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STANDARDS & DESIGN · CONTAINMENT
Cable tray sizing
Big enough is not sized: the arrangement sets the grouping factor, which changes the cable size, which changes the width you just selected. That selection also drives the fault force the cleats have to withstand and the total load at the drop rods. This guide works through the full sizing to BS 7671:2018+A4:2026, BS EN 61914 and BS 8519:2020.
Checklist
- Width. Cables fit in a single layer (multicore), spare added, rounded up to a standard size. [BS 7671, Appendix 4]
- Height. Important for coordination, spacing and builders work, and it decides whether the run stays a single layer. [BS 7671, Table 4C5 Note 2; BS EN 61537]
- Formation and spacing. Flat or trefoil, touching or spaced. Sets the occupied width, and whether a grouping factor applies at all. [BS 7671, Table 4C1 Note 2]
- Grouping (Cg). Current-carrying capacity. Table 4C1 gives four different factors for four different arrangements of the same cables. [BS 7671, Tables 4C1, 4C4, 4C5]
- Segregation. Whether Band I and Band II circuits may share the tray. [BS 7671, Regulation 528.1]
- Fixing spacing. How often the cables are clipped to the tray. [IET On-Site Guide, Table D1]
- Fault restraint. Cleat type and centres on single-core runs. The peak fault current at the tray sets the force per metre the cleats hold. [BS EN 61914]
- Supports and drop rods. Every run is supported by the bracket, fixed with a pair of rods. All services on that bracket are summed. [BS EN 61537; BS 8519:2020, Annex E and Annex I]
1.0 What to consider
Sizing a tray on width alone is the most common way to get it wrong. Adding up cable diameters, plus a spare allowance (typically 25%), is easy to calculate and select the next standard width. What about height: this is not so obvious and it is related to containment strength.
You also must decide how it is fixed.
The other factors need a bit more work: look for the relevant standards, and find a fault figure from your calculations, especially for single-core cables. This is what often gets left.
System weight must be considered for the supports – a bracket often supports multiple services. Each run must be sized individually, and then total weight calculated – this is specified in BS 8519.
Any additional loading to the support system, for example, from other services on combined services bracketry or multi-service modules, is also to be taken into account when calculating the total load on the drop rods.
BS 8519:2020, Annex E, Note 3
2.0 Establish width and height
For multicore add up the overall diameters in a single layer, apply the spare percentage, round up to the next standard width.
For single core, you must consider cable formation, for example flat or trefoil, and allow for cable spacing, which affects the grouping factor.
Reference: BS 7671:2018+A4:2026, Appendix 4. Reference Method E (multicore cable in free air), Reference Method F (single-core cables in free air).
Spare capacity. 25% is the common working allowance and it is a convention, not a standard value. Where the run includes life safety or fire-fighting cables, BS 8519 sets a range and recommends that it is confirmed on the project.
The cable systems should contain a spare capacity allowance for future system extension in the region of additional 20% to 30%; this should be confirmed in the relevant project requirements and taken into account when analysing the support system loadbearing ability in fire.
BS 8519:2020, Annex E
The exception is structured cabling. Data, AV, specialist lighting and ELV cables are not laid out in a single layer. They are bunched, and the width follows from a fill area against a usable depth. [BS EN 50174-2:2018]
Height is decided by three things at once.
Coordination and builders work. The side height sets the vertical zone the run occupies, and with it the size of the opening you ask for and the clearance to the service above. On a multi-tier bracket it also sets the tier centres, which feed back into the grouping factor.
The rating basis. Methods E and F, and the grouping factors that go with them, are single-layer arrangements. Filling the depth of the tray with a second layer of cables takes the run outside them.
Factors apply to single layer groups of cables (or trefoil groups) as shown above and do not apply when cables are installed in more than one layer touching each other. Values for such installations may be significantly lower and must be determined by an appropriate method.
BS 7671:2018+A4:2026, Table 4C5, Note 2
Strength. The side height is part of what makes the tray stiff, so it belongs with the load and span data rather than with the cable schedule. Take the permissible span and safe working load from the manufacturer’s published data for the specific product and duty, classified to BS EN 61537.
Bunched runs. For data and AV the usable depth is an input to the fill calculation, not a leftover. Set it deliberately.
3.0 Grouping (Cg)
Cables in a group have reduced heat dissipation and affect each other. This is why you need to find correct BS 7671 rating factor, Cg, applied to the tabulated current-carrying capacity, so the grouped rating is Iz = It × Cg.
Select the correct grouping factor. The choice is based on arrangement, cable type, circuit type, containment type, and method of installation. Work through it in that order.
| Question | Answer |
|---|---|
| Multicore or single core? | Multicore, Method E. Single core, Method F |
| One tray or several? | One tray, Table 4C1. Several, Table 4C4 (multicore) or Table 4C5 (single core) |
| Tray, ladder or cleats? | Separate rows in each table. Ladder and cleats are treated more favourably than perforated tray |
| Touching or spaced? | Spaced rows, and above a clearance of twice the overall diameter, no factor at all (Note 2) |
| Which cables count? | The power circuits in the group. Data, AV and ELV runs on the same bracket are not part of it |
Table 1 One table, four arrangements, five answers for the same six circuits.
| Table 4C1 item | Arrangement, cables touching | Cg at 6 circuits |
|---|---|---|
| 1 | Bunched in air, on a surface, embedded or enclosed | 0.57 |
| 2 | Single layer on wall or floor | 0.72 |
| 3 | Single layer multicore on a perforated horizontal or vertical cable tray system | 0.73 |
| 4 | Single layer multicore on cable ladder system or cleats etc. | 0.79 |
| Note 2 | Horizontal clearance greater than twice the overall diameter | none applied |
Reference: BS 7671:2018+A4:2026, Table 4C1, items 1 to 4 and Note 2.
Where horizontal clearances between adjacent cables exceed twice their overall diameter, no rating factor need be applied.
BS 7671:2018+A4:2026, Table 4C1, Note 2
Worked example.
Six identical circuits, touching, single layer, on a 300 mm perforated tray. Design current Ib = 80 A per circuit. Cable tabulated at 110 A in free air.
# item 3, single layer on perforated tray Iz = 110 × 0.73 = 80.3 A ≥ 80 A passes, by 0.3 A # item 1, bunched Iz = 110 × 0.57 = 62.7 A < 80 A fails # Note 2, clearance > 2 × overall diameter Iz = 110 A no factor applied
Tiers. One tray uses the factor from Table 4C1. More than one tray uses Table 4C4 (multicore, Method E) or Table 4C5 (single-core circuits, Method F), by number of trays and cables per tray. Both tables tabulate their values for a stated tray spacing.
Values are given for vertical spacing between cable trays of 300 mm and at least 20 mm between cable trays and wall. For closer spacing the factors should be reduced.
BS 7671:2018+A4:2026, Table 4C5, Note 3
A three-tier bracket at 225 mm tier centres is closer than the tabulated arrangement, so the factor is reduced and the reduction recorded.
Which cables count. Cg applies to the power circuits in the group. Data, AV and ELV runs on the same bracket are not part of it. They still load the rods.
The Cable Tray Sizing Calculator selects the table, the row and the factor from the cables entered, and states which it used.
4.0 Fixing and fault restraint
Cables on a horizontal tray are still clipped at the spacings for the cable, not at the spacings for the tray. Table D1 limit them by overall diameter and construction.
Table 2 Maximum spacings of clips, cables in accessible positions (mm).
| Overall diameter d (mm) | Non-armoured, horizontal | Non-armoured, vertical | Armoured, horizontal | Armoured, vertical |
|---|---|---|---|---|
| d ≤ 9 | 250 | 400 | not tabulated | not tabulated |
| 9 < d ≤ 15 | 300 | 400 | 350 | 450 |
| 15 < d ≤ 20 | 350 | 450 | 400 | 550 |
| 20 < d ≤ 40 | 400 | 550 | 450 | 600 |
Reference: IET On-Site Guide, Appendix D, Table D1, spacings of supports for cables in accessible positions.
Above 40 mm overall diameter the manufacturer’s recommendation applies. Horizontal values also apply to runs more than 30 degrees from the vertical.
Fault restraint. Under a short circuit, parallel conductors repel or attract with a force proportional to the square of the peak current, and the cleats and their fixings hold it. The formula is written for the force between conductors at a spacing S, so it is single-core runs that drive cleat selection. Two numbers set that force, and neither of them is the kA printed on the protective device. The first is the prospective fault current at the point of installation, measured or calculated. The second is the peak factor.
The distinction that gets missed. A device rating is a breaking capacity for the device. It is not a current that flows at the tray. The force on a cleat comes from the current that actually flows at that point on the route, Ipf, from IEC 60909 or a board measurement.
Peak is not the r.m.s. value, and the multiplier depends on the fault level.
Table 3 Peak factor n, where peak current = n × r.m.s. short-circuit current.
| r.m.s. short-circuit current, kA | cos φ | n |
|---|---|---|
| I ≤ 5 | 0.7 | 1.5 |
| 5 < I ≤ 10 | 0.5 | 1.7 |
| 10 < I ≤ 20 | 0.3 | 2 |
| 20 < I ≤ 50 | 0.25 | 2.1 |
| 50 < I | 0.2 | 2.2 |
Reference: BS EN 61439-1, Table 7, values for the factor n.
For reference the calculation of the forces between two conductors can be made using the formula given in BS EN 61914:2009: F = 0.17 x (ip)² / s. Where: F = force in N m-1, ip = peak prospective short circuit current in kA, S = spacing between the conductors in m.
BEAMA, Cable Ladder and Cable Tray Systems Best Practice Guide, section 2.16, p.47
Worked example.
Single-core 300 mm² AWA, 33 mm overall diameter, trefoil and touching, so the conductor spacing S = 0.033 m. Cleats at 300 mm centres.
# board A: Ipf = 6.12 kA r.m.s. n (Table 7, 5 < I ≤ 10) = 1.7 ip = 1.7 × 6.12 = 10.4 kA peak F = 0.17 × 10.4² / 0.033 = 558 N/m per cleat at 300 mm = 167 N # board B: Ipf = 25 kA r.m.s. n (Table 7, 20 < I ≤ 50) = 2.1 ip = 2.1 × 25 = 52.5 kA peak F = 0.17 × 52.5² / 0.033 = 14 199 N/m per cleat at 300 mm = 4.26 kN
Select the cleat from products type-tested to BS EN 61914 at the calculated force and the declared spacing. Note what the calculated figure does not cover.
However the ‘F Value’ is the force within the ‘loop’ of the cleat and does not indicate how much of this force transfers into the structure, or containment, which the cleat is fastened to. Hence the only certain way to assess that the cable support system is strong enough to resist the mechanical force is by testing.
BEAMA, Cable Ladder and Cable Tray Systems Best Practice Guide, section 2.16, p.47
The calculated force is what the cleat is selected against. It does not certify the tray.
5.0 Supports and drop rods
The load path runs from the cables, through the tray and the bearer, into two drop rods and the fixings above them.
Support spacing. Take the permissible span from the manufacturer’s load and span data for the specific product and duty, classified to BS EN 61537. 1.2 m is a common working figure and it is a convention, not a compliant value in itself.
Rod sizing.
If mild steel drop rods are to form the suspension element of the support system, their cross-sectional area should be determined in accordance with Annex E.
BS 8519:2020, clause 16, cable support systems
Annex E sets the allowable stress in fire conditions and tabulates the resulting static load per rod. Annex I gives the loading calculation. Two conditions in Annex E are worth reading before the numbers.
The cable systems should contain a spare capacity allowance for future system extension in the region of additional 20% to 30%; this should be confirmed in the relevant project requirements and taken into account when analysing the support system loadbearing ability in fire.
BS 8519:2020, Annex E
This annex assumes that the mechanical load is evenly distributed on the support system bearers and the two suspension threaded drop rods on each bearer are equally loaded. This might not be the case for the cable support system or the cable protective system and this is to be accounted for, either by stipulating that the cables installed on the support system be installed in such a way as to ensure even loading of the bearers or an additional safety factor included to account for the uneven load distribution.
BS 8519:2020, Annex E
Worked example. Six single-core 300 mm² AWA (33 mm, 3.938 kg/m each) on a 300 mm medium-duty tray at 3.07 kg/m. Supports at 1.2 m, spare capacity 25%, rod centres 450 mm, bearer 2.2 kg/m.
# load arriving at one support cables 6 × 3.938 × 1.2 × 1.25 = 35.45 kg tray 3.07 × 1.2 = 3.68 kg bearer 2.2 × 0.45 = 0.99 kg total = 40.12 kg per rod, load centred = 20.06 kg # rod selection ambient, M8 safe working load 168 kg passes fire route 30 or 60 min, M8 (Table E.2) 27.70 kg passes fire route 120 min, M8 (Table E.2) 18.47 kg fails fire route 120 min, M10 29.66 kg passes
Same bracket, same cables: the rod goes from M8 to M10 on the fire survival time alone. Move the tray off centre, to 280 mm from one rod on the 450 mm span, and that rod takes 24.84 kg rather than 20.06 kg.
Where the route is fire-rated, size the supports for the cable category set by the fire strategy: Cat 1 (30 min escape), Cat 2 (60 min escape) or Cat 3 (120 min fire-fighting).
6.0 Segregation on a shared bracket
Band I covers SELV, PELV, telecommunications, data and signalling. Band II covers low voltage. They may not share a wiring system unless one of six methods is adopted.
Except where one of the following methods is adopted, neither a voltage Band I nor a voltage Band II circuit shall be contained in the same wiring system as a circuit of nominal voltage exceeding that of low voltage, and a Band I circuit shall not be contained in the same wiring system as a Band II circuit:
1. every cable or conductor is insulated for the highest voltage present;
2. each conductor of a multicore cable is insulated for the highest voltage present in the cable;
3. the cables are insulated for their system voltage and installed in a separate compartment of a cable ducting or cable trunking system;
4. the cables are installed on a cable tray system where physical separation is provided by a partition;
5. a separate conduit, trunking or ducting system is employed;
6. for a multicore cable, the cores of the Band I circuit are separated from the cores of the Band II circuit by an earthed metal screen of equivalent current-carrying capacity to that of the largest core of a Band II circuit.
BS 7671:2018+A4:2026, Regulation 528.1
On a tray, method 4 is the one that gets drawn: a partition, shown on the containment drawing and repeated in the BWIC information. A separate tier on the same bracket is a separate wiring system, which is why mixed brackets are tiered by service.
Fire detection and alarm and emergency lighting circuits have their own separation requirements in BS 5839-1 and BS 5266-1. Check those against the fire strategy before fixing the tier arrangement.
7.0 Before you issue
| Check | Confirm | Reference |
|---|---|---|
| Width and height | Single layer with spare, formation allowed for, standard size, opening coordinated | BS 7671 App 4; BS EN 61537 |
| Grouping Cg | Correct table and row for the real arrangement, tier spacing checked, Iz = It × Cg ≥ Ib for every cable | BS 7671 Tables 4C1, 4C4, 4C5 |
| Segregation | Band I and Band II separated by one of the six methods, and the partition drawn | BS 7671, Regulation 528.1 |
| Fixing spacing | Clip centres within Table D1 for the diameter and construction | IET On-Site Guide, Table D1 |
| Fault restraint | Ipf at the tray, peak factor from the fault level, cleat type-tested at the calculated force | BS EN 61914; BS EN 61439-1 Table 7 |
| Supports and rods | Span within the manufacturer’s data, all services on the bracket summed, rod sized for the fire survival time | BS EN 61537; BS 8519:2020 Annex E and I |
8.0 Clarification
Support spacing of 1.2 m is common practice, not a compliant value. The permissible span comes from the manufacturer’s load and span data to BS EN 61537 for the specific product and duty.
The rod loads in Table E.2 are described by the standard itself as indicative.
The details included in Table E.2 are intended to be indicative, and the designer should identify the specific type of threaded rod to be installed and the relevant performance data.
BS 8519:2020, Annex E
The calculated cleat force is the force in the loop of the cleat, not the force into the tray. Only a test confirms the tray.
Cable Tray Sizing Calculator
The checks in this guide, run against your own cable schedule, with the clause behind every answer on screen.
9.0 References
- BS 7671:2018+A4:2026, Requirements for Electrical Installations (IET Wiring Regulations). Appendix 4; Tables 4C1, 4C4, 4C5; Regulation 528.1.
- BS EN 61914, cable cleats for electrical installations. Force between parallel conductors.
- BEAMA, Cable Ladder and Cable Tray Systems Including Channel Support Systems and other Associated Supports, Best Practice Guide, section 2.16.
- BS EN 61439-1, low-voltage switchgear and controlgear assemblies. Table 7, values for the factor n.
- BS 8519:2020, selection and installation of fire-resistant power and control cable systems. Clause 16, Annex E, Table E.2, Annex I.
- IET On-Site Guide, Appendix D, Table D1, cable support and fixing.
- BS EN 61537, cable tray and cable ladder systems; manufacturer load and span data.
- BS EN 50174-2, information technology, cabling installation planning and practices inside buildings.
- BS 5839-1 and BS 5266-1, for the separation requirements applying to fire alarm and emergency lighting circuits.
- IEC 60909, short-circuit currents in three-phase a.c. systems.
Worked figures are illustrative and rounded for the article. Cable ratings, diameters and grouping factors vary by manufacturer and installation; size against the current standard and the specific product data.
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