ToolnozaWhat is voltage drop?
Voltage drop is the reduction in voltage along a conductor carrying current. In a resistance-only model, a longer cable or a higher current produces a larger drop; a larger conductor cross-section reduces it. Compare the drop with the supply voltage to express it as a percentage.
Voltage drop formula and units
For DC or single-phase AC at unity power factor, ΔV = 2 × I × ρ × L / A. I is current in amperes, L is one-way cable length in meters, A is conductor area in mm², and ρ is resistivity in Ω·mm²/m. The factor 2 represents the outgoing and return path; do not enter twice the route length.
For a balanced three-phase circuit at unity power factor, replace 2 with √3 and use the line-to-line supply voltage. Percentage drop = 100 × ΔV / supply voltage. The Toolnoza model excludes conductor reactance and connection resistance.
Worked example: a 30 m copper circuit
Take 230 V, 20 A, 30 m one-way, 6 mm² copper and a conductor temperature of 20°C. With ρ = 0.017241 Ω·mm²/m, ΔV = 2 × 20 × 0.017241 × 30 / 6 = 3.4482 V. The percentage is 1.4992%, and the estimated receiving voltage is 226.5518 V.
With the same inputs in balanced three-phase mode, the model gives 2.9862 V, or 1.2984% of 230 V line-to-line. These are calculation examples, not a recommended circuit design.
Temperature and material change the result
The calculator uses ρ(T) = ρ20 × [1 + α × (T − 20)]. Its copper values are ρ20 = 0.017241 and α = 0.00393/°C; aluminum values are 0.028264 and 0.00403/°C. At 75°C, the copper DC example gives about 4.1935 V. Use conductor operating temperature rather than assuming it equals room temperature.
How to use the Voltage Drop Calculator
Select copper or aluminum and the circuit mode. Enter supply voltage, load current, one-way length, cross-section and temperature. Read the voltage drop, percentage and receiving voltage. Use the Cable Size Calculator if you want to solve for cross-section instead.
Common mistakes and practical limits
Avoid mixing meters with feet, conductor area with cable outside diameter, or line-to-line with phase-to-neutral voltage. Motors, long AC runs and low power factors can require a model including reactance. A voltage-drop result does not check ampacity, short-circuit withstand, protection or local installation rules.