Solar PV Array DC Combiner Run in Arizona Desert

Case Study 2: Solar PV Array DC Combiner Run in Arizona Desert

Scenario

A 25 kW rooftop solar PV system in Phoenix, AZ uses string inverters with central DC combiner boxes located 120 m from the array. The DC circuit operates at 600 V nominal (ungrounded bipolar), with peak string current of 32 A per polarity. Ambient temperatures regularly exceed 45°C; cables are installed in non-ventilated raceways on a black roof surface. NEC 2023 Article 690.31(E) mandates ≤1.5% voltage drop for DC source circuits to preserve MPPT efficiency and prevent inverter clipping. Only USE-2 and PV Wire 600 V cables (90°C rated) are permitted.

Given Data

• Voltage: 600 V (DC)
• Power: 25000 W (system max, but sizing based on worst-case string current × voltage)
• Power Factor: 1.0 (DC circuit)
• Cable Length: 120 m
• Acceptable Voltage Drop: 1.5%

Note: Though DC has no power factor, the tool accepts PF = 1.0 to compute current correctly: ( I = P/V ). For safety, we conservatively use full system power (25 kW) at 600 V — yielding 41.67 A — though actual combiner input is lower. Engineering judgment overrides nominal values: design current = 32 A × 2 (positive + negative conductors) = 64 A total loop current.

Tool inputs used:

• Voltage: 600 V
• Power: 38400 W (i.e., 64 A × 600 V — representing full loop load)
• Power Factor: 1.0
• Cable Length: 120 m
• Acceptable Voltage Drop: 1.5%

Calculation

1. Calculated Current:
   ( I = \frac{38400}{600 \cdot 1.0} = 64.00 , \text{A} )

2. Voltage Drop Iteration (tool uses DC resistive model: ( % \text{VD} = \frac{2 \cdot \rho \cdot L \cdot I}{V \cdot S} \times 100 ), with ( \rho = 22.5 , \text{m}\Omega\cdot\text{mm}^2/\text{m} )):

   • Try 16 mm²: ( % \text{VD} = \frac{2 \cdot 22.5 \cdot 120 \cdot 64}{600 \cdot 16} = 3.60% ) → too high.
   • Try 35 mm²: ( % \text{VD} = \frac{2 \cdot 22.5 \cdot 120 \cdot 64}{600 \cdot 35} \approx 1.64% ) → still exceeds 1.5%.
   • Try 50 mm²: ( % \text{VD} = \frac{2 \cdot 22.5 \cdot 120 \cdot 64}{600 \cdot 50} \approx 1.15% ) → acceptable.

   Ampacity check: 50 mm² USE-2 @ 45°C ambient (NEC Table 310.16, 90°C column, 0.56 correction factor) = 185 A × 0.56 ≈ 104 A > 64 A → compliant.

Result and Decision

The Cable Sizing Calculator returned:

• Recommended Cable Size: 50.0 mm²
• Calculated Current: 64.00 A
• Calculated Voltage Drop: 1.15%

A 50 mm² PV Wire (UL 4703, 90°C, sunlight-resistant) was specified — meeting NEC voltage drop limits, thermal derating, and UV exposure requirements. Although 35 mm² was physically installable, it violated the 1.5% threshold and risked >2% annual energy loss.

Lesson

In high-temperature DC applications, voltage drop dominates sizing — even large cables may be required. Always base calculations on actual circuit current (not system nameplate) and apply NEC ambient correction after selecting size, not before; the tool’s %VD output assumes standard resistivity, so field validation with manufacturer R-values is essential.