How to Size a Cable to AS/NZS 3008 — Free Cable Size & Voltage Drop Calculator

Undersized cable is one of the most common — and most expensive — mistakes in electrical work. It overheats, trips protection, dims the lights in the shed, and fails inspection. Our free cable size and voltage drop calculator applies the AS/NZS 3008.1.1 method for copper cables under Australian conditions and shows every candidate size side by side, so you can see exactly why a size passes or fails. You'll find the calculator embedded at the bottom of this guide, ready to use — or open the full calculator page.

How to use the calculator: step by step

Step 1 — Describe the load

Choose single-phase (230V) or three-phase (400V), then enter the load in amps — or in kilowatts with a power factor and the calculator converts it. If you don't know the load yet, work it out first with our maximum demand calculator.

Step 2 — Enter the route length and drop limit

Length is the one-way cable route, following the actual path the cable takes — not the straight-line distance. AS/NZS 3000 allows 5% voltage drop in total from the point of supply to the furthest point, shared across mains, sub-mains and final subcircuits, so most designers budget 1–3% for any one run. The default of 3% suits most sub-mains and final subcircuits.

Step 3 — Pick the cable and installation method

Choose the insulation (XLPE or PVC), construction (multicore like flat TPS and orange circular, or single cores), and how the cable is installed — clipped to a surface, in conduit in air, buried direct, in an underground conduit, or in contact with thermal insulation. The same cable carries very different current in each situation: a 16mm² XLPE two-core is good for about 97A clipped in air, but far less when it's surrounded by ceiling insulation.

Step 4 — Apply derating if it applies

If other circuits run bunched with yours, or the ambient temperature is above the standard 40°C, select those conditions and the calculator derates the capacity automatically — two touching circuits drop to 80%, three to 70%.

Step 5 — Read the result

The calculator recommends the smallest compliant size and tells you whether current or voltage drop governed the choice. The comparison table shows every size with its derated capacity, voltage drop, maximum run length and the minimum earth conductor from Table 5.1 of AS/NZS 3000 — so you can see how close the margins are before you commit to a cable order.

Common Sydney scenarios

A 63A single-phase sub-main to a garage 30 metres away lands on 16mm² XLPE. A 100A consumer main run 40 metres needs 35mm² single cores once voltage drop is counted. And a modest 32A circuit to a shed 80 metres down the back? Voltage drop pushes it two sizes up from what the current alone would need. Long runs are almost always voltage-drop problems — the calculator's "max run" column makes that obvious at a glance.

When to call a licensed electrician

The calculator covers current capacity and voltage drop — the two big sizing checks — but a compliant design also verifies earth-fault loop impedance, short-circuit protection and your distributor's requirements, and all fixed wiring in NSW must be installed by a licensed electrician. Consumer mains, metering and anything between the street and your main switch is Level 2 ASP work, which is exactly what we do.

High Demand Electrical sizes, supplies and installs compliant cabling across Sydney every day — consumer mains to 400A, sub-mains to sheds and granny flats, private power poles and underground runs. Call 0402 559 777 and bring your calculator result; we'll confirm it and quote the install.

This guide is general information for the Australian market and refers to AS/NZS 3008.1.1 and AS/NZS 3000:2018. It is not a substitute for a design check by a licensed electrician.

Try the cable size calculator

Use the calculator below — no sign-up, no watermark. Print your result and bring it to us to confirm.