Why Does Running Multiple Circuits in the Same Trench Reduce Their Safe Rating in Sydney?

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Why does running two underground circuits in the same trench reduce their safe current rating? AS/NZS 3008.1.1 Clause 3.5.2.5 requires a derating factor whenever buried cables from different circuits sit closer than 2 metres apart, because the surrounding soil cannot carry heat away from two loaded cables as effectively as it can from one on its own. The closer the spacing and the more circuits sharing the trench, the larger that reduction becomes.

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1. Soil cools a single cable differently to a group

A cable buried on its own dissipates heat into the surrounding soil in every direction. Once a second loaded cable is buried close enough, each cable starts competing for the same limited volume of cooler soil, and both run hotter than the standard's main current carrying capacity tables assume. AS/NZS 3008.1.1 Clause 3.5.2.5 addresses this directly for cables buried direct in the ground, and Clause 3.5.2.6 addresses the equivalent condition for cables inside underground wiring enclosures such as conduit.

The rule itself is simple to state. Where a single core or multicore cable is buried and separated by at least 2 metres from another circuit carrying a substantial current, no derating factor needs to be applied at all, since the soil between them is assumed to dissipate the heat adequately. Where that separation drops below 2 metres, a derating factor from Table 25(1), Table 25(2), Table 26(1) or Table 26(2) applies, depending on cable type and whether the cables sit direct in the ground or inside an enclosure.

2. How much the factor actually drops

The real numbers show how significant this can be on a trench carrying several circuits. For single core cables buried direct in trefoil formation under Table 25(1), two circuits spaced 0.15 metres apart sit at a factor of 0.78, falling to 0.61 once four circuits share that same close spacing. Widening the spacing to 0.60 metres recovers most of that capacity, with four circuits at 0.74 instead of 0.61. Multicore cables under Table 25(2) follow the same pattern, two cables at 0.15 metres sit at 0.81, improving to 0.93 once the spacing opens out to 0.60 metres.

Cables sharing an underground wiring enclosure rather than being buried direct use Table 26(1) for single core and Table 26(2) for single core or multicore cables sharing a conduit or similar enclosure, and the pattern is similar again, closer spacing and more circuits both push the factor down, while greater separation claws much of it back.

3. Trefoil versus flat matters for single core cables

For single core cables buried direct, AS/NZS 3008.1.1 distinguishes between cables grouped in trefoil, bundled together in a triangular cross section, and cables laid flat side by side. Table 25(1) gives trefoil grouping a noticeably better factor than the same number of cables laid flat at the same spacing. This is a genuine, practical choice an electrician can make at installation time that affects the final cable size needed for a given load, not just a theoretical distinction.

4. Where this shows up in real Sydney trenching

This derating factor is most relevant wherever more than one circuit genuinely shares a trench to save on excavation, a common and sensible approach on a job that involves a new underground service mains installation alongside a separate submain, or a private power pole feed trenched together with irrigation or outbuilding power in the same dig. It is distinct from the cover depth and mechanical protection rules that govern how deep a cable needs to sit and what marking tape or conduit it needs, since those rules apply regardless of whether anything else shares the trench.

A single consumer mains run on its own, more than 2 metres from any other loaded circuit, does not need this factor applied at all. It becomes relevant the moment a second or third circuit, whether that is a shed, a granny flat, a pool pump feed or a second dwelling's supply, is trenched in close alongside it to avoid a second excavation.

5. Getting it wrong undersizes the cable quietly

Soil type adds a further layer on top of this, as our guide to soil thermal resistivity and underground cable sizing explains. A trench dug once for convenience, with two or three circuits laid close together without anyone applying the correct Table 25 or Table 26 factor, can leave every cable in that trench running hotter than its design rating under real load, even though each individual cable would have been perfectly sized on its own. This is one of the less visible ways an otherwise straightforward job can end up with a genuine compliance gap, since the cables look correctly installed and pass a basic insulation resistance test, but were never actually sized for the condition they are operating in.

If you are planning any works that involve trenching for more than one circuit at a time, whether that is dialling before you dig for a private power pole feed or combining a consumer mains upgrade with a new outbuilding supply, it is worth raising the shared trench plan with your electrician before digging starts, not after the cables are already in the ground.

Why it's Level 2 ASP work

Where the shared trench carries the consumer mains or anything between the point of supply and the switchboard, sizing and certifying that section is Level 2 ASP work, since only an accredited Level 2 ASP can design, install and certify that network connected section. A standard submain to a shed or granny flat sharing the same trench still needs the correct AS/NZS 3008.1.1 factor applied, in the same way our guide to bunched circuit derating factors covers for cables grouped above ground, even though that circuit itself sits on the customer side of the installation.

How High Demand Electrical helps

High Demand Electrical plans every multi circuit trench with the correct AS/NZS 3008.1.1 derating factor applied from the start, rather than sizing each cable as if it were the only one in the ground. Servicing homes and businesses across Sydney's West, South West and Sutherland Shire, we will tell you plainly where combining circuits into one trench makes sense and where the spacing needed to avoid a heavy derating factor makes two separate runs the better option. If your project also involves a private power pole, HD Power Pole's guide to upgrading consumer mains alongside a private power pole covers the companion piece of that same job.

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