Free Cable Size & Voltage Drop Calculator — AS/NZS 3008
Long Run or Big Load? We'll Size and Install It.
From consumer mains and sub-mains to private pole and underground runs, our Sydney Level 2 ASP team sizes, supplies and installs compliant cable up to 400A — and we handle the network paperwork for you.
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Cable Size Calculator — AS/NZS 3008.1.1, Australian Conditions
How to Use the Cable Size Calculator
Step 1 — Describe the load. Pick single-phase (230V) or three-phase (400V) and enter the load in amps, or in kilowatts with a power factor. If you haven't worked out the load yet, run our maximum demand calculator first and bring the result here.
Step 2 — Enter the route length and your voltage-drop limit. Measure the actual path the cable will take, one way — not the straight-line distance. The 3% default suits most sub-mains and final subcircuits; AS/NZS 3000 allows 5% in total from the point of supply to the furthest point, shared across the whole installation.
Step 3 — Pick the cable and installation method. Choose XLPE (X-90) or PVC insulation, multicore (flat TPS or orange circular) or single cores, and how it's installed — clipped, in conduit, buried direct, underground duct, or in contact with thermal insulation. If other circuits run bunched with yours or the ambient is hotter than 40°C, set those too and the capacity is derated automatically.
Step 4 — Read the result. The calculator recommends the smallest compliant copper size, tells you whether current or voltage drop governed it, and lists every size with its capacity, drop, maximum run length and minimum earth from Table 5.1 — so you can see the margins before ordering cable.
Remember: this covers the two big sizing checks, but a compliant install also needs fault-loop, short-circuit and distributor checks — and all fixed wiring in NSW must be installed by a licensed electrician. Consumer mains and supply-side work is Level 2 ASP territory, which is exactly what we do across Sydney. Bring us your result and we'll confirm it and quote the job.
Read the full step-by-step cable sizing guide →FAQs
Explore our frequently asked questions to learn more about the specialised services provided by our certified Level 2 Electricians.
Two checks decide it: the cable must carry your load current without overheating (current-carrying capacity, from the AS/NZS 3008 tables for your installation method), and the voltage drop over the run must stay within limits. Whichever check demands the bigger cable wins — the calculator shows you both side by side.
Voltage drop = current × route length × the cable's millivolt-per-amp-metre value from AS/NZS 3008, divided by 1000. A 63A load over 30 metres of 16mm² copper drops about 5.6 volts on single-phase — around 2.4%. The calculator does this for every size and shows the maximum run length too.
AS/NZS 3000 allows 5% in total from the point of supply to the furthest point of the installation — in practice that budget is shared between consumer mains, sub-mains and final subcircuits, which is why designers commonly target 1–3% for each part. Your distributor and the NSW Service and Installation Rules can impose their own limits on consumer mains.
The insulation's temperature rating. Standard V-90 PVC cables are rated to run at 75°C, while X-90 XLPE runs at 90°C — so the same copper size carries more current in XLPE. A 16mm² two-core clipped in air is good for about 80A in PVC but 97A in XLPE. XLPE is the usual choice for mains and sub-mains.
The table ratings assume one circuit at 40°C air or 25°C soil. Run cables bunched together, in a hot roof space, or through thermal insulation and they can't shed heat as well, so their capacity is reduced by derating factors. Two touching circuits are derated to 80%, three to 70% — the calculator applies these for you.
Table 5.1 of AS/NZS 3000 sets the minimum earth for each active size — for example 6mm² earth with 16mm² or 25mm² actives, and 10mm² with 35mm². The calculator shows the minimum earth with every result. A larger earth is sometimes needed for fault-loop impedance or mechanical protection.
For a typical short run, 16mm² copper XLPE clipped in air carries 97A and handles 63A comfortably. But stretch the run to 50 metres or more and voltage drop pushes you to 25mm² or bigger — that's exactly the trade-off this calculator shows you instantly.
Commonly 25mm² or 35mm² copper single cores depending on the run length and installation method — at 40 metres, voltage drop already pushes a 100A single-phase supply to 35mm². Consumer mains are Level 2 ASP work in NSW, and we size, install and connect them daily.
Yes — on long runs voltage drop, not current, usually decides the size. Doubling the route length doubles the drop, so a shed 80 metres from the house often needs cable two or three sizes larger than the load alone would suggest. Undersized long runs cause dim lights, tripping and appliance problems.
No — it's an estimating and learning tool built on the AS/NZS 3008 method. A full design also checks fault-loop impedance, short-circuit rating, protection coordination and your distributor's requirements. Bring your result to us and we'll confirm and install it compliantly.
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