PV DC Cable Loss Calculator
Enter string Vmp/Imp, one-way run and cable size to get the DC drop and the watts lost every peak hour — the 25-year tax of undersized PV cable.
Last updated: 2026-09-28
How the calculation works
- Loop resistance doubles the one-way run: both PV+ and PV− carry the full current.
- Copper resistivity at ~70 °C conductor temperature (0.0195 Ω·mm²/m) — hot rooftop cable is ~13% worse than the 20 °C figure.
- Power lost is I²R — the watts the array generates but never reaches the inverter.
Formula
R = ρ × 2L / A Vd = I × R P_loss = I² × R
| Symbol | Meaning | Unit |
|---|---|---|
Vd | DC voltage drop | V |
ρ | Copper resistivity (~0.0195 Ω·mm²/m at 70 °C) | Ω·mm²/m |
Worked example
Interpreting the result
DC loss is pure lifetime waste: every watt lost in the cable is lost at peak sun, every day, for the array's life. The 2% design target balances cable cost against production — long runs push toward 6–10 mm² sooner than intuition suggests. Note the asymmetry: losses scale with I², so higher-voltage strings (smaller current) are dramatically kinder to cable — one reason modern 600–1000 V systems displaced 48 V string inverters in residential PV.
Assumptions
- Copper conductor at ~70 °C operating temperature.
- String operating at Vmp/Imp (peak power point).
Limitations
- Covers resistive DC loss only — connector resistance and MPP tracking offsets are separate.
- Cable ampacity, fuse ratings and temperature derating are separate checks.
Frequently asked questions
What voltage drop is acceptable for solar DC cabling?
≤ 2% is the standard design target (≤ 1% on premium installs). The loss repeats every peak hour for 25+ years, so oversized cable usually pays back.
Why do higher-voltage strings lose less in cable?
Loss scales with current squared: doubling string voltage halves the current and quarters the loss for the same power and cable — the physics behind modern high-voltage PV design.