For a steady DC circuit with equal outgoing and return conductors, cable resistance reduces the voltage at the load. Enter the stated current and a documented conductor resistance; the calculation includes both directions.

Formula: loop Ω = 2 × one-way ft × (Ω per 1,000 ft) ÷ 1,000; drop V = current × loop Ω

Worked example

At 5 A, a 50 ft one-way run whose individual conductor resistance is 1 Ω per 1,000 ft has a 0.1 Ω loop. The drop is 0.5 V, leaving 23.5 V from a 24 V supply, with 2.5 W dissipated in the loop.

Resistance must match the actual conductor

Use resistance data for the material, size and relevant temperature. OpenStax relates resistance to conductor length, cross-sectional area and resistivity. This page accepts resistance explicitly rather than assigning an AWG value or an assumed temperature.

The return path counts

The factor of two applies only when the return conductor has the same length and resistance per foot. Chassis returns, parallel conductors, joints and unequal paths need their actual total resistance. AC reactance, three-phase circuits, changing currents and source internal resistance are outside this model.

Current is a supplied operating assumption

The tool does not solve a complete load circuit. A load may change its current when voltage falls, so use an operating point supported by measurement or a circuit model. A calculated drop greater than supply voltage is rejected because those inputs do not describe this passive-load model. Results do not select wire ampacity, protective devices or code compliance.

Explore the related calculation

For a different starting point, try the resistor power calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

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DC voltage drop calculator

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