Electrical calculator
Cable voltage drop calculator
Work out what a long DC cable delivers at the far end: the voltage drop, the percentage lost and the power turned into heat in the cable, by conductor area or AWG.
This is an estimate for early design work. It uses the method and assumptions shown below the result. It leaves out some things a real build has to allow for (see “Assumptions and limits”) and it does not replace an engineer's sign-off. For a figure you can build to, send us the requirement.
Result
Why it matters
The tether is a resistor.
On an ROV the tether is often the longest conductor in the system, and its losses are easy to overlook. Thrusters that stall at depth, power dips when several thrusters speed up together, and a vehicle that behaves differently on 50 m of tether than on 10 m are all signs of voltage drop. The thrusters themselves are usually fine.
The same goes for any long DC cable: a UAV power lead, an instrument cable, a battery feed across a vehicle. The current goes out and comes back, so the resistance that counts is that of twice the run length.
How it is calculated
Resistivity, doubled.
Loop resistance is R = ρ × 2L / A, where ρ is the resistivity of annealed copper (1.724 × 10⁻⁸ Ω·m), L is the one-way run and A is the conductor cross-section. The voltage drop is that resistance times the current. The power lost is the drop times the current, and all of it becomes heat in the cable.
The usual guideline for a continuous run is a drop under about 5%. Above 10%, use a thicker conductor or raise the supply voltage. Raising the voltage is usually cheaper: for the same power delivered, doubling the voltage halves the current and cuts the loss to a quarter.
This is a DC calculation at 20 °C. Copper resistance rises about 0.4% for every degree above that, so a tether coiled on a drum in the sun loses more than the figure shown. It also does not check the cable's current rating or the temperature rating of its insulation, which are separate limits.
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