Why Do Bunched Circuits Need a Bigger Cable Size in Sydney?

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Why do bunched circuits need a bigger cable size in Sydney? When several current carrying cables are bunched together, in a conduit, on a tray or clipped in a tight group, the heat each cable gives off has nowhere to escape, so the whole group runs hotter than a single cable on its own. AS/NZS 3008.1.1 sets out exact derating factors for this in Table 22, and applying the correct factor is what decides whether your electrician specs a 2.5mm² circuit or steps up to 4mm² or 6mm² for the same load.

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1. Why grouping cables together changes their rating at all

A single cable clipped to a wall with open air on every side can shed the heat it generates easily, which is exactly how its base current rating in AS/NZS 3008.1.1 is calculated. The moment you bunch a second, third or fourth loaded cable hard up against it, each cable is now partly surrounded by other cables generating their own heat rather than cool air, so none of them can shed heat as efficiently. The practical result is that the safe continuous current for every cable in that bunch has to come down, even though nothing about the cable itself has changed.

2. The actual derating factors in Table 22

AS/NZS 3008.1.1 Table 22 sets derating factors for bunched circuits in two main arrangements relevant to most Sydney homes and small commercial jobs. For cables bunched in air, a single circuit stays at the full 1.00 factor, two circuits drop to 0.87, three to 0.75, four to 0.72, five to 0.70 and six to 0.67. For cables bunched on a surface or enclosed, which covers most conduit and cable tray runs, the drop is steeper: two circuits fall to 0.80, three to 0.70, four to 0.65, five to 0.60, six to 0.57, and the factor keeps falling as more circuits join the group, reaching 0.38 once you are bunching 20 or more circuits together. Our guide to AS/NZS 3008 cable sizing covers how this factor combines with voltage drop and ambient temperature to land on a final cable size.

3. Spacing cables instead of bunching them

Table 22 also covers what happens when cables are deliberately spaced apart rather than bunched. A single layer of cables touching on a wall or floor starts at 0.85 for two circuits and gradually settles around 0.70 for larger groups, noticeably better than the bunched on a surface figures above. Spacing the cables by at least one cable diameter improves this further, holding at 0.90 once there are three or more circuits in the group, under ceiling installations use their own, slightly lower figures again. This is one of the reasons an electrician will sometimes run cables along a wider path, or in a wider conduit, even though it looks like it uses more material. The up front cost of a wider run is often cheaper than stepping every cable up a full size to compensate for a tighter bunch.

4. Avoiding derating altogether with proper spacing

If you genuinely do not want to apply any grouping derating factor at all, AS/NZS 3008.1.1 Clause 3.5.2.2(c) sets the standard out: space the cables apart by a minimum of six times the diameter of the largest cable in the group, and the current carrying capacity tables apply exactly as published with no adjustment needed. This is rarely practical in a domestic wall cavity or a tight switchboard enclosure, which is exactly why Table 22's derating factors exist as the realistic, everyday alternative.

5. Cables inside switchboards and wiring enclosures

AS/NZS 3008.1.1 Clause 3.5.2.6 makes it clear that cables bunched inside an enclosure such as a switchboard are rated from the same unenclosed, in air current carrying tables, with the appropriate Table 22 derating factor applied once several circuits are grouped inside that enclosure. The standard gives a useful real world example: where nine circuits are bunched together inside an enclosure but only six are ever loaded at the same time, the derating factor that actually applies is the 0.57 figure for six loaded circuits, not the harsher figure for all nine. This is why an experienced electrician assessing your switchboard looks at which circuits are genuinely loaded together, not just how many cables are physically bunched inside the cabinet.

6. Parallel cables and cables passing through a group more than once

AS/NZS 3008.1.1 Clause 3.5.2.7 adds one more wrinkle: where a group of conductors forms a circuit that passes more than once through the same wiring enclosure or group of cables, or where conductors are connected in parallel to share a single load, each separate group is treated as its own circuit for the purposes of applying Table 22. Our guide to parallel cable runs covers the companion requirements for running two cables side by side to carry one load, including the minimum conductor size and equal current sharing rules that sit alongside this derating table.

7. What this means for your actual cable size

Grouping derating never acts alone. It stacks with the ambient temperature correction and any voltage drop allowance your circuit needs to meet under AS/NZS 3000's voltage drop rules, and the final cable size has to satisfy all three at once. A cable that comfortably carries a load on its own current rating can still fail once it is bunched with five other circuits in a roof space on a 40°C Sydney summer day, which is exactly why a proper load assessment looks at the whole wiring run, not just the appliance nameplate rating.

Why it's Level 2 ASP work

Applying Table 22 correctly on your consumer mains or any circuit feeding a three phase upgrade is Level 2 ASP territory, since an undersized bunched cable on the network side of your meter can overheat without ever tripping a protective device sized for the wrong assumptions. Getting the grouping, ambient temperature and voltage drop calculations right together is exactly the kind of detail a qualified Level 2 electrician is trained and accredited to get right the first time.

How High Demand Electrical helps

We are one of Sydney's most trusted Level 2 ASPs, and every cable we size on a job accounts for real world grouping, not just a textbook single cable rating pulled straight off a data sheet. Servicing homes and businesses across Sydney's West, South West and Sutherland Shire, our team regularly finds switchboards and roof spaces where cables were bunched together without the correct derating applied, usually on older installations that have had circuits added over the years without anyone reassessing the whole bundle. If you are planning a power supply upgrade or adding new circuits to an already busy switchboard, we will size every cable against the actual bunching it will sit in, not just the load it carries. HD Inner West's guide to light industrial and creative studio fitouts covers the similar cable grouping challenges that come up in commercial fitouts with multiple circuits sharing a single tray or conduit run.

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