Most electricians don’t sit there with a copy of AS/NZS 3008 open on the bench every time they run a new circuit. They size from memory or a quick mental check, and most of the time that’s fine, right up until it isn’t. A long run to a shed, a bunched conduit run behind a wall, or a hot roof space can all push a “standard” 16A circuit well outside what a 2.5mm² cable can safely carry.
This calculator walks through the same logic as the standard itself: take your design current, apply the relevant derating factors for installation method and ambient temperature, then check the resulting cable size against your voltage drop limit. It won’t replace a proper AS3008 reference table for edge cases, but for the vast majority of domestic and light commercial runs it gets you to the right ballpark fast.
Get the sizing wrong and you’re either wasting copper on an oversized cable or, worse, running a cable that overheats under sustained load. Neither is something you want to discover after the job’s signed off.
How Derating Actually Changes Your Required Cable Size
AS3008 doesn’t size cables off the nameplate current alone. It applies correction factors for how the cable’s installed and how hot its environment gets, because both change how much heat the cable can shed before its insulation starts to degrade. Bury a cable direct in the ground and it actually copes better than expected, soil acts as a decent heat sink. Bunch four circuits together in one conduit, though, and each cable loses a meaningful chunk of its rated capacity because they’re all fighting for the same airspace.
Ambient temperature works the same way. A cable rated for a 30°C environment loses real capacity once you’re routing it through a 45°C roof cavity in summer, even if the conductor itself never reaches a temperature you’d call dangerous looking at it.
The Formula, Step by Step
The core derating step is simple division: take your design current and divide it by the combined installation and temperature factors. A 32A circuit in a bunched four-circuit conduit at 40°C ambient gets divided by roughly 0.7 times 0.82, which works out to about 0.574. That pushes the effective current you need to size for up to around 55.7A, nearly double the nameplate figure.
Once you’ve got that derated figure, you check it against the standard’s current-carrying capacity tables for your chosen cable size and insulation type, then separately verify voltage drop using the formula based on cable length, resistivity, and current.
A Worked Example From a Granny Flat Job
Take a 32A circuit feeding a granny flat sub-board, 45 metres away, run in conduit underground with three other circuits bunched together. Derate for bunching and you’re looking at an effective current in the mid-50s amps. That alone pushes you from what might have been a comfortable 6mm² up to 10mm² before voltage drop even enters the picture. Run the voltage drop check on top of that and a 45m run at 32A on three-phase can still land close to the 5% threshold even on a correctly sized cable, which is exactly why long sub-board feeds so often end up oversized for current alone, purely to keep voltage drop in check.
One thing that trips people up: the standard’s tables jump in fairly large increments (4mm² to 6mm² to 10mm²), so a derated current that’s only marginally over a table threshold still forces you up an entire cable size, sometimes adding real cost for what looks like a small margin on paper.
Avoiding the Most Common Sizing Mistakes
- Always check voltage drop separately from current capacity, a cable can pass one and fail the other
- Don’t forget to derate for grouped circuits even when each individual circuit is well under its rated current
- Roof space ambient temperatures are usually higher than people assume; measure or use a conservative figure
- Three-phase voltage drop calculations use a different multiplier than single-phase, mixing them up is a common error
None of this replaces a qualified electrician’s sign-off. AS3008 compliance on anything connected to the grid needs to be checked by someone licensed, and local switchboard or main supply constraints can change what’s actually achievable on site.
FAQs
What’s the difference between current-carrying capacity and voltage drop limits?
Current-carrying capacity is about how much current a cable can handle without overheating, while voltage drop is about how much voltage is lost along the cable’s length. A cable can pass one check and still fail the other.
Why does bunching circuits together reduce cable capacity?
Bunched cables share heat with each other, so each one has less spare thermal capacity than it would have running on its own.
Is 2.5mm² always enough for a standard power circuit?
Often, but not always — long runs, bunching, or high ambient temperatures can push the same 20A or 32A circuit beyond what 2.5mm² can safely carry.
What voltage drop percentage should I be aiming for?
5% is the general allowance for most circuits, but motor circuits and some sensitive equipment need a tighter 2.5% limit.
Does AS3008 cable sizing apply to solar installations?
Yes, DC cable runs from panels to inverters still need correct sizing for current capacity and voltage drop, though the specific tables used can differ from standard AC circuits.
Why is three-phase voltage drop calculated differently to single-phase?
Three-phase systems use a different multiplier in the voltage drop formula because the current is split across three conductors rather than carried on a single active and neutral pair.
Can I use this calculator instead of a proper AS3008 lookup table?
It’s a solid estimate for planning purposes, but final sizing on any real installation should be verified against the current edition of AS/NZS 3008 by a licensed electrician.
If you’ve got the design current and run length sorted, the actual sizing decision usually comes down to whichever check, capacity or voltage drop, is more restrictive for your specific run. Run both, take the larger cable size of the two, and you’re covered either way.