Solar string sizing for a 600 V inverter

A string that is too long can push the inverter past its d.c. limit on a frosty morning. A string that is too short can drop below the MPPT window on a hot afternoon. This guide sizes one string with the PV string tool and checks its current, fuses and isolator. The temperature effects are explained in PV string voltage and temperature.

The AS/NZS 5033 figures in the tool are verified against the printed standard. Check each figure against the current standard.

The system#

• 49.5 V Voc, 41.5 V Vmp, 13.9 A Isc modules. Voc coefficient −0.27 %/K, Vmp coefficient −0.35 %/K.
• The module data sheet gives a 25 A maximum series fuse.
• A 600 V inverter with an MPPT window of 150 V to 550 V. Its plate is marked with these limits.
• A house roof. Record low −5 °C, record high 35 °C.
• Two strings in parallel on one MPPT.

Step 1. Voltage at the coldest cell#

Cl 4.2.1.2 corrects Voc to the lowest expected operating temperature. With the maker's coefficient:

1 + (−0.27 % × (−5 − 25)) = 1.081, so 49.5 V × 1.081 = 53.51 V

The inverter maximum is 600 V and the domestic limit is 1000 V (Cl 3.1). The lower one rules: 600 ÷ 53.51 = 11.2, so at most 11 modules per string.

With no maker's method, Table 4.1 gives 1.12 for −1 °C to −5 °C. Then each module reaches 55.44 V, and only 10 modules fit. The coefficient is the closer figure where the data sheet gives it.

Step 2. Voltage at the hottest cell#

The cell runs well above the air on a hot roof. AS/NZS 5033 sets no rise, so the tool asks for one: 35 K, AmpSize's assumption for a roof mount. The hottest cell is 35 + 35 = 70 °C.

1 + (−0.35 % × (70 − 25)) = 0.8425, so 41.5 V × 0.8425 = 34.96 V

150 ÷ 34.96 = 4.3, so at least 5 modules per string. The string length is 5 to 11 modules.

Step 3. Check the chosen string#

Twelve modules would reach 642 V on a cold morning, past the inverter's maximum.

Step 4. Current and fuses#

Cl 3.3.3.1 takes the maximum string current as 1.25 × KI × Isc. For a monofacial module KI is 1: 1.25 × 13.9 = 17.38 A. With two strings, a fault in one string can draw the other string's 17.38 A. That stays under the 25 A module fuse rating, so no string fuses are called for.

With three strings, the fault current is 34.75 A, past 25 A. String fuses are then called for, rated above 1.2 × 17.38 = 20.85 A and at most 25 A: a 25 A fuse fits.

Step 5. Cable and isolator#

Size the cables at these currents with the d.c. cable size tool, at the hot cable temperature. Above 120 V, keep the voltage drop to the inverter within 5 % of Vmp.

See also: PV and inverter basics and the voltage rise tool for the a.c. side.