Voltage Drop Calculator

AS/NZS 3008.1.1 gives every cable size and installation method a millivolt-per-amp-per-metre value — Vc for short. It is a single number that bundles up the cable’s resistance and reactance at operating temperature, so you do not have to calculate impedance from scratch. The values live in AS/NZS 3008.1.1 Table 42 onward, one table per insulation type, conductor material, and installation method.

Once you have Vc, the formula is short:

Where:

• Vd — voltage drop, in volts
• L — one-way cable length, in metres
• I — the load current, in amps
• Vc — the cable’s mV/A/m value from AS/NZS 3008.1.1, matched to conductor size, material, and insulation type

Divide the result by the supply voltage and multiply by 100 to get a percentage — that is what gets checked against the AS/NZS 3000 limit.

Single-phase and three-phase runs use different Vc figures for the same cable size. Single-phase values are worked out over the full active-neutral loop, so they read higher. Three-phase values reflect the balanced line-to-neutral drop and come in lower — typically around 0.866 (√3⁄2) of the single-phase figure for the same conductor. This calculator picks the right value automatically once you set the phase configuration.

AS/NZS 3000 Clause 3.6.2 sets a hard limit: total voltage drop from the point of supply to the furthest point of utilisation must not exceed 5% of the nominal supply voltage. That is one budget covering the whole installation — consumer mains, sub-mains, and final sub-circuits added together, not 5% for each section separately. A 7% limit applies only where the point of supply is a dedicated substation on the same premises as the installation, which is not the typical street-fed job.

Since the 5% has to be shared across the whole run, most designers split it up before they start pulling cable. A common approach is to budget around 2% for consumer mains and sub-mains, leaving roughly 3% for final sub-circuits — or the reverse, depending on where the long run sits. This split is a design convention, not a separate clause in the standard. AS/NZS 3000 only cares about the 5% total; how you divide it between sections is a judgement call based on where your longest or most heavily loaded run actually is.

That is why this calculator checks a run against both 3% and 5%: 3% as a sanity check against common design practice for one section of the installation, and 5% as the actual clause limit for the whole run from the point of supply.

Take a 32A single-phase circuit run in 6mm² copper over a 45m route — a fairly typical shed or workshop sub-main. AS/NZS 3008.1.1 gives 6mm² copper (V-75 insulation, single phase) a Vc of 7.50 mV/A/m.

4.70% is under the 5% total limit from AS/NZS 3000, so this run passes on the clause itself. But if this is a sub-main and the mains upstream have already used their share of the budget, 4.70% alone eats almost all of a typical 2%/3% split — worth flagging even though it technically passes. Step up to 10mm² copper and the same run drops to 6.42V, or 2.79% — comfortably inside a 3% sub-main budget with margin left for the final sub-circuit downstream. Run both sizes through the calculator above to see the numbers side by side.

Everything above deals with voltage drop — current flowing from the switchboard out to a load, with the far end of the cable sitting below supply voltage. A solar inverter exporting to the grid runs the same cable the other way: current flows from the inverter back toward the point of supply, and the far end of that cable — at the switchboard — ends up above supply voltage instead of below it. That is voltage rise, and it is what a voltage rise calculator for Australian solar jobs is really checking.

AS/NZS 4777.1 caps voltage rise at 2% of nominal voltage, measured from the point of supply to the inverter’s AC terminals, calculated at the inverter’s rated export current. On a 230V single-phase supply that is roughly 4.6V; on 400V three-phase it works out to around 8V line-to-line.

The maths is identical to the voltage drop formula above — same Vd = (L × I × Vc) / 1000, same mV/A/m tables. The only difference is which current you plug in: instead of a load current, use the inverter’s rated AC output current at full export. Enter the AC cable length from the inverter back to the switchboard, the cable size, and the inverter’s rated current as the load current in the calculator above, then check the result against 2% instead of 5%.

For sizing the DC side of a string, use the solar DC cable sizing calculator instead — DC strings run to AS/NZS 5033, not AS/NZS 4777.1. To check the full installation against CEC and AS/NZS 5033 requirements, see the solar compliance checker.

Labelling the isolators and inverter AC/DC runs while you are at the board: RBZ 3D makes custom electrical labels online — instant pricing, no minimum order.