DC voltage drop is one of the more mechanical calculations in PV design — it's pure resistance and Ohm's law — but a few details in how it's applied change the answer enough to matter at project scale, where losses of a fraction of a percent per circuit add up across hundreds of string home-runs.
Conductor resistance
The starting point is the standard resistivity relationship:
R = ρ × 2L / A
ρ is the conductor material's resistivity, L is the cable's one-way route length, and A is its cross-sectional area. Resistivity itself isn't a fixed constant — it rises with conductor temperature, which matters because a current-carrying conductor runs hotter than the surrounding air, and PV DC cables often sit in direct sun on a hot rooftop or in a warm conduit run.
Why the length gets doubled: the 2L in the formula
A DC circuit is a complete loop — current flows out through one conductor and back through the other. If a string's home-run cable measures 300 m from the array to the combiner box, current passes through 300 m going out and another 300 m coming back: 600 m of total conductor length, not 300 m. This is the single most common source of a voltage-drop calculation coming out roughly half of what it should be — using the one-way route length where the formula needs the complete-circuit length.
Copper vs aluminium
Copper has meaningfully lower resistivity than aluminium, so for the same cross-section and length, a copper conductor produces less voltage drop and less resistive loss. Aluminium is lighter and, for a given current rating, often more economical on long runs — but it needs a larger cross-section to match copper's performance, and terminations require material-appropriate connectors and installation practice. Neither material is universally “better” — the choice is a route-length, cost and mechanical decision made per project.
Voltage drop and power loss
Once resistance is known, voltage drop and resistive loss follow directly:
ΔV = I × RPloss = I² × R
Voltage drop is usually expressed as a percentage of system voltage rather than an absolute figure, because a 5 V drop means something very different on a 48 V string than on a 1500 V array. Percentage voltage drop is what design targets and comparisons are typically expressed against.
Why voltage drop alone does not size the cable
A cable that comfortably passes a voltage-drop check can still be the wrong cable. Cable sizing has to satisfy several requirements together:
- Ampacity — the cable's current-carrying capacity for its installation method, grouping with other cables, and ambient/conductor temperature, independent of voltage drop.
- Short-circuit withstand — the conductor and its protection must survive fault current for the duration the protection takes to clear it.
- Mechanical and environmental rating — UV exposure, burial depth, rodent protection, flexibility and termination compatibility for the specific installation.
A voltage-drop calculation tells you whether a given cross-section keeps losses acceptable over a given length — it doesn't, on its own, confirm the cable is safe to install. Final cable selection needs all of the above checked together against the specific product's datasheet and the applicable installation requirements for the project.
On long DC home-runs (large ground-mount and tracker arrays especially), voltage drop often becomes the binding constraint before ampacity does — the cross-section needed to keep losses acceptable ends up larger than the minimum ampacity would require. Check both, but don't assume ampacity is always the tighter limit.
Once cable sizing is settled, the same DC array's output feeds into inverter loading and DC/AC ratio considerations — covered in the companion article.