Solar Microgrid Installation in Rural Kenya

Scenario

A 12 kW off-grid solar PV microgrid is being deployed for a health clinic in Kakamega County, Kenya. The system uses a 400 V DC bus (battery bank to inverter input) with cables routed through a humid, tropical environment (ambient temperature ~35°C). Space constraints limit conduit size, and local procurement restricts cable availability to standard 6 mm², 10 mm², 16 mm², and 25 mm² Cu PVC-insulated variants. Budget prohibits oversized cables without justification.

Given Data

• Voltage: 400 V (DC)
• Power: 12,000 W (peak continuous load)
• Power Factor: 1.0 (DC system — no reactive component)
• Cable Length (one way): 42 m (from battery room to main distribution board)
• Acceptable Voltage Drop: 2.0 %

Calculation

Using the Cable Size Calculator:

1. Current calculation: For DC, (I = \frac{P}{V \times \text{PF}} = \frac{12{,}000}{400 \times 1.0} = 30,\text{A})
2. Voltage drop constraint: Max allowable drop = (2% \times 400,\text{V} = 8,\text{V})
3. Resistance limit: For two-way path (L_total = 2 × 42 m = 84 m), (R_{\text{max}} = \frac{V_{\text{drop}}}{I} = \frac{8}{30} = 0.267,\Omega)
4. Required cross-section: Using copper resistivity (ρ = 0.0172 Ω·mm²/m), (A = \frac{\rho \times L_{\text{total}}}{R_{\text{max}}} = \frac{0.0172 \times 84}{0.267} \approx 5.42,\text{mm}^2)
5. Tool input & output: Entering {"voltage":400,"power":12000,"power_factor":1.0,"cable_length":42,"acceptable_voltage_drop":2} yields:
   • Recommended Cable Size: 6.0 mm²
   • Calculated Voltage Drop: 1.92 %

Result and Decision

The calculator recommends 6 mm², which meets the 2% limit and aligns with locally available stock. However, due to ambient temperatures exceeding 30°C, derating was applied: manufacturer data shows 6 mm² Cu PVC rated at 41 A at 30°C but only 35 A at 35°C — still sufficient for 30 A. Final selection: 6 mm² single-core Cu PVC (IEC 60227), installed in shaded, ventilated trunking.

Lesson

Always validate the calculator’s output against real-world thermal derating — nominal sizing may pass voltage drop but fail ampacity in high-temperature environments. Field temperature profiling is non-negotiable for tropical deployments.