NEC PV String Voltage & Ampacity Calculator

System Parameters

Module Open-Circuit Voltage (Voc) Rated Voc at Standard Test Conditions (STC)

Module Short-Circuit Current (Isc) Rated Isc at Standard Test Conditions (STC)

Voc Temperature Coefficient (%/°C) Typically a negative percentage (e.g., -0.30)

Record-Low Ambient Temperature (Tmin) Expected extreme cold in Celsius (°C)

Modules in Series Number of panels wired in a single string

System Max Voltage Limit (V) Inverter/Equipment limit (e.g., 600, 1000, 1500)

Calculated Results

Max String Voltage (NEC 690.7)

-- V

Corrected for extreme cold.

Max Series Modules Allowed

Maximum possible without exceeding system limit.

Max Circuit Current (NEC 690.8)

-- A

Isc × 1.25 (125% continuous limit)

Min Ampacity & OCPD Size

-- A

Isc × 1.5625 (125% × 125%)

About the NEC PV String Voltage & Ampacity Calculator

The NEC PV string calculator assists solar installers, electrical engineers, and designers in sizing arrays safely. By factoring in record cold temperatures, you can pinpoint the maximum expected voltage and safely size string lengths, overcurrent protection devices, and wiring ampacity in strict compliance with the National Electrical Code.

How it works

Input the solar module's open-circuit voltage, short-circuit current, and temperature coefficient.
Select the lowest expected ambient temperature for the specific installation site.
Compute the cold-weather voltage to determine the maximum number of panels allowed in series.

Frequently asked questions

Why does cold weather increase solar panel voltage?

Solar cells have a negative temperature coefficient for voltage. As the temperature drops below standard test conditions, the open-circuit voltage rises, which can damage downstream equipment.

What is the NEC 690 rule for PV circuit ampacity?

NEC Article 690 requires circuit current to be calculated as 125% of the short-circuit current. An additional 125% continuous duty factor means wiring is effectively sized at 1.56 times Isc.

Can I just use the nameplate open-circuit voltage to size my string?

No. Using the nameplate voltage without applying a temperature correction factor violates the NEC and risks over-voltaging the inverter during a cold winter morning.

What happens if a PV string voltage exceeds the inverter maximum?

Over-voltaging an inverter can cause catastrophic hardware failure, void the manufacturer's warranty, and create an arc flash or fire hazard.

References

NFPA 70 National Electrical Code (NEC) Article 690 - Solar Photovoltaic Systems
ASHRAE Extreme Annual Design Conditions for Temperature