Cable Sizing Calculator (AS/NZS 3008)
Estimate the minimum conductor cross-section needed to keep voltage drop within your allowable limit for a single-phase, two-wire run. It uses the AS/NZS 3008 style voltage-drop relationship so you can pick a starting cable size before checking current-carrying capacity.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the load current in amps and the one-way cable run length in metres.
- Enter the maximum voltage drop you will allow across the run, in volts.
- Leave resistivity at 0.0225 ohm.mm2/m for copper, or enter your own value (e.g. ~0.036 for aluminium), then read the required cross-section and round up to the next standard size.
How it works
For a single-phase, two-wire circuit both the active and neutral carry the current, so the loop length is twice the one-way run. The required cross-section is csa = (2 x rho x L x I) / Vd, where rho is conductor resistivity, L the one-way length, I the load current and Vd the allowable voltage drop. Larger current, longer runs or a tighter voltage-drop budget all demand a bigger conductor.
Worked example
Worked example. For a 32 A load over a 30 m run with a 5 V allowable drop and copper (rho = 0.0225): csa = (2 x 0.0225 x 30 x 32) / 5 = 43.2 / 5 = 8.64 mm2. You would select the next standard size up, 10 mm2, then verify its current-carrying capacity.
Common mistakes
- Entering the total loop length instead of the one-way run — the formula already doubles it.
- Treating the raw result as a real cable size; always round UP to the next standard cross-section.
- Ignoring current-carrying capacity and derating — voltage drop is only one of the checks AS/NZS 3008 requires.
Frequently asked questions
Does this pick my final cable size?
No. It gives the minimum cross-section for voltage drop only. You must round up to a standard size and then separately confirm the cable's current-carrying capacity with grouping, ambient and install-method derating.
What resistivity should I use?
About 0.0225 ohm.mm2/m for copper at typical operating temperature, or roughly 0.036 for aluminium. Standards use temperature-corrected values, so treat the result as indicative.
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