Solar PV Array Combiner to Inverter Run in Southern Spain

Scenario

A 240 kW ground-mounted solar farm near Seville uses string inverters (each 60 kW, 400 V AC output). One inverter feeds into a 45 m underground duct bank containing three parallel 4-core cables (L1/L2/L3/N), buried at 0.8 m depth in sandy loam (ambient 55°C summer peak). Soil thermal resistivity is high (2.5 K·m/W), requiring careful voltage drop control to preserve inverter efficiency and grid compliance (UNE 21103-2 limits voltage drop to ≤2.5% for generation feeders). DC-side losses are managed separately; this case focuses on the final AC interconnection.

Given Data

• Voltage: 400 V
• Power: 60,000 W (per inverter)
• Power Factor: 0.98 (inverter-rated, unity assumed for worst-case reactive margin)
• Cable Length: 45 m
• Acceptable Voltage Drop: 2.5% (tighter than default due to grid code)

Calculation

1. Full-load current:
   ( I = \frac{60{,}000}{1.732 \times 400 \times 0.98} \approx 88.5 , \text{A} )

2. Voltage-drop-driven minimum area (copper, ρ = 0.0172):
   ( A = \frac{1.732 \times 0.0172 \times 45 \times 88.5 \times 100}{400 \times 2.5} \approx 29.3 , \text{mm}^2 )

3. Tool input & output:
   Inputs → voltage=400, power=60000, power_factor=0.98, length=45, acceptable_voltage_drop=2.5
   Output → cable_size=35.0 mm², calculated_voltage_drop=2.38%

Thermal validation: 35 mm² Cu XLPE (PRV-0.6/1 kV) has 135 A base rating (IEC 60502-1). Buried in high-resistivity soil with adjacent circuits → derating factor ≈ 0.72 → 135 × 0.72 = 97.2 A > 88.5 A. Passes.

Result and Decision

The tool recommends 35.0 mm², meeting both voltage drop (2.38% ≤ 2.5%) and derated ampacity. Market availability was confirmed: 35 mm² 4-core XLPE SWA cable is standard. No upsizing needed — 35 mm² balances cost, installation ease (bend radius, weight), and performance. Installed as specified.

Lesson

For renewable energy AC feeders, grid-mandated voltage drop limits often govern sizing more stringently than ampacity — especially in hot, resistive soils. Always use the actual inverter power factor (not default 0.8) and tighten acceptable drop per local grid code; small reductions (e.g., 3% → 2.5%) can shift recommended size by one full standard gauge.