Calculator Power
Voltage drop calculator
Work out voltage drop on any cable run using the AS/NZS 3008 method, and check it against the 5% limit in the Wiring Rules. Australian figures, 230 V nominal.
Supply
The current the circuit actually carries.
One-way distance along the cable run. Do not double it for the return.
Cable figure from its data sheet
From your cable's data sheet. Match the row to its construction and phase, not just its size: the same size is a different figure across cable types.
230 V is standard Australian mains; three-phase is 400 V between phases.
Voltage drop
3.91 %
18 mV/A/m × 20 A × 25 m ÷ 1000 = 9 V9 V dropped · within the 5% limit
DropV = mV/A/m × A × m ÷ 10001φ / DCmV/A/m = 2 × (Rc·cosθ + Xc·sinθ)3φmV/A/m = √3 × (Rc·cosθ + Xc·sinθ)Limit5% of nominal (AS/NZS 3000)
How voltage drop is calculated
Every conductor has resistance, so a cable delivers slightly less voltage at the load than it received at the switchboard. The longer the run and the higher the current, the bigger the loss.
AS/NZS 3008.1.1 handles this with a per-cable figure in millivolts per amp per metre, published in its cable tables. You take the figure for your cable, multiply by the current and the route length, and divide by 1000 to get volts. Route length is the one-way distance of the circuit, not there and back.
The formulas
-
Voltage drop
Vc is the mV/A·m figure for your cable, I the circuit current in amps, L the route length in metres.
-
As a percentage
Compare the result against the 5% the Wiring Rules allow across the whole installation.
9.2 V on 230 V = 4%
What the Wiring Rules actually require
The limit in AS/NZS 3000 is 5% of nominal voltage, and it applies to the whole path: point of supply through to any point in the installation. At 230 V nominal that is 11.5 V across everything, not per circuit.
The 3% you see everywhere is a design allocation, not a requirement. Designers split the 5% budget so the sub-mains take part of it and the final subcircuit takes the rest, which leaves room for the run you are actually working on. It is good practice. It is not what the standard says.
Where it bites in a smart home
Mains circuits usually have headroom. Extra-low-voltage runs often don't, because the percentage is measured against a much smaller number. On a 24 V LED strip, 5% is 1.2 V. On 12 V it is 0.6 V, which a long garden run in thin cable will eat before the far end lights up, and the visible symptom is dim or colour-shifted light rather than anything that looks electrical.
The same maths sits behind Ohm's law, and if you need to get from a device's wattage to the current it draws first, start with watts, amps and volts.
Frequently asked questions
How do you calculate a voltage drop?
Multiply the cable's mV/A·m figure by the circuit current in amps and the route length in metres, then divide by 1000. That gives the drop in volts. Divide by the nominal supply voltage for a percentage.
How do I calculate voltage drop in Australia?
Use the AS/NZS 3008.1.1 tables for the figure, and check the result against the 5% limit in AS/NZS 3000 clause 3.6.2. Australian cable is sized in mm², not AWG, so American calculators give the wrong answer.
What is the rule of thumb for voltage drop?
Designers commonly hold sub-mains to about 2% and the final subcircuit to about 3%, so the total stays inside the 5% the Wiring Rules allow. The split is convention. Only the 5% is a requirement.
Does voltage drop matter on 12 V LED strip?
More than on mains. 5% of 12 V is 0.6 V, so a long run in undersized cable dims the far end. Feed long strips from both ends, or run a higher-voltage strip.
Still need a hand?
We answer most messages within one business day. If your question is about an existing order, have your order number ready. It speeds things up.
Contact us