What This Tool Calculates
This calculator determines the voltage drop and I²R power loss along a DC PV circuit — string home-run, combiner-to-inverter, or any DC cable run — for a given current, one-way route length, conductor material, cross-section and operating temperature. It can also search upward through standard cable sizes to find the smallest cross-section that meets a target voltage-drop percentage.
How To Use It
- Enter DC current (or power and system voltage) for the circuit.
- Enter the one-way cable route length — the tool doubles it for the complete circuit automatically.
- Choose conductor material (copper or aluminium) and expected operating temperature.
- Enter a cross-section directly, or switch to target mode to find the minimum size meeting a voltage-drop limit.
- Review voltage drop (V and %), and I²R power loss (W).
Engineering Principle
Conductor resistance follows the standard resistivity relationship, using the complete-circuit (2L) length:
R = ρ × 2L / A
where ρ is resistivity (temperature-corrected), L is the one-way route length, and A is conductor cross-sectional area. Voltage drop and power loss follow directly from Ohm's law and the I²R relationship once resistance is known:
ΔV = I × RPloss = I² × R
Formulas
R = ρ × 2L / AConductor resistanceL is one-way route length; 2L accounts for the return path.
ΔV = I × RVoltage dropΔV% = (ΔV / System Voltage) × 100Voltage drop percentagePloss = I² × RI²R power lossEngineering Notes
This tool checks voltage drop and resistive loss only — it does not verify current-carrying capacity (ampacity), short-circuit withstand, derating for grouping/burial/conduit fill, or mechanical/environmental cable ratings. A cable must satisfy all of these together, not voltage drop in isolation.
Resistivity and temperature coefficient values used are standard reference figures for copper and aluminium at the stated conditions — always confirm against the actual cable manufacturer's datasheet for the specific product being installed.
FAQ
Why is the route length doubled in the resistance formula?
Current has to travel out to the load and back through the return conductor — a DC circuit is a complete loop. If you measure 500 m from the array to the combiner box, the current actually passes through 1000 m of conductor (2L), so the one-way route length is doubled before calculating resistance.
Copper or aluminium — which has lower voltage drop?
Copper has lower resistivity than aluminium, so for the same cross-sectional area and length, copper always produces less voltage drop and less I²R loss. Aluminium is lighter and often cheaper, which is why it still gets used, typically at a larger cross-section to compensate — this calculator lets you compare both directly for the same route.
Does a lower voltage drop always mean the cable is correctly sized?
No. Voltage drop is one constraint among several. The cable also has to meet current-carrying capacity (ampacity) for the installation method and ambient/conductor temperature, short-circuit withstand, and mechanical/environmental requirements (UV, rodent, burial depth, etc.). A cable that passes the voltage-drop check can still be undersized on ampacity or protection grounds — this tool checks voltage drop and I²R loss only.
Why does conductor temperature matter, not just ambient temperature?
Resistivity itself rises with conductor temperature, and a current-carrying conductor runs hotter than the surrounding air. Using the STC/ambient resistivity figure without correcting for actual expected operating temperature understates both voltage drop and loss — this calculator applies the temperature coefficient explicitly rather than assuming 25°C throughout.