Cable tray sizing is decided by three dimensions, and each is governed by a different constraint: width by how many cables must fit, depth by how much of them there is, and bend radius by the largest and most bend-sensitive cable on the route. Sizing by width alone is the standard error — it produces a tray that looks correct, passes a visual check, and then damages the cable during installation or fails the fill requirement on review. This guide works through all three in the order they should be considered.
Dimension 1: Width — Set by Fill
Width is determined by how much cable cross-sectional area the tray must accommodate, with enough free space that cables can be laid without forcing and without excessive bunching.
How Fill Is Assessed
Fill is normally assessed as the aggregate cross-sectional area of the cables expressed against the tray’s usable cross-sectional area, with the permissible value depending on the tray type and the applicable wiring rules. Under NEC 392, for example, fill limits differ for ladder versus solid-bottom tray and for power versus control and signalling cables — the rules are specific, and using the wrong one is a common compliance failure.
Practically, the assessment has two steps:
- Sum the cable areas. Use the actual outside diameter from the cable datasheet, not the nominal conductor size. The difference between nominal and actual diameter is where most fill calculations go wrong.
- Compare against the tray’s usable area at the applicable fill percentage for the tray type and cable classification.
Note that some tray types also impose a width-based constraint alongside the area-based one — a tray can satisfy the area limit and still fail because the cables do not physically lay side by side within the available width. Both conditions must be checked.
Cable Diameter: Get the Real Number
This is the single most common source of sizing error. A cable described as “4 × 50 mm²” has an outside diameter determined by the cable’s construction — conductor class, insulation thickness, bedding, armour where present, and outer sheath. Two cables with the same conductor size from different constructions can differ materially in diameter, and therefore in area.
- Use the manufacturer’s datasheet outside diameter, not a table of nominal values.
- Account for armour and any additional sheath — these add substantially to diameter.
- Where the exact cable is not yet selected, size on the largest credible construction and confirm before ordering tray.
Dimension 2: Depth — Set by Layer Count and Heat
Depth determines how many layers of cable the tray can carry in a usable arrangement, and it also affects heat dissipation.
Practical Layer Guidance
Single-layer installation is generally preferred wherever space allows. It gives the best heat dissipation, the simplest installation, and the easiest future access. Multiple layers are common where space is constrained, but they reduce dissipation and — importantly — require derating, because cables in a bundled or layered arrangement run hotter than a single layer in free air.
Derating for grouping and layering is set out in the applicable wiring rules and depends on the arrangement, spacing, and number of circuits. This is not optional: laying three layers of power cable in a tray and applying the free-air current rating is a genuine safety issue, not a documentation quibble.
Typical Tray Depths
| Depth | Typical application | Note |
|---|---|---|
| 50 mm | Light routes, data and instrumentation, small power | Usually single layer |
| 75 mm | General purpose mixed routes | Common default |
| 100 mm | Power distribution routes, multiple layers | Most common for industrial power |
| 150 mm | Heavy power, large-diameter cable, high fill | Check load capacity — deeper tray carries more weight |
Heat: The Constraint People Forget
Adding depth to solve a fill problem is tempting, and it works up to a point. But a deep tray packed with power cable has poor dissipation at the bottom of the stack, and the derating factor becomes more severe. Where fill is genuinely high, the better answer is often a wider tray in a single layer, or a second tray run, rather than a deeper tray with more layers.
Dimension 3: Bend Radius — Set by the Cable, Not the Tray
Bend radius is the dimension most often missed entirely, and it is the one that causes physical damage to cable during installation.
Every cable has a minimum bending radius, declared by the cable manufacturer and dependent on the cable’s construction — armoured cable, cable with a large conductor, and cable with certain insulation types all require larger radii. The tray’s fittings must provide a radius not smaller than the most restrictive cable on the route.
How to Apply It
- Identify the governing cable — the one with the largest minimum bending radius, usually the largest power cable or any armoured cable.
- Confirm the fitting radius — every bend, tee, riser, and offset in the route must provide at least that radius. A horizontal bend is obvious; the inside of a vertical riser is where it gets missed.
- Check the sweep, not the angle — a 90° bend and a 45° bend of the same radius are equally acceptable as far as the cable is concerned; it is the radius that matters.
- Remember the cable is pulled through the bend — pulling tension combined with a tight bend is what damages cable, so the installed radius and the pulling method must both be right.
Illustrative Minimum Radii
| Cable type | Illustrative minimum bend radius | Source |
|---|---|---|
| Unarmoured PVC-insulated power cable | Multiple of overall diameter, lower than armoured | Cable manufacturer datasheet |
| Armoured cable | Materially larger multiple of overall diameter | Cable manufacturer datasheet |
| Data / communications cable | Specified per category; tight bending degrades performance | Cable manufacturer datasheet |
| Fibre optic cable | Specified minimum, plus a larger dynamic radius during pulling | Cable manufacturer datasheet |
Illustrative only. The cable manufacturer’s declared minimum bending radius is the governing value, and it varies by construction, size, and standard. Always obtain it from the datasheet.
The practical consequence: a route carrying a large armoured power cable may require a bend radius that rules out a compact fitting. If the route was designed around the space available rather than around the cable, this is where the problem surfaces — usually on site, during the pull.
Worked Example (Illustrative)
A route must carry:
- 6 × 4-core 50 mm² XLPE/SWA power cable — outside diameter approximately 33 mm each
- 12 × data cables — outside diameter approximately 6 mm each
- Power and data segregated into separate trays
Power tray — area: 6 × π × (33/2)² ≈ 6 × 855 ≈ 5,130 mm² of cable.
Width check: Six cables at 33 mm laid side by side need at least roughly 200 mm of usable clear width, plus spacing. A 300 mm wide tray satisfies both the area test at typical fill limits and the physical lay-out test.
Depth: Single layer, so 75 mm is sufficient structurally; 100 mm gives margin for a future addition without re-layering.
Bend radius: The armoured power cable’s declared minimum radius governs. If the datasheet gives a multiple of overall diameter, apply it to 33 mm and confirm every fitting in the route provides at least that radius — including the vertical riser at the end of the run.
Data tray: 12 × π × (3)² ≈ 12 × 28 ≈ 339 mm² — a modest area, but data cable is often sized on count and future additions rather than current fill. A 200 mm wide tray with spare capacity is a common choice, on a separate route from the power tray.
Illustrative example only. Use actual datasheet diameters, the fill rules applicable in your market, and the cable manufacturer’s declared bend radius.
Application Case: Industrial Plant Route With Mixed Cable
Scenario Constraints
A plant expansion adds a route carrying four 95 mm² armoured power cables and a bundle of control cables to a new production line. The route runs 60 m along a pipe rack, turns 90° twice, and rises vertically 4 m to a mezzanine distribution board. Space on the rack is limited, and the rack’s spare load capacity is modest. Power and control must be segregated.
Selection Approach
Width and depth are driven by the four armoured power cables — their diameter and weight, not the control bundle. Because the rack has limited spare capacity, tray weight matters, which favours a lighter tray type and a single-layer arrangement rather than a deep multi-layer tray.
The two 90° bends and the vertical riser are the sizing-critical items. The 95 mm² armoured cable has a substantial minimum bend radius; the horizontal bends need fittings providing that radius, and the transition from horizontal to vertical at the riser base is the point where an undersized radius does the most damage. Specify the riser fitting by radius, not by angle.
Segregation: separate tray for the control cables, running parallel with the required separation, rather than a divided tray — unless space genuinely forbids two runs, in which case a tray with a fixed divider is used and the separation is documented.
Common Mistakes
- Sizing on nominal conductor size instead of actual outside diameter. The most common error, and it understates fill by a wide margin on armoured cable.
- Ignoring bend radius until the cable arrives on site. By then the fittings are installed and the route is fixed.
- Checking the horizontal bends but not the riser. The base of a vertical riser is where the tightest effective radius usually occurs.
- Solving fill by adding layers instead of width — which worsens derating and heat dissipation.
- Not applying derating for the layered arrangement. Multi-layer tray is not free-air.
- Exceeding the rack’s load capacity — tray plus cable is heavy; confirm the structure can carry it before finalising width and type.
Acceptance Checks
- Fill calculation documented using actual datasheet outside diameters, checked against both the applicable area limit and the physical lay-out width.
- Derating factors applied for the actual layer and grouping arrangement, recorded in the cable schedule.
- Minimum bend radius confirmed for the governing cable, and verified at every bend, tee, and riser on the installed route.
- Support spacing confirmed against the declared span for the chosen width, depth, and load class, with the loaded weight verified against the supporting structure’s capacity.
- Segregation verified — power and control maintain required separation along the full route, including at crossings.
- Cable pulling method confirmed as compatible with the installed radii and the declared pulling tension.
Frequently Asked Questions
How do I know what width tray I need?
Calculate the aggregate cross-sectional area of all cables using their actual outside diameters, compare against the tray’s usable area at the fill limit applicable to that tray type and cable class, and separately confirm the cables physically lay side by side within the usable width.
Can I stack cables to save width?
Yes, but it is not free. Multi-layer arrangements require derating and dissipate heat less effectively. Where space allows, a wider tray in a single layer is usually the better engineering answer.
What tray depth should I use?
Deep enough for the intended layer arrangement with margin. 100 mm is common for industrial power routes; 50–75 mm for data and light routes. Do not use depth as the primary way to solve a fill problem.
Why does bend radius matter so much?
Bending a cable below its minimum radius damages insulation, conductor, and — for data and fibre — transmission performance. The damage may not be visible and may not appear until later. It is one of the few installation errors that cannot be inspected away.
Whose bend radius applies — the tray’s or the cable’s?
The cable’s. The tray fitting must provide a radius at least as large as the most restrictive cable’s declared minimum. The tray’s own radius is a product property; the cable’s requirement is the constraint.
Do I need to derate cables in tray?
Yes, where the arrangement is not equivalent to the reference conditions the rating was established under — layered, grouped, or closely spaced cables dissipate heat less well. Apply the factors in the applicable wiring rules for the actual arrangement.
Should power and control cable share one tray?
Generally no — segregate onto separate trays or use a tray with a fixed divider. Where a divided tray is used, document the arrangement and confirm it meets the applicable separation requirement.
Size Your Cable Tray Route with BANGE Electric
BANGE Electric supplies cable tray in widths from 50 mm to 900 mm and depths of 50–150 mm, with declared fill data, load classes, and fitting radii so you can verify every dimension before ordering. Send us your cable schedule with actual outside diameters and the minimum bend radius of the largest cable, plus your span and support conditions, and we will size the route and specify the fittings.
Work through the fill calculation or request a quotation.
Requirements may vary by application, market, and applicable standard. Figures given here are illustrative examples; always confirm against manufacturer data and your project specification.