Temperature-Corrected Voc & String Sizing (NEC 690.7)

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Here's one that matters more than its name lets on — Temperature-Corrected Voc & String Sizing. If you remember one thing, make it this: module voltage goes up when it gets cold, so the longest string you can build is set by the coldest morning of the year, and the shortest is set by the hottest afternoon. Get this right and the inverter lives a long life; get it wrong and you either void a warranty, break a Code limit, or watch the inverter drop offline every summer afternoon.

Educational content only — not electrical design, engineering, or a substitute for a licensed electrician, electrical contractor, or professional engineer. Designs must follow the NEC (NFPA 70) edition adopted by your Authority Having Jurisdiction (AHJ) and any local amendments, the equipment manufacturers' installation instructions, and your utility's interconnection rules. Code section numbers below refer to the 2023 NEC unless noted; numbering and requirements change between editions.

Reading the module datasheet

Everything starts with the module's electrical ratings at standard test conditions (STC): 1,000 W/m² irradiance and a 25°C cell temperature. The numbers you need:

• Voc — open-circuit voltage
• Vmp — voltage at maximum power
• Isc — short-circuit current
• Imp — current at maximum power
• Temperature coefficient of Voc (in %/°C or V/°C), and of Pmax (and sometimes Vmp)
• Maximum system voltage of the module and its maximum series fuse rating

Temperature coefficients for crystalline silicon Voc are negative, often in the range of roughly -0.25 to -0.35 %/°C. Negative means voltage rises as temperature falls below 25°C.

What 690.7 requires

690.7 says the maximum PV system DC voltage is the sum of the series-connected modules' Voc, corrected for the lowest expected ambient temperature. It gives three paths: use the manufacturer's temperature coefficient (the preferred method in practice), use the correction factors in Table 690.7(A) for crystalline and multicrystalline modules when a coefficient isn't used, or, for larger systems (100 kW or more), a documented engineering calculation by a licensed professional engineer.

For the low temperature, the Code's informational note points designers to the ASHRAE "Extreme Annual Mean Minimum Design Dry Bulb Temperature" for the nearest weather station. Many AHJs publish the value they expect.

690.7 also sets the ceilings: 600 V for PV systems on one- and two-family dwellings, 1,000 V for other buildings, and up to 1,500 V for systems not on or in buildings. Module electronics can change how voltage is determined, so follow the listing and instructions for MLPE.

The formula and a worked example

Corrected Voc = Voc x [1 + (Tmin - 25) x coefficient]

Module: Voc = 49.5 V, coefficient = -0.27%/°C. Site design low: -15°C.

• Temperature difference: -15 - 25 = -40°C
• Correction: -40 x -0.0027 = +0.108
• Corrected Voc: 49.5 x 1.108 = 54.8 V

Inverter maximum DC input: 600 V. Maximum modules in series: 600 / 54.8 = 10.9 → 10 modules. Always round down. A string of 10 reaches about 548 V on the coldest morning. Eleven would reach about 603 V — over the inverter rating and the dwelling limit.

The minimum string: the hot side

The other end of the window is the inverter's MPPT minimum voltage (and its start-up voltage). On a hot roof, cells run well above ambient — often 25 to 35°C hotter in full sun, more for flush-mounted arrays with little airflow. Voltage sags.

Same module: Vmp = 41.6 V. Assume a design high ambient of 38°C plus 35°C rise = 73°C cell. Use the Vmp or Pmax coefficient if provided; say -0.35%/°C.

• Difference: 73 - 25 = 48°C; 48 x -0.0035 = -0.168
• Hot Vmp: 41.6 x 0.832 = 34.6 V

Inverter MPPT minimum: 200 V → 200 / 34.6 = 5.8 → 6 modules minimum. Round up. The valid string window is 6 to 10 modules. Real designs add margin for module degradation and inverter derating at the low end.

Going Deeper (Intermediate)

Inverter manufacturers publish string-sizing tools, and they are worth using — but know the hand calculation so you can check the tool and answer the inspector. Make sure each MPPT input has strings of equal length (paralleled strings must match voltage), and that the strings match the MPPT's current limit. With optimizers, the string length rules come from the optimizer and inverter manufacturer, not simply the module Voc, because the optimizer regulates string voltage; with microinverters, each module sees only its own micro, and the check is that module Voc at the cold temperature stays inside the micro's input range.

When you cannot find a temperature coefficient, Table 690.7(A) gives multipliers that rise as the temperature range falls. The table is conservative compared to most modern modules' coefficients.

Advanced / Pro-Level

Pros document the source of every number: the datasheet revision, the weather station and its ASHRAE value, and the inverter input limits. They check the module's own maximum system voltage (1,000 V or 1,500 V) as well as the inverter's. On bifacial modules they read the datasheet for rear-side gain, because higher current affects 690.8 calculations even though voltage behavior is similar. On commercial and utility work, engineers may use site-specific irradiance-weighted temperature data under the engineering method, which can allow longer strings — but only with a PE stamp and the AHJ's acceptance.

Practice Challenge

A residential design uses a module with Voc = 40.8 V and a coefficient of -0.29%/°C. The AHJ's design low is -20°C. The inverter's maximum input is 600 V. What is the longest allowable string? (Answer: difference = -45°C; correction = -45 x -0.0029 = +0.1305; corrected Voc = 40.8 x 1.1305 = 46.1 V; 600 / 46.1 = 13.0 → 13 modules gives about 599.7 V, which is just under 600 V. Many designers would drop to 12 for margin, and you should confirm the exact numbers before committing.)

In Practice

Put the string-sizing calculation on the plan set's calculations page, showing the module, the coefficient, the temperature source, and the result. Inspectors approve what they can check, and a clear calculation shortens the review.

Common Mistakes to Avoid

• Using STC Voc with no cold-temperature correction
• Rounding the maximum string length up instead of down
• Ignoring the hot-side minimum, so the inverter drops out of MPPT range in summer
• Mixing string lengths on one MPPT input
• Using a generic coefficient instead of the actual module's datasheet value

Takeaway: Cold sets the longest string and heat sets the shortest — correct Voc to the site's design low under 690.7, round down, check the hot-side minimum against the MPPT window, and show your work on the plan set.