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Engineering Tool

Cable Sizing Calculator

Find the minimum conductor size that keeps a DC string or AC run under its voltage-drop limit, then round up to the next standard cable size, with the resulting drop shown.

Sources listedReviewed July 2026Our methodology

Inputs

View result
One way; the tool adds the return path.
Enter the project criterion that applies to this circuit.
String operating voltage (DC) or nominal AC voltage.

Live result

Edit inputs

Minimum area

1.93mm²

Standard size

2.5mm²

Drop at that size

0.77%

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Key takeaways
  • Sizes a conductor for voltage drop only: the minimum cross-sectional area that keeps drop under your target limit, then the next standard size up.
  • This is not a current-carrying-capacity (CCC) check. You must still confirm the conductor's rating against AS/NZS 3008.1 tables for your install method, grouping and ambient temperature.
  • The voltage-drop target is supplied by the user; the tool does not infer which design criterion applies.

How it works

This sizes a conductor for voltage drop only. Rearranging the voltage-drop formula for the minimum cross-sectional area:

  • A_min = (k × length × current × ρ) ÷ V_drop, where k = 2 for a DC or single-phase two-wire run (active + return) or √3 for three-phase, ρ is resistivity (~0.0175 Ω·mm²/m copper, ~0.0282 aluminium near 20 °C), and V_drop is the allowed drop in volts (drop % × system voltage ÷ 100).
  • The result is rounded up to the next standard conductor size (1.5, 2.5, 4, 6, 10, 16 mm²…), and the drop that size actually achieves is shown.

Enter the voltage-drop criterion that applies to the actual circuit and project. The tool deliberately does not infer a compliance limit. Treat the result as a first check, not a cable schedule.

Important. This is not a current-carrying-capacity check. A conductor must also carry its load current safely for the install method, cable grouping, insulation and ambient temperature. Confirm the current-carrying capacity against the AS/NZS 3008.1 tables (this tool does not reproduce them), and have a licensed electrician sign off the final design.

Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.

Source ledger2 sources

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AS/NZS 3008.1StandardAS/NZS 5033Standard

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Frequently asked

Does this size the cable for current-carrying capacity?
No. It sizes purely for voltage drop. Current-carrying capacity depends on install method, cable grouping, insulation and ambient temperature, and must be checked against the AS/NZS 3008.1 tables. This tool does not reproduce those tables.
Which resistivity does it use?
The tool discloses fixed copper and aluminium resistivity inputs in its method. A complete design must instead use the applicable conductor data and AS/NZS 3008.1 tables for the cable, installation method and design conditions.
Is it free, and is my data stored?
It is free with no sign-up. The calculation runs in your browser, and nothing is sent to Wattbench unless you sign in and deliberately select Save result.
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