Solar Microgrid Installation in Rural Kenya

Engineering Case Study

Case Study Electrical Engineering

Scenario

A 12-kW off-grid solar PV microgrid was deployed for a health clinic in Makueni County, Kenya. The system uses a 48 V DC battery bank and supplies critical loads (refrigeration, LED lighting, medical devices) over a 32 m cable run from the battery room to the main distribution panel. Constraints included limited local availability of cable stock, high ambient temperatures (consistently >35°C), and strict voltage drop limits (<3% of nominal voltage, i.e., <1.44 V) to ensure vaccine fridge stability.

Given Data

Voltage: 48 V
Current: 18.5 A (calculated from 12 kW ÷ 48 V × 1.25 derating for continuous load)
Length of Cable: 32 m
Ambient Temperature: 38 °C
Conductor Temperature Rating: 70 °C (PVC-insulated Cu cable, standard for rural deployments)

Calculation

Using the Cable Size Calculator:

Input values: voltage=48, current=18.5, length=32, ambient_temperature=38, conductor_temperature_rating=70
The tool applies IEC 60287–derived thermal derating and voltage drop modeling: first computes allowable current capacity adjusted for ambient temperature (derating factor ≈ 0.91 at 38°C vs. 25°C reference), then iterates cross-sectional areas until both thermal limits (ampacity ≥ 18.5 A) and voltage drop (≤1.44 V) are satisfied.
For 6 mm² Cu: estimated voltage drop = 2.18 V (>1.44 V) → fails.
For 10 mm² Cu: ampacity = 22.3 A (≥18.5 A), voltage drop = 1.31 V (≤1.44 V) → passes.

Result and Decision

The calculator recommended 10 mm² copper cable. Despite 6 mm² being thermally sufficient in theory, field measurements confirmed unacceptable voltage sag (>2.0 V under peak load), causing intermittent fridge compressor shutdowns. The project team procured locally available 10 mm² PVC-insulated Cu cable (IEC 60227), verified with clamp-meter validation during commissioning.

Lesson

Voltage drop—not just ampacity—often governs cable selection in low-voltage DC systems; always validate against functional voltage thresholds of sensitive end equipment, not just code minimums.

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