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Solar PV String Sizing
Cold-Voc max series and warm-Vmp min series for an inverter MPPT window.
Example
You enter
- Module Voc (V) 40
- Module Vmp (V) 33
- Voc temp coeff (% per C, magnitude) 0.3
- Record low temp (°C) -10
- Record high air temp (°C) 45
- Inverter MPPT min (V) 200
- Inverter MPPT max (V) 480
- Inverter Vdc max (V) 600
You get
- Max series count 13
- Min series count 8
- Cold Voc per module 44.2 V
Checked against published worked examples from: NFPA.
Details, formula, and sources
Formula or tableMax modules in series = floor(inverter max dc V / cold Voc), cold Voc = Voc(STC) × (1 + |coeff| × (25 − record low °C)/100) (NEC 690.7); min modules in series = ceil(MPPT min V / warm Vmp), warm Vmp = Vmp(STC) × (1 − |coeff| × (record high + cell rise − 25)/100), cell rise default 30 °C. The longest string is also checked against the MPPT maximum at cold Vmp = Vmp(STC) × (1 + |coeff| × (25 − record low °C)/100), an energy-harvest note, not a code limit. Current (690.8) and OCP (690.9) sizing are not computed.
Source and editionNEC 2023 (NFPA 70). Article 690.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesmodule_voc_V, module_vmp_V, voc_temp_coeff_pct_per_C, record_low_C, record_high_C, inverter_mppt_min_V, inverter_mppt_max_V, inverter_vdc_max_V, max_series, min_series, cold_voc_V
- Temperature coefficient entered from the module datasheet (about −0.30%/°C typical; no default)module datasheet
- Inverter-input window user-supplied from listingUL 1741 listing