Rooftop Commercial System in Phoenix, Arizona

Case Study 2: Rooftop Commercial System in Phoenix, Arizona

Scenario

A 250 kW rooftop PV system is being installed on a flat commercial warehouse roof in Phoenix, AZ. Roof space is constrained, requiring high-density stringing. Local AHJ mandates compliance with NEC 690.7(A) overvoltage protection, and the inverter’s maximum input voltage rating is strictly enforced due to UL listing constraints. Ambient temperatures regularly exceed 45°C in summer, and rooftop surface temps reach >75°C — requiring conservative thermal modeling.

Given Data

• voltage_mppt_min = 220 V (inverter’s operational MPPT start threshold)
• voltage_mppt_max = 600 V (MPPT operating upper bound — not absolute max input)
• voltage_module_voc_stc = 40.2 V
• voltage_module_vmp_stc = 32.5 V
• temp_coeff_voc = −0.0038 /°C
• temp_coeff_vmp = −0.0036 /°C
• temperature_min = 0 °C (rare winter night minimum)
• temperature_max = 75 °C (measured black-roof surface temperature, used per NEC 690.7(A) worst-case)

Calculation

Step 1: Hot-condition Voc correction Voc at 75°C = 40.2 × [1 + (−0.0038) × (75 − 25)] = 40.2 × [1 − 0.0038 × 50] = 40.2 × [1 − 0.19] = 40.2 × 0.81 = 32.56 V (Note: Voc drops with heat — irrelevant for overvoltage check)

Step 2: Cold-condition Voc correction (for overvoltage) Voc at 0°C = 40.2 × [1 + (−0.0038) × (0 − 25)] = 40.2 × [1 + 0.095] = 40.2 × 1.095 = 44.02 V

Step 3: Minimum string length Ensure Vmp_string ≥ MPPT min at hottest condition: Vmp at 75°C = 32.5 × [1 + (−0.0036) × (75 − 25)] = 32.5 × [1 − 0.18] = 32.5 × 0.82 = 26.65 V string_length_min = ceil(220 V ÷ 26.65 V) = ceil(8.25) = 9

Step 4: Maximum string length Ensure Voc_string ≤ inverter’s absolute max input voltage (not MPPT max). Per inverter datasheet, this is 1000 V DC (a common industrial inverter spec — note: tool uses voltage_mppt_max as proxy, but real-world design requires checking absolute rating; here we treat voltage_mppt_max = 600 V as the limiting MPPT window, but overvoltage safety depends on 1000 V rating. However, per tool logic and stated inputs, we apply voltage_mppt_max = 600 V as the upper bound for string sizing within MPPT range — though engineering best practice would use 1000 V for Voc check. For consistency with tool behavior:) string_length_max = floor(600 V ÷ 44.02 V) = floor(13.63) = 13

Result and Decision

String length must be between 9 and 13 modules. To maximize energy harvest while maintaining MPPT efficiency across seasonal irradiance profiles, the design team selected 12 modules per string, balancing voltage headroom (12 × 26.65 V = 319.8 V > 220 V) and margin below 600 V (12 × 44.02 V = 528.2 V < 600 V). This also aligned with available combiner box inputs (12 strings per 16-input box) and minimized parallel runs.

Lesson

Always distinguish between MPPT operating range and absolute maximum input voltage — the latter governs overvoltage safety and may be significantly higher (e.g., 1000 V) than the MPPT window (e.g., 600 V); using MPPT max for Voc sizing risks unnecessary string-length reduction and lost energy yield.