Free voltage drop calculator
Add each part of the route, from the consumer mains to the final subcircuit, and see the total drop against the 5% limit, how long each run could be and the cable size you’d need. Single or three phase, copper or aluminium.
Check a cable run
No sign-up. Works offline, nothing is uploaded, and the working is shown for every run.
Open the calculatorOpens in a new tab. Works on a computer, tablet or phone.
What it does
- Adds up the whole route: consumer mains, submains and final subcircuits as separate runs, totalled against the 5% limit (or your own).
- Single and three phase: each run can be either, so a three-phase submain feeding a single-phase circuit adds up correctly.
- Copper or aluminium, 1 to 630 mm²: at 75 °C (V-75 PVC, most TPS), 90 °C (X-90) or 110 °C, with up to four cables in parallel.
- Amps or kW: enter the load either way, with a power factor for kW.
- Max length and max load for each run: how far that run could go, or how much current it could carry, with the others as they are, before you hit the limit.
- Voltage at the load: how many volts are left at the end of the route, and after each run along the way.
- Size up when it’s over: the smallest cable size that brings the total back within the limit, plus the total you’d get with each of the next sizes up.
- Quick add: EV chargers, shed and granny flat sub-boards, air con, hot water, pool pumps and three-phase motors, ready to adjust.
- Shows the working: mV/A·m × A × m ÷ 1000 for every run, so you can check it.
- A one-page report for the job file: your logo, business and licence number, the customer and date, the route with the voltage after each run, every run’s cable, load and drop, the method and a sign-off line. Or copy a short summary into a quote or email.
Voltage drop limits at a glance
| Supply | Nominal voltage | 5% of nominal |
|---|---|---|
| Single phase | 230 V | 11.5 V |
| Three phase | 400 V | 20 V |
The 5% is the total from the point of supply to the furthest point, not per cable. Some equipment, such as solar inverter cabling, has its own tighter limits from the manufacturer or other standards.
How to use it
- Set the limit. 5% is filled in. Change the voltages if your supply is different.
- Add each run. Start with the consumer mains or submain if there is one, then the final subcircuit.
- Enter length, load and cable. Use the route length (one way), not the total length of conductor.
- Read the total. Green is fine, amber is close, red is over. If it’s over, it tells you which size to go up to.
- Print the report or copy the summary. Add your details once and they’re kept for the next job.
Common questions
How is the voltage drop worked out?
Vd = mV/A·m × current × length ÷ 1000. The mV/A·m figure comes from the conductor’s standard resistance (IEC 60228), raised to the cable’s operating temperature, plus a typical reactance for multicore cable. It’s 2 × the impedance for single phase and √3 × for three phase. The working is shown under every run so you can check it.
What is the voltage drop limit in Australia?
AS/NZS 3000 allows a total of 5% of the nominal voltage from the point of supply to any point in the installation: 11.5 V on 230 V, or 20 V on 400 V. That 5% covers the consumer mains, any submains and the final subcircuit together, so add each one as a run.
Does it tell me the right cable size?
It tells you the smallest size that passes on voltage drop. It doesn’t check the current-carrying capacity for how the cable is installed (grouping, insulation, ambient temperature), short-circuit performance or earth fault loop impedance. Check those against AS/NZS 3000 and AS/NZS 3008.1.1 too.
Why might my figure differ slightly from the AS/NZS 3008 tables?
The tables use reactance values for each cable type and arrangement. This calculator uses a typical multicore value, so results for large single-core cables can differ a little. For cables up to about 16 mm² the difference is very small.
Can I enter the load in kW?
Yes. Switch the load to kW and enter the power factor. It works out the current at the voltage you’ve set, for single or three phase.
Can I print the calculation for the job file?
Yes. Print report gives a one-page A4 report with your logo, business and licence number, the customer, the route from the supply to the load, each run’s cable, load and drop, the method used and a sign-off line. Copy summary puts a short text version on the clipboard for a quote or email.
Is anything uploaded?
No. It all runs in your browser, works offline once open, and keeps your last calculation on your device.
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