DC Cable Voltage Drop Calculator
Solar DC string and battery cable voltage drop with the 1% / 2% design limits used for PV array and battery circuits.
Inputs
Use Isc × 1.25 for PV strings
PV cable in sunlight commonly reaches 70–90 °C
Formula
- ΔV = 2 × ρ_T × L × I / A (DC always uses the go-and-return path)
- ρ_T = ρ₂₀ (1 + α (T − 20))
- Energy loss ratio = ΔV / V
DC circuits in PV systems carry current continuously at peak sun, so even a small percentage drop compounds into a real annual yield loss. Industry practice limits array DC drop to 1% and battery-to-inverter drop to 2%, because low-voltage battery circuits carry very high current where every milliohm matters.
Step-by-step calculation
Resistivity at temperature
ρ₂₀(1 + α(T − 20))
0.02058 Ω·mm²/m
Voltage drop
2 × ρ × L × I / A
4.939 V
Percentage
ΔV / V × 100
0.823%
Required area
2ρLI / (target% × V)
3.29 mm²
Worked example
- String of 12 A over 40 m of 4 mm² copper at 70 °C, 600 V DC.
- ρ₇₀ = 0.02058 Ω·mm²/m; ΔV = 2 × 0.02058 × 40 × 12 / 4 = 4.94 V = 0.82% — within the 1% target.
Assumptions
- Double-insulated PV cable rated 1.5 kV DC with a 90–120 °C conductor rating.
- Current is taken as the continuous design current, not the instantaneous peak.
Tips
- Keep string cable runs short by placing combiner boxes near the array.
- For battery banks, use short, equal-length cables to keep parallel strings balanced.
Warnings
- A 48 V battery bank carrying 200 A loses 2% in only a few metres — always oversize battery cables.
- PV DC cables must be UV-resistant and rated for the full open-circuit voltage at the lowest site temperature.
Standards & references
- IEC 62548 (PV array design)
- IEC 60364-7-712
- IS 16221
Frequently asked questions
What is the maximum voltage drop for solar DC cables?
1% from array to inverter and 2% from battery to inverter are the accepted design targets; codes allow up to 3% but yield suffers.
Why is DC drop calculated with a factor of 2?
Because current flows out along the positive conductor and back along the negative, so the total resistive path is twice the route length.
Should I use Isc or Imp for string cables?
Size the conductor for 1.25 × Isc for safety, but calculate voltage drop and yield loss at Imp, the actual operating current.
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