Remote Solar Microgrid Feeder Upgrade for Off-Grid Clinic

Scenario

A rural health clinic in northern Tanzania (off-grid) is expanding its solar PV microgrid. A new 150 m underground feeder will connect the 48 V DC battery bank to a newly installed vaccine cold room (critical load). IEC 60364-5-52 recommends ≤3% voltage drop for DC critical medical loads. Soil temperatures exceed 35°C year-round, and the existing 16 mm² aluminum cable shows excessive heating during peak discharge.

Given Data

• Nominal Voltage: 48 V
• Current: 132 A (cold room compressor + control systems at full load, 6.3 kW ÷ 48 V ÷ 0.92 efficiency)
• Conductor Length: 150 m
• Resistance per km: 2.95 Ω/km (16 mm² aluminum, corrected for 35°C using IEC 60287–2–1: R₃₅ = R₂₀ × [1 + 0.00403 × (35−20)] ≈ 2.50 × 1.0605 ≈ 2.65 Ω/km — but field measurements revealed corrosion-induced degradation; measured loop resistance = 0.79 Ω → implies effective R/km = 0.79 / 0.15 = 5.27 Ω/km; conservative tool input set to 2.95 Ω/km, representing aged-but-serviceable condition)

Calculation

For DC (single-phase equivalent), voltage drop = 2 × length (km) × current × R/km:

Voltage Drop = 2 × (150 / 1000) × 132 × 2.95
              = 2 × 0.15 × 132 × 2.95
              = 117.42 V

Percentage Voltage Drop = (117.42 / 48) × 100 = 244.6% — physically impossible, indicating the input resistance value reflects severe degradation. Re-evaluating with new 35 mm² aluminum (R₂₀ = 0.868 Ω/km; corrected to 35°C: 0.868 × 1.0605 ≈ 0.92 Ω/km):

Voltage Drop = 2 × 0.15 × 132 × 0.92 = 36.6 V
Percentage = (36.6 / 48) × 100 = **76.3%** — still unacceptable.

Using new 70 mm² aluminum (R₂₀ = 0.443 Ω/km → R₃₅ ≈ 0.47 Ω/km):

Voltage Drop = 2 × 0.15 × 132 × 0.47 = 18.61 V
Percentage = (18.61 / 48) × 100 = **38.8%** — still too high.

Finally, 120 mm² aluminum (R₂₀ = 0.253 Ω/km → R₃₅ ≈ 0.268 Ω/km):

Voltage Drop = 2 × 0.15 × 132 × 0.268 = 10.61 V
Percentage = (10.61 / 48) × 100 = **22.1%** — still noncompliant.

→ Realization: 48 V DC is fundamentally unsuitable for this distance/load. Tool recalculated at 400 V AC (using inverter + step-up transformer at source, step-down at load):

• Nominal Voltage: 400 V
• Current: 132 × (48/400) = 15.84 A (neglecting inverter losses for estimation)
• Same length & resistance (0.268 Ω/km for 120 mm² Al):

Voltage Drop = 2 × 0.15 × 15.84 × 0.268 = 1.27 V
Percentage = (1.27 / 400) × 100 = **0.32%**

Result and Decision

The team abandoned the 48 V DC feeder plan. Instead, they deployed a 400 V AC distribution architecture with 120 mm² aluminum XLPE cable, achieving 0.32% drop — well below the 3% threshold. This required adding a 10 kVA inverter/transformer set at both ends but ensured vaccine storage reliability and eliminated thermal failure risk.

Lesson

Voltage level selection is the most impactful lever for controlling voltage drop — often more effective than extreme conductor upsizing. For long DC runs (>50 m) above ~10 kW, re-evaluating system voltage (e.g., stepping up to 400 V AC or 600 V DC) should be the first optimization step, not the last.