Cable size calculator — AS/NZS 3008 with mandatory 5% drop

Use the calculator

Pick the AS/NZS preset, enter load current, run length, and your installation method. The calculator returns the smallest mm² cable from the AS/NZS ladder that satisfies both ampacity and the 5% drop limit, plus the recommended circuit-breaker rating.

The two AS/NZS checks

Cable selection in Australia and New Zealand involves two independent tests; both must pass.

For three-phase, the leading factor is √3 instead of 2 and L·I gives line-to-line drop. For DC and small-cable single-phase, reactance X is negligible (X ≈ 0) and the formula collapses to V_drop = 2·L·I·R.

The 5% is on the combined drop from the consumer mains to the load — usually split as 1–2% across the consumer mains and 3–4% across the sub-mains and final sub-circuits. Voltage drop usually controls cable size for runs over 30 m and for low-voltage DC; ampacity controls short, hot-environment runs.

How to size cable to AS/NZS 3008, step by step

1. Compute the design current. For a single load, design current is the rated current. For a panel feeder, sum the individual load currents and apply the diversity factor allowed by AS/NZS 3000 Section 2.7. Continuous loads (EV chargers, hot-water systems, fixed lighting ≥ 3 hr) take the full rated current with no diversity.
2. Identify the installation method. Pick the AS/NZS 3008 method letter that matches your route: B1 conduit on a wall (most common indoor), C clipped direct, D buried direct or in conduit underground, E perforated cable tray. The method changes ampacity by up to 30% for the same mm² — pick honestly.
3. Apply ambient and grouping correction factors. AS/NZS 3008 Tables 27 and 22 give correction factors. Australian summers, roof cavities, and buried installations all need correction. Multiply tabulated ampacity by f_T (ambient) × f_G (soil thermal resistivity, buried only) × f_N (grouping) to get the working ampacity.
4. Pick the smallest cable whose corrected ampacity ≥ design current. Use the AS/NZS 3008 ampacity tables (or the embedded calculator) to find the smallest standard mm² cable that passes ampacity. Standard sizes per IEC 60228: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400 mm².
5. Verify the 5% voltage-drop limit (AS/NZS 3000 Cl. 3.6.2). Compute the drop from point of supply to point of use using AC resistance and reactance from AS/NZS 3008 Tables 30 and 35. Total drop must not exceed 5% of nominal voltage (230 V single-phase, 400 V three-phase). If it fails, step up the cable size and recompute. The 5% limit is mandatory, not advisory.
6. Pick the protective device and verify the loop impedance. Round design current up to the next standard MCB rating (10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 A). Verify earth-fault loop impedance at the load is below the AS/NZS 3000 Section 3 maximum to ensure the MCB trips within the required disconnection time (0.4 s for final sub-circuits, 5 s for distribution circuits).

Standard cable sizes (IEC 60228 / AS/NZS)

The voltage drop between the point of supply for the low-voltage installation and any point in that installation shall not exceed 5% of the nominal voltage at the point of supply.

Aluminium cable size chart (AS/NZS 3008)

Aluminium consumer mains and large feeders are common in Australian dwellings — typically AA-8000 alloy with anti-oxidant compound and torque-checked terminations.

Aluminium needs roughly one IEC standard size larger than copper for the same ampacity. The cost-per-amp-metre is still ~30% lower than copper for sizes ≥ 35 mm², which is why most consumer-mains feeders ≥ 100 A in Australia are aluminium.

Installation methods (the most-used codes)

AS/NZS 3008 selects ampacity by installation method letter. The five most common in low-voltage installations:

The installation method dominates the ampacity for a given mm² — buried cable carries 30% more current than the same cable in conduit on a hot wall, simply because the soil dissipates heat better than air. Pick your method honestly; over-stating cooling is one of the most common AS/NZS sizing errors and leads to overheated cable in service.

AS/NZS 3008 vs NEC — what is different

If you are sizing for both Australian and US projects, the table below summarises the key differences.

Variants and special cases

Solar PV (AS/NZS 5033)

Solar string conductors must be sized to 1.25× short-circuit current (Isc) after a temperature correction for the typical 70°C cable temperature. AS/NZS 5033 Clause 4.3 requires X-90 or rated-PV cable for above-ground PV runs. The 5% AS/NZS 3000 voltage-drop limit applies to the AC side; the PV side commonly targets 2% to maximise harvest.

EV charging

AS/NZS 3000 treats EV charging as a continuous load and requires the cable to be sized for the rated current with no diversity factor. A 32 A continuous EV charger needs a 4 mm² Cu V90 cable in B1 method (or 6 mm² for runs over 25 m to keep 5% drop). Multiple chargers on a shared submain require a load-management calculation.

Three-phase installations

Three-phase reduces voltage drop dramatically because the line-to-line voltage (400 V) is 1.73× the phase voltage (230 V). For the same load in kW, three-phase pulls less line current and the drop along the conductor is correspondingly smaller. Most commercial AS/NZS sizing exercises start with three-phase as the default for any submain over ~50 A.

Marine, caravan, and automotive battery cable

AS/NZS 3001 (Caravans) and AS/NZS 3004 (Marine) extend AS/NZS 3000 with stricter rules for vibration, corrosion, and isolation. Tin-plated finely-stranded copper cable is required for marine; standard V90 is acceptable for caravans but with reduced ambient corrections. Automotive battery cable sizes follow the same DC-dominated voltage-drop logic — a 100 A starter draw at 12 V over a 2 m run is comfortable on 4 mm² (≈ 8 AWG), but inverter feeds from a 12 V battery bank to a 1 kW continuous load typically use 25 mm² or 35 mm². The calculator above runs the DC variant for any 12 V / 24 V battery cable size chart lookup.

Earthing and protective earth conductors

An earthing cable (protective earth, PE) sized to AS/NZS 3000 Section 5 is a function of the active conductor size, not load current — Table 5.1 maps the largest active in the circuit to the minimum protective earth size. Common pairings: 2.5 mm² active → 2.5 mm² PE; 6 mm² active → 6 mm² PE; 25 mm² active → 16 mm² PE; 95 mm² active → 50 mm² PE. Use the calculator for the active sizing first, then pick the PE row directly from Table 5.1 — never undersize the earth.

Related calculators and references

Frequently asked questions

How do I calculate cable size?

Two checks must both pass. Ampacity: tabulated ampacity from AS/NZS 3008 (by mm², insulation, installation method) × correction factors for ambient, soil, and grouping must be ≥ design current. Voltage drop: the drop from point of supply to load must be ≤ 5% per AS/NZS 3000 Clause 3.6.2. The smallest standard cable that satisfies both is the answer — that is what the calculator above produces automatically.

What is the maximum allowable voltage drop in AS/NZS 3000?

AS/NZS 3000 Clause 3.6.2 sets 5% from the point of supply (consumer mains) to any point of utilisation as the mandatory limit. This is a hard limit, unlike the NEC 3% branch / 5% combined target in the US, which is informational. Many distribution-network operators in Australia further restrict the consumer-mains drop to 1–2% so the sub-circuits have headroom.

How does AS/NZS 3008 differ from NEC 310.16?

Both are ampacity tables for cables of given size, material, and insulation. Differences: AS/NZS uses mm² (IEC 60228 standard sizes), NEC uses AWG. AS/NZS organises tables by installation method (in air, in conduit, buried direct, in cable tray) — NEC has a single Table 310.16 for "in raceway." AS/NZS treats the 5% drop as mandatory; NEC treats 3 / 5% as informational. Insulation classes use V90 / V75 / X-90 nomenclature in AS/NZS rather than the NEC THHN / THWN code.

What size cable for a 32 A circuit (Australia)?

4 mm² Cu V90 in conduit (installation method B1) carries 32 A at 30°C ambient under AS/NZS 3008 Table 14 — the standard 32 A submain or large appliance circuit. For long runs the 5% drop limit may push to 6 mm². Common alternatives: 2.5 mm² for 20 A, 6 mm² for 40 A, 10 mm² for 50 A, 16 mm² for 63 A, 25 mm² for 80 A, 35 mm² for 100 A subboard feed.

Do I need to derate for ambient temperature in Australia?

Yes. AS/NZS 3008 Table 27 gives ambient correction factors. Standard ampacity assumes 30°C in air or 25°C in soil; Australian and NZ summers commonly exceed both. Roof-cavity wiring in Northern Australia can sit at 50–60°C, where the V90 correction factor drops to ~0.65–0.71 — the cable carries about two-thirds of its tabulated current. The calculator on this page applies the correction automatically once you select the ambient.

What is the difference between V90 and V75 cable?

V90 (also "TPS V90" or "X-90") is PVC insulation rated 90°C continuous. V75 is the older 75°C grade. Both are PVC; V90 has a higher-melt-point compound. V90 is the modern standard for new work; V75 still appears in repair and legacy stock. X-90 is cross-linked polyethylene (XLPE), used for higher temperatures and some HV cable.

When should I use aluminium cable?

Aluminium is the standard for service entrance and large submain feeders ≥ 100 A in Australia. It is 30–40% cheaper than copper for the same ampacity and ~50% lighter — important on long pole-to-pole or pit-to-pit runs. Use AA-8000 alloy aluminium with anti-oxidant compound and torque-checked terminations per AS/NZS 3000. Sub-circuits and branch wiring stay copper.

Sources and methodology

1. Standards Australia / Standards New Zealand. AS/NZS 3008.1.1:2017 Electrical installations — Selection of cables.
2. Standards Australia / Standards New Zealand. AS/NZS 3000:2018 Electrical installations (Wiring Rules).
3. IEC. IEC 60228:2004 Conductors of insulated cables.
4. Standards Australia. AS/NZS 5033:2021 Installation and safety requirements for photovoltaic (PV) arrays.
5. Standards Australia. AS/NZS 3001:2008 Electrical installations — Connectable installations and supply (caravans, RVs).
6. Standards Australia. AS/NZS 3004.2:2014 Electrical installations — Marinas and recreational boats.