Solar PV Array Interconnection in Arizona Desert

Case Study 2: Solar PV Array Interconnection in Arizona Desert

Scenario

A 2.4 MW ground-mount solar farm near Yuma, AZ required DC string interconnection from combiner boxes to the central inverter. High ambient temperatures (up to 55°C), long trench runs across sandy soil, and NEC 2023 Article 690.7(A) mandates drove design. Critical constraint: DC voltage drop must remain ≤1.5% to preserve inverter MPPT efficiency and avoid clipping losses — stricter than the default 3% lighting guideline.

Given Data

• Nominal Voltage: 1000 V (ungrounded DC system)
• Current: 212 A (maximum string current after 125% overcurrent protection factor)
• Cable Length (one way): 215 m (trenched, direct burial, 30°C earth temp but 55°C ambient exposure at surface)
• Initial Cable Size Considered: 150 mm² Al, XLPE-insulated, USE-2/RHHW-2 rated

Calculation

Per the tool’s resistive model (using aluminum resistivity ρ = 0.02826 Ω·mm²/m at 20°C, adjusted to 0.0341 Ω·mm²/m at 55°C):

• R = (ρ × L) / A = (0.0341 Ω·mm²/m × 215 m) / 150 mm² = 0.0489 Ω
• Voltage Drop (DC) = I × R = 212 A × 0.0489 Ω ≈ 10.37 V
• Percentage Voltage Drop = (10.37 V / 1000 V) × 100 = 1.04%

The tool output:

• voltage_drop: 10.37 V
• percentage_voltage_drop: 1.04%

Result and Decision

Although 1.04% met the 1.5% target, field measurements during commissioning revealed 1.8% drop due to unaccounted connector resistance (0.3 mV per MC4) and elevated daytime conductor temperature (>60°C). The engineering team revised to 185 mm² Al, reducing calculated drop to 0.85% — providing margin for aging, contact degradation, and peak summer conditions. UL-listed 185 mm² USE-2 cable was procured, staying within budget and trench width limits.

Lesson

Always include real-world parasitic resistances (connectors, terminations, splices) in voltage drop validation — the calculator provides a baseline, but field conditions often demand ≥20% safety margin for critical DC renewable systems.