants, commercial buildings, and utility installations. A tray section consists of longitudinal side rails joined by a bottom surface, rungs, or mesh, depending on the type. The system carries cable weight over spans between supports, provides ventilation for heat dissipation, and allows cable entry and exit along the route without pulling through enclosed conduit.

Manufacturers produce tray sections from coiled flat strip or sheet using roll forming, press-brake bending, or extrusion (aluminum). Roll forming dominates high-volume production of steel ladder trays and perforated channel trays because it yields consistent flange heights, repeatable bend radii, and long straight lengths suitable for automated punching and cutting downstream.

2. Tray vs Ladder Terminology (IEC 61537)

IEC 61537 (European adoption EN 61537; third edition published 2023) defines the umbrella term cable tray system for metallic and non-metallic support structures. Within that family, naming follows the continuity of the cable-bearing surface.

2.1 IEC Product Family

Under IEC 61537, a cable ladder is a subtype of cable tray system, not a separate product category. In North American project language, engineers often say “ladder tray” when specifying rung-based sections and “channel tray” for solid or ventilated bottom types. The distinction matters for load testing, marking, and procurement documents because IEC and NEMA use different rating methods.

2.2 IEC Safe Working Load (SWL)

IEC 61537 assigns a Safe Working Load rather than a letter class. SWL is the lower of: (a) the uniformly distributed load at which mid-span deflection reaches 1/100 of the support span, or (b) the test load divided by a safety factor of 1.7. Lateral deflection must stay below 1/20 of tray width. Temperature range, electrical continuity class, and corrosion-protection class are also marked on the product. Because deflection limits and safety factors differ from NEMA VE 1, an IEC SWL in kg/m cannot be converted arithmetically to a NEMA class; the tray must be tested to the governing standard on the project.

3. NEMA VE 1 Load and Span Classes

NEMA Standard Publication VE 1 — Metallic Cable Tray Systems — defines construction, testing, and marking for steel, aluminum, and stainless trays sold in North America. NEMA VE 2 covers installation guidelines referenced alongside VE 1 in specifications.

3.1 Class Designation Format

NEMA class designation combines support span (feet) with a working-load letter:

Example: Class 20C indicates a tray tested to span 20 ft (6.1 m) between supports while carrying a cable static load in the 75–100 lb/ft band. Eaton specification documents cite this format when defining flex tray and steel ladder products.

3.2 NEMA Test Requirements

Project engineers fill blank load and span fields in manufacturer spec templates (e.g., Eaton Series 1 steel) with calculated cable fill weight plus future capacity margin. Manufacturer test reports per the latest VE 1 revision are typically available on request.

4. Tray System Types

4.1 Solid Bottom Tray

Solid bottom trays use a continuous metal base welded or bolted between side rails. They shield cables from falling debris and reduce electromagnetic coupling compared with open ladder types. Trade-off: reduced natural convection; NEC and project specs may require ventilation covers or derating for dense power fills.

4.2 Ventilated (Perforated) Tray

Perforated trays combine a continuous base with punched or rolled ventilation patterns. They balance cable support with airflow and are common for mixed power/control runs in manufacturing plants. Hole patterns are often produced inline on the roll forming line before the strip enters forming rolls.

4.3 Ladder Tray

Ladder sections have two side rails connected by transverse rungs spaced at 6, 9, or 12 in. on center (150, 230, or 300 mm). Standard straight lengths are 10 ft (3 m) or 12 ft per manufacturer catalogs. Rung spacing in horizontal bends is often fixed at 9 in. measured at tray centerline. Ladder tray dominates petrochemical, power generation, and heavy industrial cable routing because rungs allow top and side cable entry over long spans.

4.4 Wire Mesh (Basket) Tray

Wire mesh systems use welded steel wire grids or formed mesh panels. They are lightweight, quick to install, and common in data centers and ceiling plenum routes. NEMA VE 1 class designations apply to metallic mesh trays when rated per VE 1 testing; fiberglass trays follow NEMA FG 1.

5. Materials and Finishes

5.1 Pre-Galvanized Steel (GI)

Mill galvanized strip per ASTM A653 (designations such as G90) is the economical baseline for indoor and mild outdoor tray. Yield strength is commonly Grade 33 (230 MPa minimum) for 14 gauge and heavier, or Grade 33 Type 2 for lighter gauges per ASTM A1008 in manufacturer specs. GI side rails roll-form cleanly at thicknesses from 0.8 to 1.5 mm used on many production lines.

5.2 Hot-Dip Galvanized (HDG)

Hot-dip galvanizing after fabrication per ASTM A123 applies to cut, punched, and welded tray sections when cut edges and welds need full zinc coverage. HDG is specified for outdoor corrosive atmospheres. Mill-galvanized covers are generally not accepted when the project calls for post-fabrication HDG on ladder components.

5.3 Stainless Steel

AISI Type 304 and 316 stainless side rails and rungs serve food, pharmaceutical, and coastal environments. Eaton Series 3 and 4 stainless specifications reference Type 304 or 316 for straight sections and fittings. Roll forming stainless requires higher forming force and polished rolls to avoid galling; tooling is often made from tool steels with hard chrome plating.

5.4 Aluminum Alloy 6063

Aluminum tray side rails and rungs are extruded from Aluminum Association Alloy 6063 per Eaton Series 2–5 aluminum specifications. Extrusion yields precise flange geometry; lighter trays use aluminum when weight reduction or non-magnetic support is required. Some manufacturers also roll-form aluminum strip for custom profiles, though 6063 extrusion dominates catalog sections.

6. Typical Dimensions and Rung Spacing

Standard tray widths include 150, 230, 300, 450, 600, 750, and 900 mm (6, 9, 12, 18, 24, 30, and 36 in.). Loading depth (side rail height) is typically 3–6 in. (75–150 mm). The table below summarizes commonly cataloged dimensions for steel ladder tray used with roll-formed side rails.

7. Roll Forming of Side Rails and Rungs

Steel cable tray side rails are C-channel or hat-shaped profiles cold-formed from coiled strip. A typical dedicated production line includes hydraulic decoiler, leveling unit, servo-fed punch press (for rung attachment holes or bottom ventilation), roll forming mill, tracking or flying shear cut, and output table. Believe Industry and similar equipment builders document lines for 0.8–1.5 mm strip with 18–25 roll stations, 70–90 mm shaft diameter, and forming speeds of 8–20 m/min depending on punching complexity.

7.1 Side Rail Profile Development

Side rails require vertical flanges for structural depth, horizontal lips for rung attachment or cover retention, and consistent bend radii so splice plates align at joints. Roll pass design progresses from flat strip through gradual bending stages to avoid edge cracking on galvanized coating. Width and height adjustment on some lines use motorized spacer systems so one mill produces multiple tray sizes without full roll changeover.

7.2 Inline Punching and Rung Integration

Ladder tray production often punches rung mounting holes in side rails before forming, or stamps rungs separately and welds them after cut-to-length. Perforated bottom trays integrate CNC punching prior to the forming section so hole arrays stay registered to flange geometry. Servo feeders synchronize punch index with strip speed to hold hole pitch tolerance for automated assembly robots downstream.

7.3 Tooling and Machine Parameters

After roll forming, side rails are cut to standard lengths, rungs are welded or bolted, and splice hardware is packaged. For HDG projects, cut and welded sections may be sent for hot-dip galvanizing as a batch after fabrication.

8. Specification and Selection

Selection starts with cable fill calculation: sum of cable diameters and weights per NEC fill rules, plus future expansion. Span between supports determines required load class (NEMA) or SWL (IEC). Environment selects material finish: GI for dry indoor, HDG or stainless for outdoor or corrosive areas, aluminum where weight limits apply.

When procuring roll-formed tray sections, require mill certificates for steel grade and zinc coating mass, and factory test reports traceable to NEMA VE 1 Section 5 or IEC 61537 load tests. Splice plate hardware must match tray finish (electro-zinc vs chromium-zinc per ASTM F1136 for HDG systems) to avoid galvanic corrosion at joints.