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How this is calculated#

The tool works out the drop for one cable, one run length and one current. It compares the drop with the limit for the system.

1. Resistance and reactance. It reads the conductor resistance at its operating temperature and the reactance for your size, material, insulation and core count. The Standard select decides which part it reads. It reads AS/NZS 3008.1.2 for New Zealand or AS/NZS 3008.1.1 for Australia. Cables in parallel divide both values.
2. Drop per ampere metre. It combines the two with the power factor. On one phase it takes 2 × (R cos φ + X sin φ). On three phases it takes √3 × (R cos φ + X sin φ). The result is in mV/A·m.
3. Drop. It multiplies that figure by the current and the run length. It divides the volts by the nominal voltage to get the percent.
4. Unbalanced load. On three phases with a neutral current, it works the drop on one conductor. It adds the neutral current over the neutral length and compares the drop with the phase voltage.
5. Limit. It compares the percent with the limit for the system. It also gives the run length at which the drop reaches the limit.
6. Single‑core cable. If you do not know the formation, the tool uses the highest reactance for single‑core cable and says so.

Tables without a standard named are in the part of AS/NZS 3008.1 you pick. Each result step names the edition it read.

The limit#

The tool checks one run at a time. When a submain feeds a final subcircuit, add the two drops and compare the total with the limit.

Common circuits, worked with the tool#

Each line states its route. Every line uses multicore copper cable with PVC insulation and the New Zealand tables. The power factor is 0.9, except where a line names a different value.

• Shed supply, 32 A single‑phase over 40 m on 6 mm². It drops 3.8 % against the 5 % limit. The drop reaches the limit at 52.58 m.
• Socket‑outlets, 20 A single‑phase over 30 m on 2.5 mm². It drops 4.25 % against the 5 % limit. The tool warns that the drop is close to the limit.
• Pump, 16 A single‑phase over 80 m on 2.5 mm². It drops 9.08 % against the 5 % limit, so it fails. The same run on 6 mm² drops 3.8 %.
• Three‑phase submain, 100 A at 400 V over 60 m on 35 mm². It drops 1.59 % against the 5 % limit.
• Extra-low voltage, 5 A at 24 V over 20 m on 2.5 mm². It has a power factor of 1. It drops 7.51 % against the 10 % limit.

New Zealand and Australia#

The Standard select picks the part of AS/NZS 3008.1 the tool reads. The limits come from AS/NZS 3000, which is the same in both countries.

• The shed supply on the Australian tables. It drops 3.8 % against the 5 % limit on AS/NZS 3008.1.1:2025, the same as on the New Zealand tables.

d.c. circuits#

Set Supply to d.c. for a battery, PV or ELV d.c. run. The tool reads the d.c. resistance of each size from AS/NZS 3008.1.1:2025 Tables 4.6(A), 4.6(B), 4.8(A) and 4.8(B). These are encoded at the maximum conductor temperature: 75 °C for PVC and 90 °C for XLPE. Single‑core cable reads Table 4.6, multicore Table 4.8. The tool takes 2 × current × resistance × length for the two wires. It uses no reactance, no power factor and no phases. AS/NZS 3008.1.2:2017 prints no d.c. tables, so New Zealand reads the 2025 part too. The limit follows the System select, as on a.c.

• Caravan feed, 30 A at 12 V over 6 m on 10 mm². It drops 6.69 % against the 10 % limit.
• Battery to inverter, 100 A at 48 V over 5 m on 35 mm² single‑core. It drops 1.33 % against the 10 % limit.
• PV array cable, 15 A at 600 V over 40 m on 4 mm². It drops 1.18 % against the 3 % limit.

The DC cable size calculator reads the printed d.c. mV/A.m of Tables 4.16 to 4.23 instead, which is twice the resistance rounded. Its drop can differ from this tool in the last digit.

Keep the result#

Every input sits in the page address, so the link in your browser reproduces the calculation. The PDF button prints the result with its steps and table references. Saving a calculation to a project needs a free account.

See also: AS/NZS 3000 Clause 3.6.2: voltage drop limit, Voltage drop with the mV/A·m method and Voltage drop basics.