By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After burning a 15mm cable on a 50-metre run to my shed in the heat, I learned that voltage drop kills more batteries than bad wiring ever will.”
Ensure safety and efficiency with the right cable choices for your Australian power setup.
Sizing cable correctly is the unglamorous half of any off-grid build. Get it wrong and your inverter trips, your pumps run hot, or worse — your cabling becomes the weakest link in the system. This guide covers the working spec you need on site: how to calculate current, how to apply AS/NZS 3008.1, what voltage drop actually costs you, and when to pick up the phone to a licensed sparky.
AS/NZS 3008.1:2006 is the standard that governs cable selection for Australian installations. It covers current-carrying capacity, voltage drop limits, derating factors for ambient temperature, installation method, and grouping. Every cable I pull on a job traces back to this standard — not the back of a beer coaster, and not a YouTube comment thread.
If you're sizing cable in Australia, this is your reference document. Anything else is a guess.
Before you touch a cable drum, you need the current. The basic relationship is:
Current (A) = Total wattage (W) ÷ Voltage (V)
For a 230V/50Hz single-phase run (which is the Australian default), divide your total wattage by 230. A 2,300W load at 230V draws 10A. A 4,600W load draws 20A. This gives the nameplate figure, but the cable must be sized on actual worst-case current, not the neat number off the nameplate. Use these for each circuit type:
These are the figures you work your cable sizing backward from, alongside the run length.
Voltage drop is the difference between what your source supplies and what your load receives. It's caused by resistance in the conductor and scales with both current and cable length.
The working form you'll actually use on site comes from AS/NZS 3008.1 itself, expressed per metre so you can feed the cable table directly:
Vd (V) = I × L × 2 × (mV/A/m from AS/NZS 3008.1 Appendix A) ÷ 1000
Where I is circuit current in amps, L is the one-way run length in metres, ×2 accounts for the active plus neutral (or positive plus negative on DC) return path, and mV/A/m is the volt-drop per ampere per metre value for the chosen cable from the standard's Appendix A (e.g. 6mm² two-core + earth copper V-90 is around 7.3 mV/A/m). The original compact form Vd = I × R × 2 is true, but R is a per-metre loop resistance looked up from the table, not a number you can guess at.
AS/NZS 3008.1 recommends 3% on subcircuit length and 5% total from origin to load as the design limits. The 5% figure is the absolute ceiling from the standard, not a working target.
| System Type | Recommended Maximum |
|---|---|
| General subcircuits | 3% |
| AS/NZS 3008.1 total (origin to load) | 5% |
| Critical systems (water pumps, refrigeration) | Aim ≤3% where practical |
Exceeding 3% on a subcircuit or 5% total is non-compliant with AS/NZS 3008.1 and causes poor motor starting, dimming under load, inverter low-voltage cutouts, and reduced battery charge efficiency on long DC runs. Critical loads — anything that controls temperature or runs a motor under load — should be held as close to ≤3% as the cable table allows. I use 5% as an absolute last resort, never a default.
Below is a working table for typical off-grid runs. These are starting points derived from common installation scenarios — always cross-check against AS/NZS 3008.1 with your specific run length, ambient temperature, installation method, and applicable derating factors. The reference current-carrying capacities quoted are for V-90 (PVC) two-core + earth copper at the standard 25 °C ambient with no derating — your on-site figure will be lower once method, grouping, and ambient are applied.
| Current (A) | Run Length Assumption | Cable Size | Budget Tier |
|---|---|---|---|
| 30 | Short run, standard conditions | 6mm² | Low |
| 50 | Short run, standard conditions | 10mm² | Low |
| 80 | Medium run, standard conditions | 16mm² | Medium |
| 100 | Longer run / heavier load, derated conditions | 35mm²+ | High |
For reference, 6mm² V-90 two-core + earth copper clipped direct has a current-carrying capacity (CCC) of around 41A at the standard 25 °C reference — not 30A and not a value you can use without checking the installation method. A 100A run on 25mm² only holds up on a short, lightly derated run; in most realistic off-grid installs with conduit, grouping, or roof-space ambient, 35mm² or larger is the correct starting point. Don't rely on this table in place of the standard.
Use DC-rated flexible cable for all battery and PV interconnects: V-90, Solar PV1-F (TUV 2 PfG 1169), or welding cable with appropriate insulation. Never use AC twin-and-earth (TPS) on DC battery strings — it's not DC-rated and the insulation breaks down under sustained DC voltage stress.
Larger currents need larger conductors — non-negotiable. Undersized cable on a high-current run is a heat build-up waiting to happen.
Cable doesn't perform in a vacuum. Three factors consistently shift sizing decisions on Central Queensland jobs:
If your cable is running through insulation, bundled with three other circuits, in a 45 °C ceiling space, you're not going to carry the same amps as the table value suggests. Step up.
In my experience these systems, the cable sourcing is straightforward. Common stockists carrying AS/NZS-rated cable include Bunnings, Jaycar, Westinghouse, and Legrand. Buy from a supplier who can show you the standard rating on the drum or coil — don't accept unmarked or imported product without traceable certification.
If you're doing this regularly, a few tools pay for themselves:
A calculator and a thermometer cover most of the planning work. The insulation testers are for verification after install and during periodic checks.
Hardwired 230V AC work in Australia is not a DIY space, and neither is hardwired DC battery work above 50V DC. Under AS/NZS 3000, all battery and inverter-to-board wiring at these voltages is licensed electrical work. Call a licensed electrician when:
Following AS/NZS standards and using properly rated gear isn't optional. Exceeding 3% voltage drop is non-compliant with AS/NZS 3008.1; aim for 1–2% for critical systems where the cable table allows it. Use larger cables for high currents, long runs, or high ambient temperatures to prevent overheating. Choose the installation method to match the derating factors from AS/NZS 3008.1, and use mechanical protection where required by AS/NZS 3000. Never use AC twin-and-earth on DC battery strings — use DC-rated flexible cable only. Test insulation resistance and replace damaged cables immediately.
Cable doesn't get installed and forgotten:
If anything looks heat-damaged, cracked, or UV-brittled, replace it. Cable is cheap relative to the load it's protecting.
What's the maximum voltage drop I should allow?
3% on subcircuits and 5% total from origin to load per AS/NZS 3008.1. For critical systems like water pumps and refrigeration, aim ≤3% where the cable table allows; 5% is the standard's absolute ceiling, not a working target.
How do I calculate current for my circuit?
Start from Current (A) = Total wattage (W) ÷ Voltage (V), then size the conductor on worst-case current: inverter surge for AC, I = W ÷ Vmin for DC battery circuits, and Isc × 1.25 for PV strings (AS/NZS 5033). At 230V AC, a 2,300W load draws 10A and a 4,600W load draws 20A as nameplate figures.
Why does the voltage drop formula include ×2?
Single-phase AC and 2-wire DC circuits both have a return path. Current travels down the active (or positive) conductor and back along the neutral (or negative), so the total conductor length is twice the physical run distance.
What size cable do I need for a 50A run?
For a short run in standard conditions, 10mm² is the starting point in the low-budget tier — verify the CCC against AS/NZS 3008.1 for your installation method and derating factors. On a longer run or in a derated installation, step up.
How often should insulation resistance be tested?
Every 5 years is the minimum for established installations. Test annually if the cable is exposed to mechanical risk, UV, or extreme heat.
Can I use smaller cable if I keep the run short?
No — current is the primary sizing factor, not length. A short high-current run still needs the conductor cross-section to handle the amperage without overheating.
Can I use AC twin-and-earth on a DC battery string?
No. AC TPS is not rated for sustained DC voltage stress. Use DC-rated flexible cable only — V-90, Solar PV1-F, or welding cable with appropriate insulation — for all battery and PV interconnects.
Worth a watch: How To Size a Wire For Off-Grid Solar Power - Calculation Examples · Cleversolarpower by Nick