NEC 690 PV Calculations for Apprentices: Cold-Weather String Voltage, the 125% Current Factors and Rapid Shutdown Limits

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.

Calculation 1: maximum string voltage

A PV module’s open-circuit voltage (Voc) is printed at standard test conditions, 25°C. As the cells get colder, the voltage goes up. The code gives two ways to correct for the lowest expected ambient temperature:

• Module temperature coefficient. Use the voltage temperature coefficient from the module’s instructions (NEC 690.7(A)(1)).
• Table method for crystalline silicon modules. Multiply by the factor from Table 690.7(A), which rises above 1.00 as the temperature falls (NEC 690.7(A)(2)).

Worked example, table method. Crystalline silicon modules with Voc = 40.0 V. The lowest expected temperature is -8°C, so the factor from Table 690.7(A) is 1.14. The system is on a one-family dwelling, where PV dc circuits are limited to 600 V.

1. Corrected Voc per module: 40.0 × 1.14 = 45.6 V.
2. Maximum modules in series: 600 ÷ 45.6 = 13.2, so 13 modules. Always round down.
3. String voltage: 13 × 45.6 = 592.8 V.

Same modules, coefficient method. The instructions list a Voc coefficient of -0.28% per °C.

1. Temperature difference: 25 – (-8) = 33°C.
2. Correction: 1 + (0.0028 × 33) = 1.0924.
3. Corrected Voc: 40.0 × 1.0924 = 43.696 V, and 13 modules give 13 × 43.696 = 568.0 V. Keep the unrounded value until the last step.

The two methods give different answers. On an exam, use the method the question names. The installer must post the highest maximum dc voltage on a permanent label at the dc PV disconnecting means, the electronic power conversion equipment or the associated distribution equipment (NEC 690.7(D)).

Calculation 2: current and conductors

1. Maximum circuit current. For one string, 125% of the module short-circuit current (Isc). Where modules or strings are in parallel, add their Isc values first, then apply 125% (NEC 690.8(A)(1)).
2. Conductor without adjustment. At least 125% of that maximum current (NEC 690.8(B)(1)). 1.25 × 1.25 = 1.5625, so 156.25% of Isc.
3. Conductor with adjustment and correction. After temperature correction and bundling adjustment, the ampacity must still be at least the maximum circuit current from step 1 (NEC 690.8(B)(2)). Choose a conductor that passes both the step 2 check and this check.

Worked example. One string of modules with Isc = 13.0 A.

1. Maximum current: 13.0 × 1.25 = 16.25 A.
2. Without adjustment: 16.25 × 1.25 = 20.3125 A, so the conductor needs at least that much ampacity.
3. On a hot roof the conductor ampacity after correction and adjustment must still be at least 16.25 A.

Calculation 3: inverter output breaker

The inverter output circuit current is the inverter’s continuous output current rating (NEC 690.8(A)(1)(c)). The overcurrent device must be at least 125% of it (NEC 690.9(B)(1)).

An inverter has a continuous output rating of 24 A: 24 × 1.25 = 30 A minimum, which is a standard size.

Safety rules that come with the numbers

• Arc-fault protection. PV dc circuits at 80 V dc or more need listed PV arc-fault protection, with a limited exception (NEC 690.11).
• Rapid shutdown. Outside the array boundary, controlled conductors drop to not more than 30 V within 30 seconds. Inside the boundary, one option limits them to not more than 80 V within 30 seconds (NEC 690.12). A permanent label states that the system has rapid shutdown (NEC 690.12(D)).
• Ground-fault protection. PV dc circuits over 30 V or 8 A need listed ground-fault detection and interruption (NEC 690.41(B)).
• Wiring. Exposed single conductors in the array are PV wire or other listed types (NEC 690.31(C)(1)), and dc wiring methods on or in buildings are marked at intervals of not more than 10 feet (NEC 690.31(D)(2)).
• Connectors. Readily accessible connectors over 30 V dc must be the latching or locking type that takes a tool to open (NEC 690.33(C)).

Check yourself

1. Why does PV voltage rise in winter? Colder cells produce a higher open-circuit voltage.
2. A string has Isc = 10 A. What is the minimum conductor ampacity without adjustment? 10 × 1.5625 = 15.625 A.
3. An inverter is rated 40 A continuous output. What is the minimum breaker? 40 × 1.25 = 50 A.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Article 690 and Table 690.7(A)