Zerdly

DC Cable Loss Calculator

Work out the loop resistance, voltage drop, power loss and loss percentage of a DC cable run — ideal for battery, solar and low-voltage wiring where even a small drop matters.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the DC current, the one-way run length and the conductor cross-section.
  2. Enter your system voltage (e.g. 12, 24 or 48 V) so the tool can express the drop as a percentage.
  3. Leave resistivity at 0.0225 ohm.mm2/m for copper, or override it, then review the resistance, voltage drop, power loss and loss percentage.

How it works

Both conductors carry the current, so the loop resistance is R = 2 x rho x L / csa. Voltage drop is I x R, power lost as heat in the cable is I^2 x R, and the loss percentage is the voltage drop divided by system voltage times 100. Because DC systems often run at low voltage, a modest drop can be a large percentage of the supply.

Worked example

Worked example. For 20 A over a 15 m run on 6 mm2 copper at 48 V: R = 2 x 0.0225 x 15 / 6 = 0.1125 ohm; voltage drop = 20 x 0.1125 = 2.25 V; power loss = 20^2 x 0.1125 = 45 W; loss percentage = 2.25 / 48 x 100 = 4.69%.

Common mistakes

  • Entering the round-trip length — use the one-way run, the formula doubles it for the loop.
  • Forgetting that a small voltage drop is a big percentage on a 12 V or 24 V system.
  • Overlooking the power-loss figure, which is wasted energy and heat inside the cable.

Frequently asked questions

Why is the resistance doubled?

In a DC circuit the current flows out along one conductor and back along the other, so both add to the resistance the current sees. Using 2 x rho x L / csa captures the full loop.

What loss percentage is acceptable?

It depends on the application, but many solar and battery installers aim to keep DC voltage drop under about 2-3% on critical runs. Larger conductors reduce both the drop and the wasted power.

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Tip: Enter any known values to calculate the remaining results.

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