NEC 690.7 PV string sizing & maximum system voltage

NEC 690.7 requires the maximum PV system voltage to be the sum of the module open-circuit voltages (Voc) corrected to the lowest expected ambient temperature — cold raises Voc — and it must not exceed the inverter's maximum DC input voltage.

The method

Corrected Voc per module = Voc × [1 + (T_min − 25) × (β / 100)], where β is the temperature coefficient of Voc (%/°C, negative). Maximum system voltage = corrected Voc × modules in series. It must stay below the inverter's max DC voltage (commonly 500–600 V for residential string/hybrid inverters).

Worked example: a 40 V Voc module, β = -0.28%/°C, at a −20 °C design low, 12 in series → 540.5 V cold (45.04 V/module). That fits a 600 V inverter but exceeds a 500 V one.

Max modules per string (Voc 40 V, β -0.28%/°C)

Design low temp	500 V inverter	600 V inverter	1000 V inverter
-10°C (14°F)	11	13	22
-20°C (-4°F)	11	13	22
-30°C (-22°F)	10	12	21

Use the Extreme Annual Mean Minimum Design Dry Bulb Temperature for the site (ASHRAE) as T_min. Related: 690.8 conductor & OCPD sizing.

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