How Many Cables Can You Run Together on a Cable Tray in a Sydney Commercial Fitout?

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How many cables can you run together on a cable tray before they need to be derated? AS/NZS 3008.1.1 sets separate derating tables for cables on trays, ladder supports, racks or cleats, Table 23 for single core cables and Table 24 for multicore cables, and the factor that applies depends on both how many circuits are bunched together and the type of support they sit on. A ladder support holds its rating far better than an enclosed, unperforated tray carrying the same number of circuits.

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1. Why bunched cables need a different current rating

Every current carrying capacity listed in AS/NZS 3008.1.1's main tables assumes a single, independently cooled circuit. The moment a second or third circuit sits close enough to share the same heat, each cable in the group runs hotter than it would alone, because its neighbours are adding heat to the same small volume of air or tray space rather than letting it dissipate freely. Clause 3.5.2.1 sets this out plainly, where a number of circuits are grouped in free air, on a surface, buried in the ground or in the same enclosure so that they are not independently cooled, a derating factor applies.

For cables run horizontally on a cable tray, ladder support, rack or cleat, that derating factor comes from Table 23 for single core cables or Table 24 for multicore cables, under Clause 3.5.2.3. It applies whenever a cable is installed in close proximity to another circuit's cable and either touching it or spaced closer than the minimum clearance set out in Clause 3.5.2.2.

2. Tray type changes the number more than people expect

AS/NZS 3008.1.1 does not treat every cable tray the same way, and the difference between an enclosed tray and an open one is bigger than most non electrical people assume. For multicore cables touching on a single tier, Table 24 gives an unperforated, solid tray a factor of 0.97 at two cables, falling to 0.68 once nine cables are bunched together. A perforated tray carrying the same nine cables sits at 0.73, and a ladder support, rack or cleat arrangement holds up best of all at 0.78, because far more air can move freely around each cable.

The pattern is similar for single core cables under Table 23. On an unperforated tray with three circuits per tier touching, the factor drops to 0.84. A ladder support carrying the same three circuits sits close to full rating at 0.94. That gap is exactly why the choice of tray, not just the cable size, is part of getting a commercial cable run right the first time, the same principle our guide to bunched circuit derating factors covers for cables grouped without a tray at all.

3. Spacing the cables apart is often the cheaper fix

AS/NZS 3008.1.1 also distinguishes between cables that are touching and cables that are spaced apart by at least one cable diameter. Where that minimum spacing in Clause 3.5.2.2 is maintained, no derating factor needs to be applied at all for some arrangements, and in others the reduction is far smaller than for a tightly bunched group. On a wide enough tray, simply spreading cables out with proper clearance can avoid the need to step up a cable size purely to compensate for grouping, which is often the more practical and cheaper outcome on a real job.

For the cable sizing fundamentals behind all of this, our guide to how AS/NZS 3008 cable sizing works is worth reading alongside this one. This is one of the first things worth checking on an existing commercial installation where extra circuits have been added to a tray over the years without anyone reviewing the original cable sizing. A tray that started with two or three well spaced circuits can end up heavily bunched after several rounds of additions, quietly pushing every cable on it past the condition its original sizing assumed.

4. Where this shows up on real Sydney jobs

Cable tray derating is most relevant on commercial and light industrial fitouts, plant rooms, switchrooms and comms rooms where several circuits genuinely do run together on a shared tray between a switchboard upgrade and the loads it feeds. A retail fitout feeding multiple tenancy switchboards from a shared riser, a warehouse with lighting and machinery circuits sharing a tray, or a commercial kitchen's dedicated equipment circuits are all typical examples where this calculation genuinely changes the cable size an electrician specifies.

It is less relevant to a standard single circuit run in a typical Sydney home, where cables are rarely bunched in anything like the numbers these tables address. It becomes genuinely important the moment a job moves into power supply upgrade territory with multiple new circuits sharing one route, or a three phase upgrade where several new submains leave the switchboard together along the same tray.

5. Getting the cable size wrong has real consequences

Ambient temperature compounds the same problem, since our guide to ambient temperature cable derating explains how a hot plant room pushes a cable's safe rating down further still. A cable sized without accounting for genuine bunching on a shared tray can run hotter than its insulation is rated for under sustained load, accelerating insulation ageing and increasing the risk of a fault developing over time. This is exactly the kind of error a verification test under Section 8 is designed to help catch during insulation resistance testing, but the better outcome is specifying the correct derated size at the design stage rather than relying on testing to flag a marginal installation after the fact.

Because the practical effect of tray type, spacing and bunching compounds quickly on a run with several circuits, this is genuinely specialist calculation work rather than a simple lookup, and getting it wrong on a commercial job can mean a cable that passes an initial insulation resistance test but still runs warmer than it should under real daily load.

How High Demand Electrical helps

High Demand Electrical is a licensed local team, not a call centre, and we size every multi circuit cable tray run to the actual bunching and tray type on site rather than a generic rule of thumb. We service commercial and light industrial fitouts across Sydney's West, South West and Sutherland Shire, and on a job with several circuits sharing a tray we will confirm the right Table 23 or Table 24 factor applies before the cable is pulled, not after. For a broader fitout involving a cafe or restaurant kitchen with multiple dedicated equipment circuits, HD Inner West's guide to switchboard planning for a cafe or restaurant fitout covers the switchboard side of the same kind of project.

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