Solar PV String Sizing: Why Your Inverter Clips at 2pm on the Hottest Day

A colleague in Perth called me about a rooftop PV system that kept throwing inverter overvoltage faults every winter morning. The system had been designed and installed by an experienced solar company. The string voltage calculation showed 470V at STC — comfortably below the inverter's 500V maximum input.

The problem: Perth's minimum morning temperature drops to 2°C in July. At that temperature, the open-circuit voltage of the string rose to 530V — exceeding the inverter maximum by 30V. The inverter's overvoltage protection tripped, shutting down the system until the modules warmed up. On the coldest mornings, the system didn't start generating until 10am.

The string had been sized at STC (25°C) only. Nobody had calculated the voltage at the minimum site temperature.

STC vs Reality

Solar PV modules are rated at Standard Test Conditions (STC):

• Cell temperature: 25°C
• Irradiance: 1,000 W/m²
• Air mass: AM 1.5

These conditions exist for perhaps 15 minutes per year in most locations. In reality:

• Module operating temperature in the field is 20–35°C ABOVE ambient temperature (the NOCT offset)
• On a hot day with 35°C ambient, module cell temperature reaches 55–70°C
• On a cold morning with 0°C ambient and clear sky irradiance, module temperature might be only 5–10°C

Where NOCT is the Nominal Operating Cell Temperature (typically 43–48°C for crystalline silicon) and G is the irradiance in W/m².

The Temperature Coefficients

Every PV module datasheet lists three temperature coefficients:

The critical insight: Voc has a NEGATIVE temperature coefficient. This means voltage goes UP when temperature goes DOWN. Engineers intuitively expect hot weather to be the worst case for PV systems. For voltage, it's the opposite — cold weather produces the highest voltages.

Worked Example: String Voltage at Temperature Extremes

Module specifications (typical 400W mono-crystalline):

• Voc (STC) = 48.0V
• Vmp (STC) = 40.0V
• α_Voc = -0.30%/°C
• α_Vmp = -0.40%/°C (often approximated from α_Pmax and α_Isc)

String configuration: 10 modules in series

Site temperature range: -10°C to +45°C ambient (Southern Australia)

At STC (25°C):

• String Voc = 10 × 48.0 = 480V
• String Vmp = 10 × 40.0 = 400V

At minimum temperature (-10°C, cell temperature approximately -5°C):

Temperature difference from STC: -5 - 25 = -30°C

At maximum temperature (45°C ambient, cell temperature approximately 70°C):

Temperature difference from STC: 70 - 25 = +45°C

Now compare these against a typical string inverter:

• Maximum input voltage: 500V
• MPPT range: 140–500V

Cold morning: 523V exceeds the 500V maximum — inverter will trip or be damaged.

Hot afternoon: 328V is within the MPPT range — this is fine, but we've lost 18% of the Vmp, which reduces generation capacity.

The fix: reduce to 9 modules per string. Voc(cold) = 9 × 48 × 1.090 = 471V (safe). Vmp(hot) = 9 × 40 × 0.82 = 295V (still within MPPT range).

IEC 62548, Clause 7.3 — String sizing voltage limits with temperature correction

The NEC Approach

NEC/NFPA 70, Section 690.7 — Maximum voltage for PV source and output circuits

NEC 690.7(A) requires the maximum PV system voltage to be calculated based on the lowest expected ambient temperature. Table 690.7(A) provides voltage correction factors for crystalline and thin-film modules at various temperatures:

NEC requires using the manufacturer's temperature coefficient if available (more accurate than Table 690.7(A)), and explicitly states that the corrected voltage must not exceed the maximum rated voltage of the inverter, conductors, disconnects, and other equipment.

The Australian Approach

AS/NZS 5033, Clause 4.3.3 — Maximum PV string voltage

AS/NZS 5033 requires voltage calculation using the actual temperature coefficients and the minimum ambient temperature for the site location. The Bureau of Meteorology provides design minimum temperatures for Australian locations. Importantly, AS/NZS 5033 uses a slightly different cell temperature model for the cold case — it assumes the cell temperature equals the ambient temperature (no irradiance heating) at the coldest moment, which gives a conservative result.

DC Cable Sizing Trap

While discussing PV string sizing, there's a related trap that catches many designers. PV DC cables must be sized to carry the short-circuit current multiplied by a safety factor, not the operating current:

NEC/NFPA 70, Section 690.8 — Circuit sizing and current

This catches designers who size the DC cable based on the maximum power current (Imp). The short-circuit current is typically 5–10% higher than Imp, and the 1.25× factor adds another 25%. The combined effect means DC cables should be rated for approximately 35% more current than the expected operating current.

Additionally, PV cables operate at elevated temperatures (rooftop mounting, direct sun exposure). The cable derating for high ambient temperature must be applied on top of the current safety factor.

Intentional Inverter Clipping: DC:AC Ratio

Counterintuitively, experienced PV system designers often INTENTIONALLY oversize the PV array relative to the inverter's AC output capacity. This is called the DC:AC ratio (or oversizing ratio), typically 1.1 to 1.3.

For example, a 10kW inverter might be paired with 12–13kW of PV modules (DC:AC ratio of 1.2–1.3).

Why? Because:

1. PV modules rarely produce their full STC rating — clouds, soiling, temperature, and low sun angles reduce output
2. A 10kW array connected to a 10kW inverter only clips for a few minutes per year (at solar noon in perfect conditions)
3. The additional morning and afternoon generation from the oversized array more than compensates for the midday clipping losses
4. Annual energy yield increases by 5–15% with a 1.2:1 ratio, despite the midday clipping

This intentional clipping is different from the VOLTAGE clipping discussed earlier. DC:AC oversizing clips the POWER output (the inverter limits its AC output to its rated capacity). Voltage clipping occurs when the string voltage exceeds the inverter's input voltage limit — this causes the inverter to shut down entirely, which is never desirable.

The Tropical Context

In Indonesia, the cold morning overvoltage risk is minimal — ambient temperatures rarely drop below 18°C even in the highlands. The dominant PV sizing challenge in tropical locations is:

1. High operating temperatures: module temperatures regularly reach 65–75°C, reducing Vmp significantly and requiring string lengths that keep Vmp above the inverter's MPPT minimum even at peak temperature
2. Irradiance variability: rapid cloud transitions cause rapid voltage swings as modules heat and cool; the MPPT tracker must handle this
3. Humidity and salt air: coastal installations in tropical Indonesia face aggressive corrosion; all DC connections and junction boxes need IP65+ rating and marine-grade materials

String Sizing Checklist

1. Calculate maximum Voc at the lowest site temperature — this must not exceed the inverter maximum input voltage OR the maximum system voltage rating of cables and connectors (typically 600V or 1000V)
2. Calculate minimum Vmp at the highest cell temperature — this must remain within the inverter MPPT window
3. Verify the DC cable current rating using Isc × 1.25 (NEC) or × 1.20 (IEC/AS), with temperature derating
4. Check the DC:AC ratio — intentional oversizing of 1.1–1.3:1 is normal and beneficial
5. Document the temperature data source — use official meteorological records for the site, not assumptions

Getting these calculations right at design stage prevents costly commissioning failures and ensures the system generates optimally across all seasons.

Related Resources

• California Solar Farm: Arc Fault — DC arc fault risks in PV string design
• Cable Sizing: The 50m Office Feeder — Cable sizing principles apply to PV DC circuits too
• Cable Derating: 12 Cables in a Tray at 40°C — Rooftop PV cables face extreme temperature derating
• View all worked examples →