A common-looking 550 Wp module and a 10 kW three-phase inverter, checked against IEC 62548's string voltage window and per-MPPT current limit — string length and module current turn out to be two completely separate problems.
| Module | Voc=49.5V, Vmpp=41.5V, Isc=13.9A, Impp=13.0A, Pmax=550Wp, tempCoeff(Voc)=-0.29%/°C |
| String configuration | 20 modules in series (Ns), 1 string in parallel (Np) |
| Site temperature range | 5°C (cold extreme) to 65°C (hot extreme) |
| Inverter MPPT window | 200 V to 850 V |
| Inverter max DC voltage / max DC current per MPPT | 1000 V / 16 A |
| Inverter AC rating | 10 kW, 3-phase, 400 V |
PV module voltage rises as temperature falls below the 25°C STC rating, and falls as temperature rises above it — both extremes have to be checked against the inverter's limits.
IEC 62548 applies a 1.25x safety margin to the module's rated Isc for this check — and this figure depends only on the module and the 1.25x factor, not on how many modules are wired in series.
| Cable | Design current | Selected size | Derated ampacity | Voltage drop |
|---|---|---|---|---|
| DC string cable | 17.375 A | 4 mm² | 36.4 A | 1.38% (≤2% limit) — PASS |
| AC output cable | 18.04 A (10kW/√3/400V x 1.25) | 2.5 mm² | 30 A | 2.06% (≤3% limit) — PASS |
Dropping one module from the string lowers both Vmax and VmppCold proportionally.
IscDesign depends only on the module and the 1.25x margin, not on Ns — so this failure is untouched by shortening the string.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Max string voltage (cold) | ≤ 1000 V | 1047.42 V (Ns=20) | ✗ FAIL |
| MPPT window (cold ceiling) | ≤ 850 V | 878.14 V (Ns=20) | ✗ FAIL |
| Per-MPPT design current | ≤ 16 A | 17.375 A | ✗ FAIL |
| DC cable voltage drop | ≤ 2% | 1.38% | ✓ PASS |
| AC cable voltage drop | ≤ 3% | 2.06% | ✓ PASS |
Key insight: String voltage/MPPT-window checks and per-MPPT current checks are governed by entirely different design levers — series module count controls the first, module short-circuit current controls the second — so a design that 'fixes' one by changing string length can still fail the other outright. Both need to be checked independently rather than assuming that resolving one automatically resolves the array.
Every input in this example is editable in the live calculator — free, no signup.
Open Solar PV Sizing calculator →PV voltage rises as temperature falls, so the coldest expected site temperature produces the highest string voltage — this is exactly the condition that risks exceeding an inverter's absolute maximum DC input voltage rating, which is why IEC 62548 requires the cold-extreme calculation for this specific check even though the array will spend far more of its life at higher temperatures.
Not on this calculator's free-tier per-MPPT current check — that check compares a single string's Isc-based design current against the inverter's per-MPPT limit, because each string is assumed to land on its own MPPT input. A second string only helps if it is wired to a separate MPPT channel with its own headroom, not if both strings are combined onto the same overloaded input; combining multiple strings onto a shared combiner is exactly what the subscriber-tier multi-string OCPD sizing in this calculator is for.