Cable Power Loss Calculator
Power lost as heat in a cable run, and what that waste costs over a year. Use it to decide whether a larger conductor pays for itself.
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The formula
R = ρ × 2L ÷ A P_loss = I² × R
Loss depends on the square of the current, which is the single most important fact about power distribution. It is why the grid transmits at hundreds of kilovolts: raising the voltage lets you deliver the same power with less current, and the loss falls with the square of that reduction.
What to watch out for
Three consequences of loss going as I²R:
- Current matters far more than length. Doubling the run doubles the loss; doubling the current quadruples it.
- Low voltage is expensive. Delivering 1 kW at 12 V needs 83 A; at 230 V it needs 4.3 A. The loss in the same cable is about 370 times greater on the low-voltage run.
- Upsizing has diminishing returns. Loss is inversely proportional to area, so going from 4 mm² to 6 mm² cuts it by a third, while 6 to 10 mm² only saves another 40% of what remains.
Frequently Asked Questions
How do I calculate power loss in a cable?
Work out the loop resistance — resistivity times twice the run length, divided by the conductor area — then multiply by the square of the current. 20 A through 30 m of 4 mm² copper loses about 103 W.
Why does cable loss go up with the square of current?
Because power is I²R. Doubling the current doubles the voltage dropped across the cable resistance and doubles the current through it, so the power dissipated goes up fourfold.
Is it worth using thicker cable to save energy?
On long runs with continuous high current, often yes — the loss falls in inverse proportion to area. On short domestic circuits that run intermittently, the saving is usually a few kWh a year and will not repay the cable.