Voltage Drop Calculator
Every wire that carries current loses a small amount of voltage due to the wire's own resistance. That loss is called voltage drop, and over a long cable run it can be enough to cause equipment to malfunction, motors to run hot, or LED lighting to appear dim. This calculator works out the round-trip voltage drop for a given wire run using the current flowing, the one-way cable length, the wire gauge (AWG), and the supply voltage at the source. The round-trip length is used because current must travel to the load along the active conductor and return along the neutral conductor, so the effective length of copper is twice the one-way run. You can choose the wire size from a list of common AWG gauges; the conductor diameter and cross-sectional area are looked up automatically. The resistivity of annealed copper is 1.724 x 10 to the minus 8 ohm-metres at 20 degrees Celsius. The formula is Vdrop = 2 x rho x L x I / A, where L is the one-way length in metres, I is the current in amperes and A is the cross-sectional area of the wire in square metres. The calculator also expresses the drop as a percentage of the supply voltage and shows the power wasted as heat in the cable. For general building wiring in New Zealand under AS/NZS 3000:2018, a drop of up to 5 percent is typically acceptable. For DC circuits such as solar or automotive wiring, 3 percent or less is a common design target. This tool is for planning and estimation only. Final wiring design should comply with the relevant New Zealand standard and be checked or installed by a licensed electrician where required.
Uses copper resistivity at 20 C. AWG 12 conductor diameter is 2.053 mm, cross-section 3.309 mm squared. For estimation only; consult a licensed electrician for NZ wiring compliance.
How it works
The wire cross-sectional area in mm squared comes from the AWG gauge selected. The round-trip resistance for a one-way run of L metres is R = 2 x rho x L / A, where rho = 1.724 x 10^-8 ohm-metres and A is the area in m squared. The voltage drop is Vdrop = I x R. The percentage drop is Vdrop / Vsupply x 100. Power wasted in the cable is P = I squared x R. The formula assumes copper at 20 degrees Celsius; resistance rises by about 0.4% per degree of temperature increase.
Worked example
A 10 A load is fed by a 50 m one-way cable run using AWG 12 wire (conductor diameter 2.053 mm, cross-sectional area 3.309 mm squared). The round-trip cable length is 100 m. Resistance is 2 x 1.724 x 10^-8 x 50 / (3.309 x 10^-6) = 0.521 ohms. Voltage drop is 10 x 0.521 = 5.21 V, which is 5.21 / 240 x 100 = 2.17% of a 240 V supply. The power wasted as heat in the cable is 10 squared x 0.521 = 52.1 W. These figures match the default values pre-filled above.
Related calculators
- Wire Gauge AWG Calculator: find diameter, resistance per metre and current capacity for any AWG size.
- Volts Amps Watts Ohms Calculator: solve any two of V, I, R and P.
- Voltage Divider Calculator: calculate the output voltage of a resistor voltage divider.
- kVA to kW Calculator: convert between apparent and real power using power factor.