Voltage drop calculator
Voltage drop calculator for New Zealand and Australia. Conductor impedance comes from AS/NZS 3008.1.2 or 3008.1.1, and the limit from AS/NZS 3000 Clause 3.6.2.
Reads AS/NZS 3008.1.2:2017 (NZ), AS/NZS 3008.1.1:2017 and 2025 (AU) and AS/NZS 3000:2018. Tables checked 27 Sept 2026.
Try a job 13
Each job fills in example values. Replace them with your values.
- Check a long run for voltage dropThe voltage drop on the run, against the limit.
- Check a 12 V d.c. run for dropThe voltage drop on a 30 A caravan feed over 6 m, against the 10 % ELV limit.
- Check the drop on a PV array cableThe d.c. drop of a 15 A string at 600 V over 40 m on 4 mm².
- Check the drop on a submainThe voltage drop on a 100 A three‑phase submain over 80 m.
- Check the drop on a lighting circuitThe drop on an 8 A lighting circuit over 45 m.
- Check the drop to a rural pumpThe drop on 10 A over a 250 m run.
- Check the drop on aluminium mainsThe drop on 200 A over 60 m of aluminium mains.
- Check a 24 V control circuitThe drop on 2 A at 24 V over 60 m.
- Check a 48 V battery cableThe drop on 100 A over 3 m at 48 V.
- Check the drop with a loaded neutralThe drop on an unbalanced circuit with 20 A in the neutral.
- Check an extension leadThe drop on 10 A through a 30 m lead.
- Estimate a 160 A submain for a tenderThe drop on 160 A over 90 m, three phase.
- Check the drop at a 3 % limitThe drop on 32 A over 40 m against a 3 % limit you set.
Inputs
A worked example with the default inputs.
- Supply
- a.c.
- Phases
- Three phase
- Voltage
- 400 V
- Design current
- 50 A
- Run length
- 60 m
- Power factor
- 0.9
- Standard
- NZ · AS/NZS 3008.1.2
- Cable
- 16 mm² Copper PVC 75 °C
- Cores
- Multicore
- Voltage drop limit
- 5 %
- System
- Low voltage (limit you set)
- Neutral current
- 0 A
Voltage drop
1.69%
- Voltage drop
- 6.74V
- Drop per amp metre
- 2.25mV/A·m
- Maximum length at limit
- 177.98m
- Voltage drop limit
- 5%
This result is a design aid. Verify it against the current standard.
Calculation steps
- Resistance1.4Ω/km
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- Reactance0.09Ω/km
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- mV/A·m2.25mV/A·m
AS/NZS 3008.1.2:2017 Cl 4.5
- Voltage drop6.74V
- Voltage drop percentResult1.69%
- Voltage drop limit5%
AS/NZS 3000:2018 Cl 3.6.2
- Maximum length177.98m
About this calculator
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.
- 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.
- 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.
- Drop. It multiplies that figure by the current and the run length. It divides the volts by the nominal voltage to get the percent.
- 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.
- Limit. It compares the percent with the limit for the system. It also gives the run length at which the drop reaches the limit.
- Single‑core cable. If you do not know the formation, the tool uses the highest reactance for single‑core cable and says so.
| Step | Table or clause | What it decides |
|---|---|---|
| Resistance | Tables 34 and 35 | The conductor resistance at its operating temperature |
| Reactance | Table 30 | The reactance for the cable and, for single core, its formation |
| Drop per ampere metre | Cl 4.5 | How resistance, reactance and power factor combine |
| Unbalanced load | Cl 4.6 | The drop with a neutral current on three phases |
| Low voltage limit | AS/NZS 3000 Cl 3.6.2 | The limit for a low voltage circuit |
| Extra-low voltage limit | AS/NZS 3000 Cl 7.5.7 | The limit for an extra-low voltage circuit |
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
| System | Limit the tool applies | Clause |
|---|---|---|
| Low voltage | The limit you set, 5 % by default | AS/NZS 3000 Cl 3.6.2 |
| Extra-low voltage | 10 % of the nominal voltage | AS/NZS 3000 Cl 7.5.7 |
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.
Questions
The result is a design aid. Verify against the current standard.
What does the voltage drop calculator do?
The tool reads the resistance and reactance of the conductor. It combines them at the power factor to get the drop in millivolts per amp metre. It then reports the drop in volts and the longest run that still meets the limit. It also gives the drop as a percent of the supply voltage.
What voltage drop limit does AS/NZS 3000 set?
The default limit is 5 % and you can change it. Choosing extra-low voltage sets the limit to 10 % from AS/NZS 3000:2018 Clause 7.5.7. A result above the limit is marked as a fail and a warning appears above 80 % of the limit. Impedance comes from AS/NZS 3008.1.2:2017 Table 30 and Tables 34 to 35.
Which inputs change the voltage drop most?
Design current and run length scale the drop directly. Conductor size, material and insulation set the resistance and reactance. Power factor sets how much reactance adds to the drop. Cables in parallel divide both resistance and reactance by the number of cables.
Why does a three phase result differ from a single phase result?
The tool multiplies by the square root of three for a three phase result. It multiplies by two for a single phase result, because the current flows out and back. The same cable and current therefore give a different drop on each supply type. For a d.c. circuit, use the DC cable size tool.
Can I work out d.c. voltage drop?
Yes. Set Supply to d.c. The tool reads the d.c. resistance of AS/NZS 3008.1.1:2025 at the maximum conductor temperature, in both countries. It takes 2 × current × resistance × length for the two wires, with no reactance or power factor. To size a d.c. cable, use the DC cable size calculator.
Worked examples
- Homeowner shed supply voltage drop over 60 mA 20 A single‑phase supply running 60 m to a shed on 6 mm² copper, with the drop, the percentage and the maximum length the AmpSize voltage drop engine returns.
- Voltage drop single phase vs three phaseThe same 6 mm² cable at 32 A over 50 m, worked on a three phase supply and on a single phase supply, with both drops, both percents and both run limits.
- Submain voltage drop estimate for a 160 A tenderA price stage check on a 160 A submain over 80 m on 70 mm² XLPE. See the mV/A·m figure, the 1.89 % drop and the length the budget allows.
- 12 V caravan cable: d.c. voltage dropA 12 V caravan feed at 30 A over 6 m, checked on 4, 6 and 10 mm² twin against the 10 % ELV limit, then a 48 V battery-to-inverter run.
- 24 V DC voltage drop on an instrument loopA 4–20 mA loop runs 200 m on 2.5 mm² at 24 V. See the mV/A·m figure, the 7.51 % drop and the maximum length the engine returns for a 10 % limit.
- Total voltage drop: mains, submain and finalAdd the voltage drop of the consumer mains, a shed submain and a final subcircuit against the 5 % limit, and fix a long submain before the shed circuits fail.
Background
- The 5 % voltage drop limit from supply to loadWhat AS/NZS 3000 Cl 3.6.2 limits, how the drop of the mains, submains and final subcircuits adds by percent, the four exceptions and why any split is yours.
- Voltage drop basicsOhm's law on a cable, the mV/A·m shortcut, the percent limit and the maximum length, worked on a 25 A run over 35 m of 4 mm² copper.
- Control panels and PLC wiringSegregation of LV and ELV in a control panel, terminal and numbering practice, panel earthing, and the 24 V DC voltage drop that long instrument loops fail on.
- DC voltage drop and conductor resistanceWhy d.c. drop is 2 × I × R × L, why it has no reactance or power factor, and how the hot resistance, the cable tables, the limit and parallel cables change it.
- Motor circuits and startingHow full load current sizes the cable and starting current sizes the device on a motor circuit, with a worked 22 kW 400 V motor and the cable the tool selects.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 3.6.2Voltage drop limit
- AS/NZS 3000:2018 Cl 7.5.7Extra-low voltage wiring, voltage drop limit
- AS/NZS 3008.1.2:2017 Cl 4.5Voltage drop method
- AS/NZS 3008.1.2:2017 Cl 4.6Voltage drop with an unbalanced load
- AS/NZS 3008.1.2:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2025 Table 4.1(A), 4.1(B), 4.5(A), 4.5(B)Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2025 Table 4.6(A), 4.6(B), 4.8(A), 4.8(B)d.c. resistance at the maximum conductor temperature
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
Related tools
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- DC cable size calculatorSize a d.c. cable up to 1500 V for current and voltage drop.AS/NZS 3008.1.1
- Maximum circuit length calculatorLongest run that voltage drop and the fault loop allow.AS/NZS 3000AS/NZS 3008.1.2
Was this useful?
A question, or a value that looks wrong? Contact us.