About the Voltage Drop
Voltage drop in a solar wiring system is the reduction in voltage along a cable due to its resistance. Excessive voltage drop wastes energy (converting it to heat in the cable), can push inverter input voltage below the MPPT window, and reduces system output. Indian and IEC standards require DC cable voltage drop ≤ 1% and AC cable drop ≤ 1.5%.
Formula
Voltage drop (V) = 2 × L × I × ρ ÷ A
- L
- One-way cable length from array to inverter (m)
- I
- Maximum current through the cable (A) — Isc for DC strings, rated AC current
- ρ
- Resistivity of copper: 0.0168 Ω·mm²/m; aluminium: 0.0282 Ω·mm²/m
- A
- Cable cross-sectional area (mm²)
Voltage drop (%) = Voltage drop (V) ÷ System voltage (V) × 100
- System voltage
- String Vmpp for DC; line voltage (230/415 V) for AC
How to calculate it
- 1Measure or estimate the cable run length (one-way) from string combiner to inverter (DC) or inverter to grid (AC).
- 2Determine maximum current: string Isc (DC) or AC rated output current.
- 3Select a cable cross-section from the standard range (4/6/10/16/25 mm²).
- 4Apply the formula to get voltage drop in volts and as a percentage.
- 5If the percentage exceeds 1% (DC) or 1.5% (AC), upsize the cable cross-section.
Worked example
DC cable run 30 m (one-way), 10 A string current, 6 mm² copper. Vdrop = 2 × 30 × 10 × 0.0168 ÷ 6 = 1.68 V. On a 360 V string: 1.68/360 = 0.47% — within the 1% limit. The same run with 4 mm² cable: Vdrop = 2.52 V → 0.70% — still within limit but cutting margin thinly for a longer future run.
What you can calculate
- DC & AC circuits
- ≤1% DC limit
- ≤2.5% AC limit
- Temperature correction
Frequently asked questions
What cable size should I use for a 3 kW solar rooftop system?
For standard 3 kW residential rooftop runs under 20 m: 4 mm² DC solar cable is usually sufficient. For longer runs (20–50 m) or higher-current strings: 6 mm². AC output cable from inverter to distribution board: typically 10 mm² copper for single-phase 3 kW output. Always use UV-resistant double-insulated solar cable (H1Z2Z2-K or equivalent) for outdoor DC runs.
Why is DC voltage drop more critical than AC voltage drop in solar?
DC voltage drop has a double penalty: it directly reduces the string voltage at the inverter, which can push it below the MPPT window and cause the inverter to shut down or de-rate. AC voltage drop only reduces the voltage at the grid connection point, which the inverter compensates for by adjusting output. Both matter, but DC cable sizing deserves more care.
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