AS/NZS 3008 and Cable Sizing: Why Your Sparky Recommends a Bigger Cable Than You Expected

Why does your electrician sometimes recommend a bigger cable than you expected? Cable size in Australia is not a guess. It is calculated to AS/NZS 3008, the standard that sets the minimum size for both current carrying capacity and voltage drop. Two circuits carrying the same load can end up needing different cable sizes depending on how they are installed, how far they run, and how hot the surrounding space gets.

This guide walks through how AS/NZS 3008 actually works, why the bigger cable is usually the correct one, and how to check a job yourself using our free cable size calculator.

1. Two rules decide cable size, not one

AS/NZS 3008 requires a cable to pass two independent checks, and the final size is whichever check needs the larger conductor.

The first check is current carrying capacity. Every cable type and installation method has a tabulated rating in AS/NZS 3008, and the cable must be able to carry the design current without exceeding its maximum operating temperature.

The second check is voltage drop. Under AS/NZS 3000, the total voltage drop between the point of supply and any point in the installation should not exceed 5% of the nominal voltage. Over a long run, a cable that easily passes the current rating check can still fail on voltage drop, because resistance adds up over distance.

Most Sydney homes never notice this because their circuits are short. It becomes obvious on longer runs, such as a power supply upgrade to a rear granny flat, a workshop, or an EV charger circuit run from a switchboard at the front of the block.

2. How installation method changes the cable size

The same cable does not carry the same current in every situation. AS/NZS 3008 publishes different current ratings depending on whether a cable is:

• Clipped directly to a surface in free air
• Enclosed in conduit on a wall or in a stud cavity
• Buried directly underground or run in an underground enclosure
• Fixed to the underside of a ceiling
• Grouped alongside other current carrying cables

A 6 mm² cable clipped to a surface can carry meaningfully more current than the same 6 mm² cable buried underground or grouped tightly with several other circuits. This is one reason a switchboard upgrade quote might size cabling differently to what a general contractor expects. It is not overengineering, it is the standard doing its job.

3. Why voltage drop can force a bigger cable than current alone

Voltage drop is measured in millivolts per ampere metre, and AS/NZS 3008 provides tables for working it out based on cable type, installation method and load power factor. The longer the run and the higher the current, the more voltage is lost along the way.

This matters in practice for things like EV charger circuits run from a distant switchboard, sheds and workshops at the back of a block, and any circuit feeding equipment that needs a stable supply voltage to run properly. On these jobs, current rating alone would suggest a smaller cable than is actually needed, and a licensed electrician will size up to keep voltage drop within limits.

4. What derating actually removes from a cable's rating

Derating factors reduce a cable's tabulated current rating to account for real world conditions that push more heat into the conductor. The main ones a Sydney electrician deals with are:

• Ambient temperature, since Sydney summers regularly push roof space temperatures well above the standard's 40°C reference
• Grouping, when multiple current carrying cables run together and share heat
• Thermal insulation, which is the biggest single factor and can more than halve a cable's rating when it is fully surrounded

This is exactly why a cable that looks oversized on paper is often correctly sized once real installation conditions are factored in.

5. Try it yourself: our AS/NZS 3008 cable size calculator

Rather than working through the tables by hand, our free cable size calculator below applies the current rating and voltage drop method from AS/NZS 3008 directly. It covers copper and aluminium conductors, PVC and XLPE insulation, the ambient temperature and grouping derating factors, and suggests a matching circuit breaker size.

Why it's Level 2 ASP work

Cable sizing itself is standard electrical work, but it often comes up alongside jobs that sit squarely in Level 2 territory, such as consumer mains upgrades, private power pole connections, and any work that touches the point of supply. A Level 2 electrician is accredited to work on both sides of the meter, so sizing and installing the cable is handled as one job rather than being split between two trades.

How High Demand Electrical helps

Our team sizes every cable run to AS/NZS 3008 as a matter of course, whether that is a straightforward circuit or a full switchboard upgrade. We factor in the real installation conditions on your property, not just the load, so the cable you get is the cable the job actually needs. If you already have a size in mind from our calculator, we are happy to confirm it on site before any work starts.

Not sure what size cable your job needs? Get a power supply upgrade quote from High Demand Electrical, or call 0402 559 777 to talk it through. Every job still needs a site assessment before we can confirm final scope and pricing.