Voltage drop: route length, conductor data and useful checks

What causes voltage drop

Every real conductor has impedance. When current flows, part of the source voltage appears across that impedance rather than at the load terminals. The amount depends on current, route length, conductor resistance, reactance, phase arrangement and power factor.

That means a voltage-drop result is only as reliable as the conductor data used. A resistance value at one temperature or a mV/A/m figure for one circuit geometry should not be silently applied to another.

Three supported approaches

R/X data keeps resistance and reactance visible. It is useful when traceable ohm-per-kilometre values are available. For balanced three phase, the familiar form is ΔV = √3 × I × L × (R cosφ + X sinφ) with compatible units.

mV/A/m data is compact and convenient, but the published value normally already represents a particular phase arrangement and conductor geometry. The calculator therefore asks for a phase-specific value and does not invent an I²R loss from it.

Simplified resistivity estimates resistance from material, cross-sectional area and temperature. It is useful for learning and early comparisons, not as a substitute for manufacturer cable data.

Route length must be interpreted correctly

The input is the physical one-way distance from source to load. The calculator applies the relevant circuit geometry once. Entering the full out-and-back conductor length into a formula that already includes the return path doubles the drop incorrectly.

Parallel runs also belong in the calculation once. The simplified model assumes equal paths share current appropriately. Actual current sharing depends on installation symmetry, conductor characteristics and termination quality.

Read more than the percentage

If your on-site reading differs from the predicted value, compare measurement points, simultaneous current, conductor temperature and the impedance source before changing the route-length input. The measured-versus-calculated guide walks through that check.

A useful result includes volts dropped, percentage drop and estimated end voltage. When R/X or resistivity data supports it, conductor losses can also be estimated. In mV/A/m mode, the source value does not separate resistance from reactance, so power loss is deliberately reported as unavailable.

Voltage drop is one constraint among several. Ampacity, fault performance, equipment starting behavior, protective-device operation and local requirements need their own checks before a conductor is selected.