Cable trays and fill: tray width, conduit size and occupancy ratio
Tray and conduit fill sizing means summing the cross-sectional area of every cable on a route and comparing it to the usable area of the support, so the chosen tray width or conduit size stays within the practical occupancy limit.
When to use
Use it as soon as the cable list and the routing of an electrical project are defined, to pick the commercial tray width or conduit diameter for each run. It tells you whether a route is comfortable, in the warning band or overcrowded, flags class segregation conflicts (MV, LV and control on the same tray), and estimates the supported weight per metre for the bracket and hanger design. It is also the tool to diagnose existing routes that overheat or cannot take new cables: an over-filled tray loses heat dissipation and forces a derating of the conductor ampacity.
What cable tray and conduit fill sizing is
Sizing a cable support is not picking a tray width from habit: it is checking that every cable on a route actually fits — with room to dissipate heat, to be spaced apart and to grow — inside the usable area of the chosen tray or conduit. The method compares two areas on each route: the occupied area, which is the sum of the external cross-section of all the cables, and the usable area of the support, which is its geometric area scaled down by a practical fill rate. The ratio between them is the occupancy, and it is the single number that tells you whether the run is comfortable, tight or overcrowded.
The most common field mistake is to size the tray by the conductor cross-section instead of the cable outer diameter. A 240 mm² conductor lives inside a cable whose outer diameter is around 26.6 mm — an external area more than twice the copper section. Count only the copper and the tray looks half empty when it is in fact full.
The occupied area
The occupied area is the sum of the external circle of every cable on the route:
A_occ = Σ [ π·(d_ext,i/2)²·n_i ]
The outer diameter d_ext comes from the manufacturer catalogue, per family and section — for example, an Afumex 0.6/1 kV single-core cable goes from 4.8 mm at 1.5 mm² to 30.6 mm at 300 mm². It is the finished diameter, jacket included, because that is what physically occupies the tray. The quantity n multiplies each distinct cable, and the sum runs over the whole route.
The usable area of the support
A support is never filled to 100% of its geometric area. Two practical limits apply, one per support type.
For a perforated tray or ladder:
A_tray = W · H · 0.50
where W is the width and H the useful height in millimetres. The 0.50 factor is the practical fill rate that preserves spacing and heat dissipation and leaves capacity for future cables.
For a circular conduit:
A_cond = π·(D/2)²·0.40
where D is the internal diameter. The 0.40 factor is the NBR 5410 / NEC fill rule for three or more conductors, which also leaves room to pull the cables. Two conductors raise the limit to about 31% and a single conductor to about 53%. Never apply the 50% factor to a conduit or the 40% factor to a tray.
The occupancy ratio
With both areas known, the occupancy is simply:
fill[%] = (A_occ / A_usable) · 100
The result is read against three bands:
- Below 60% — comfortable (green): good heat dissipation and spare capacity for growth.
- 60% to 85% — warning (amber): the run works, but there is little room for new cables; check the grouping derating.
- At or above 85% — critical (red): widen the tray to the next commercial width, step up the conduit, or stack another tier.
Commercial tray widths follow a standard ladder — 50, 100, 150, 200, 300, 400 and 600 mm — so the design rule is to pick the next width above the fill limit, never the one that just barely passes.
Why occupancy controls ampacity
Occupancy is not only a geometric check. The more tightly cables are packed, the worse each one dissipates heat, and the hotter it runs for the same current. Standards capture this with grouping derating factors (IEC 60364-5-52 / NBR 5410): a cable rated for a given ampacity in free air must have that ampacity reduced when bunched with others. Keeping the tray within the practical fill limit preserves the spacing that the ampacity tables assume — an over-filled tray quietly erodes the very current rating the cables were chosen for.
Class segregation
Cables fall into classes by voltage and function: medium voltage (MV), low voltage (LV) and control/instrumentation (CII). Running them on the same tier without a barrier induces electromagnetic interference on the signal cables and complicates maintenance. NBR 5410 requires segregation: when there is no physical divider, different classes must run on distinct stacked trays. The sizing tool inspects each route and flags a tier that mixes classes, so the designer can split MV, LV and control into their own trays.
The supported weight
A full tray is heavy. The distributed load per metre is the tray self-weight plus the mass of all the cables:
q = q_support + Σ (m_cable,i · n_i)
A 300×100 mm steel tray weighs around 5.8 kg/m empty; loaded with power cables it can exceed several tens of kilograms per metre. This distributed load sets the spacing of brackets and hangers and the structural check of the support. Ignoring it leads to under-spaced supports that sag or detach under load, so the weight estimate is part of the sizing, not an afterthought.
Practical design considerations
- Always use the outer diameter, never the conductor section, to compute the occupied area.
- Match the factor to the support: 50% for trays, 40% for conduits with three or more conductors.
- Leave spare capacity: target the green band (< 60%) on new runs so future cables fit without rework.
- Segregate by class: MV, LV and control on distinct tiers when there is no barrier.
- Size the support structure: feed the weight per metre into the bracket and hanger spacing.
- Align standard and method: tray systems follow NBR 14306 / IEC 61537; conduit systems NBR 15465 / NBR 5597; grouping and fill criteria NBR 5410 / IEC 60364-5-52.
Following this chain — occupied area from outer diameters, usable area from the practical fill rate, occupancy band, class segregation and supported weight — yields a cable support design that fits the cables today, dissipates their heat, respects segregation and still has room to grow.
Formulas and fundamentals
A_cable = π·(d_ext/2)²·n External area occupied by a cable group on the route. d_ext is the cable outer diameter [mm] taken from the manufacturer catalogue (not the conductor section), and n is the number of parallel cables. The full external circle is counted, including insulation and jacket — that is what physically takes up room on the tray.
A_occ = Σ [ π·(d_ext,i/2)²·n_i ] Sum of the external area of every cable routed through the tray or conduit. The index i runs over each distinct cable in the route. This is the numerator of the fill ratio.
A_tray = W · H · 0.50 Practical usable area of a ladder/perforated tray. W is the tray width and H its useful height [mm]; the 0.50 factor is the practical fill rate that leaves room for spacing, heat dissipation and future cables. A tray is not filled to 100% of its geometric area.
A_cond = π·(D/2)²·0.40 Usable area of a circular conduit. D is the conduit internal diameter [mm] and 0.40 is the maximum fill fraction for three or more conductors (NBR 5410 / NEC fill rule), leaving room for pulling and ventilation.
fill[%] = (A_occ / A_usable) · 100 Fill percentage of the support. A_usable is A_tray for trays or A_cond for conduits. Below 60% the run is comfortable (green); 60–85% is the warning band (amber); at or above 85% it is critical (red) — widen the tray, step up the conduit or stack another tier.
q = q_support + Σ (m_cable,i · n_i) Distributed load on the support [kg/m]. q_support is the tray/conduit self-weight per metre, m_cable the mass per metre of each cable and n the quantity. This load feeds the spacing of brackets and hangers and the structural check of the support.
Standards & methods
- ABNT NBR 14306 — Perforated cable tray and ladder systems — Requirements
- ABNT NBR 15465 — Conduit systems of insulating material for electrical installations
- ABNT NBR 5410 — Low-voltage electrical installations (cable grouping, conduit fill and segregation)
- ABNT NBR 5597 — Rigid steel electrical conduit (commercial dimensions)
- IEC 61537 — Cable management — Cable tray and cable ladder systems
- IEC 60364-5-52 — Selection and erection of wiring systems (grouping and derating)
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Practical tray fill rate | 50% of W·H | Leaves room for spacing, heat dissipation and future cables. |
| Conduit fill — 3 or more conductors | ≤ 40% of the internal area | NBR 5410 / NEC. Two conductors ≤ 31%; one conductor ≤ 53%. |
| Comfortable occupancy (green) | < 60% | Good heat dissipation and spare capacity for growth. |
| Warning band (amber) | 60% to 85% | Acceptable, but little room for new cables; check grouping derating. |
| Critical occupancy (red) | ≥ 85% | Widen the tray, step up the conduit or add another stacked tier. |
| Commercial tray widths | 50, 100, 150, 200, 300, 400, 600 mm | Standard heights 50–100 mm; pick the next width above the fill limit. |
| Vertical class segregation | MV / LV / control on separate tiers | Without a barrier, different classes must use distinct stacked trays. |
Worked example
Power tray on a motor control centre feeder run
Inputs
- Tray width
- W = 300 mm
- Tray height
- H = 100 mm
- Cable section
- 240 mm² (1-core) mm²
- Cable outer diameter
- d_ext = 26.6 mm
- Number of cables
- n = 18 cables
Results
- Area per cable
- ≈ 555.7 mm²
- Total occupied area
- A_occ ≈ 10003 mm²
- Usable area (50%)
- A_tray = 15000 mm²
- Occupancy ratio
- fill ≈ 67 %
- Status
- Warning (amber) —
Each 240 mm² cable occupies π·(26.6/2)² ≈ 555.7 mm²; eighteen of them sum to A_occ ≈ 10003 mm². The usable area of the 300×100 mm tray is W·H·0.50 = 300·100·0.50 = 15000 mm², so the fill is 10003/15000 ≈ 67%. That lands in the warning band (60–85%): the run works, but there is little room left for new cables and the grouping derating should be checked. Stepping up to a 400×100 mm tray (usable 20000 mm²) drops the fill to ≈ 50% and brings the route back into the comfortable green band with spare capacity.
Common mistakes
- Sizing a tray by conductor cross-section instead of cable outer diameter — the insulation and jacket take far more room than the copper, and the tray fills up faster than expected.
- Filling a tray to 100% of its geometric area: the practical limit is around 50%, otherwise heat dissipation collapses and the conductor ampacity must be derated.
- Applying the 40% rule to a tray or the 50% rule to a conduit — the two supports follow different fill criteria.
- Mixing medium-voltage, low-voltage and control cables on the same tier with no barrier, violating class segregation and inducing interference on instrumentation.
- Ignoring the supported weight per metre, so the bracket spacing is too wide and the tray sags or detaches under the cable load.
- Leaving no spare capacity: a route filled to the limit on day one cannot take the inevitable future cables without a full rework.
Frequently asked questions
Why size by outer diameter instead of conductor cross-section?
Because what physically fills the tray is the whole cable — conductor, insulation and jacket — not just the copper or aluminium core. A 240 mm² cable has an outer diameter around 26.6 mm, so its external area (≈ 556 mm²) is more than double its conductor section. Using the conductor section would grossly under-count the occupancy and lead to an over-filled tray.
What is the difference between the 50% tray rule and the 40% conduit rule?
They are practical fill limits for two different supports. A perforated tray or ladder is sized to about 50% of its geometric area (W·H) to keep spacing and heat dissipation. A circular conduit follows the NBR 5410 / NEC rule of 40% of the internal area for three or more conductors, which also leaves room to pull the cables. Never swap the two factors between supports.
What occupancy is safe?
Below 60% the run is comfortable (green), with good heat dissipation and room to grow. From 60% to 85% it is the warning band (amber): it works but there is little spare capacity and you should check the grouping derating. At or above 85% it is critical (red) — widen the tray, step up the conduit or stack another tier.
How does occupancy relate to conductor ampacity?
An over-filled tray packs cables tightly, so each cable dissipates heat worse and runs hotter for the same current. Standards handle this with grouping derating factors (IEC 60364-5-52 / NBR 5410): the more cables grouped, the lower the allowed ampacity. Keeping the tray within the practical fill limit preserves spacing and the rated ampacity.
Why does the tool flag class segregation?
Mixing medium-voltage, low-voltage and control/instrumentation cables on the same tier without a barrier induces electromagnetic interference on the signal cables and complicates maintenance. NBR 5410 requires segregation: when there is no divider, different classes must run on distinct stacked trays. The tool warns when classes share a tier so you can split them by tier.
Why does the calculator estimate the supported weight per metre?
The distributed load (tray self-weight plus the mass of all cables) sets the spacing of brackets and hangers and the structural check of the support. An over-filled tray can weigh tens of kilograms per metre; ignoring it leads to under-spaced supports that sag or detach. The weight estimate feeds the support design directly.
Glossary
- Occupancy ratio (fill)
- Percentage of a support's usable area taken up by the cables routed through it — the occupied area divided by the usable area, times 100.
- Cable tray
- Open perforated or ladder support that carries cables along a route; sized to about 50% of its W·H geometric area in practice.
- Conduit fill
- Fraction of a conduit's internal cross-section occupied by conductors; limited to 40% for three or more conductors by NBR 5410 / NEC.
- Outer diameter (d_ext)
- Overall external diameter of a finished cable including insulation and jacket, taken from the manufacturer catalogue and used to compute the occupied area.
- Class segregation
- Separation of cables by class (medium voltage, low voltage, control/instrumentation) onto distinct tiers or with a barrier to avoid interference.
- Grouping derating
- Reduction of a cable's rated ampacity when several cables are bunched together and dissipate heat to one another (IEC 60364-5-52 / NBR 5410).
- Usable area
- Effective area available for cables in a support after applying the practical fill rate (50% for trays, 40% for conduits).