Can You Run Two Cables in Parallel Instead of One Bigger Cable in Sydney?

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Can you run two cables in parallel instead of one bigger cable in Sydney? Yes, AS/NZS 3000 allows two or more cables to share a circuit's current instead of installing one oversized cable, provided each cable is at least 4 square millimetres, the cables are grouped so their cooling is not affected, and the load current shares evenly between them. It is a genuine design option used on heavier submains and three phase feeders, but several conditions have to be met before the capacities of the separate cables can simply be added together.

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1. Why an installer would choose parallel cables over one bigger cable

Once a load gets large enough, the single cable size needed to carry it can become impractical to buy, bend, terminate or fit into existing conduit and switchboard gland plates. Running two or more smaller cables in parallel, each sharing part of the total current, is often a more practical and sometimes more economical way to deliver the same capacity, particularly on heavier power supply upgrades and single phase to three phase upgrades where the alternative single cable would be unwieldy to install through existing routes.

2. The minimum size and cooling conditions parallel cables must meet

AS/NZS 3000 Clause 3.4.3 sets three conditions before parallel cables are permitted at all. Every cable in the group must be not less than 4 square millimetres. Grouping the cables must not affect the cooling of each cable or group by the surrounding air or the ground, meaning they cannot simply be bundled so tightly that heat cannot escape. The load current sharing between each cable or group must be sufficient to prevent any single cable or group overheating, which is the condition that drives most of the detailed design work described below.

3. How equal load current sharing is actually achieved

The standard gives a clear example of how equal sharing is normally achieved. Conductors should be of the same material and cross sectional area, with a minimum size of 4 square millimetres. The cables should follow the same route and achieve the same length as each other, since a shorter or more direct cable will naturally want to carry more of the current. The conductors of each parallel cable or group should be effectively joined together at both ends. The relative position of phase and neutral conductors in and between parallel groups also needs to account for mutual impedance between the cables, which is why AS/NZS 3008.1 recommends trefoil groupings, keeping each phase and the neutral bundled together within every group rather than separated out by phase across different groups.

4. The exception for unequal load sharing

Unequal load current sharing between parallel cables or groups is permitted as an exception, but only where the design current and overcurrent protection requirements for each individual cable or group are assessed on their own terms rather than assumed to share evenly. This is a more involved design exercise, generally reserved for larger commercial installations where a licensed designer has calculated the actual current distribution rather than relying on the simpler equal sharing method most residential and light commercial jobs use.

5. Coordinating parallel cables with their overcurrent protection

Whichever sharing method is used, the combined current carrying capacity of the parallel cables still has to be coordinated with the protective device for the circuit, the same coordination rule that applies to any single cable under Clause 2.5.3. A circuit breaker's rating must not exceed the cable's current carrying capacity, and an HRC fuse's rating must not exceed 90 percent of it. Our guide to circuit breaker discrimination covers the related question of how breakers at different points in the same switchboard are coordinated with each other, which becomes more important on the larger boards where parallel cables typically appear. Correct neutral sizing also needs checking separately where harmonic loads are present, covered in our guide to neutral conductor sizing.

6. Where parallel cables typically show up in Sydney properties

Parallel cables are rarely needed for an ordinary household circuit, where a single cable size is almost always the simpler and cheaper option. They become a genuine consideration on heavier submains feeding a large shed, workshop or secondary dwelling, on three phase upgrades supplying machinery or EV charging infrastructure, and on commercial and industrial switchboards where the total demand current is large enough that a single cable size would be oversized for available conduit or gland plate space. Voltage drop still has to be checked over the full route either way, which our guide to voltage drop explains in more detail, and cable selection more broadly is covered in our AS/NZS 3008 cable sizing guide.

Why it's Level 2 ASP work

Deciding between parallel cables and a single larger cable is standard electrical design work for the wiring within a property. Where that decision affects the size or configuration of the consumer mains connection itself, or where a heavier switchboard upgrade requires a larger connection at the point of supply, that part of the job falls within Level 2 ASP accreditation, and a genuinely complex parallel cable design for a larger commercial site may also call for a Level 3 design from a qualified electrical engineer.

How High Demand Electrical helps

We are a licensed local team, not a call centre, and we work through the sizing, routing and load sharing calculations properly whenever a heavier circuit makes parallel cables worth considering, rather than defaulting to whichever option is quickest to quote. Servicing homes and businesses across Sydney's West, South West and Sutherland Shire, we regularly design submains and three phase feeders for sheds, workshops and commercial fitouts where cable size and conduit space genuinely matter. HD Inner West's guide to commercial three phase supply upgrades covers the related sizing considerations that come up on a heavier inner west fitout, where available roof and riser space often makes parallel cables worth comparing against one larger run.

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