Understanding NEC 690 and Cold Weather Voltage
Solar panels operate differently based on the ambient temperature. As the temperature drops, a panel's voltage increases. Therefore, the most dangerous time for a solar inverter is a freezing, clear, sunny winter morning.
NEC 690.7 requires installers to calculate the Maximum System Voltage based on the lowest expected ambient temperature for the installation site. You cannot simply use the Open Circuit Voltage (Voc) printed on the panel's spec sheet, as that number is tested at standard room temperature (25°C).
How to Calculate Maximum String Voltage
To find the true worst-case voltage, you must apply the panel's temperature coefficient for Voc to the difference between 25°C and the record cold temperature for your ZIP code.
The Formula: Max Volts per Panel = Voc x [1 + (T_min - 25°C) x (TempCoeff / 100)]. Then multiply this by the number of panels in the series string.
Worked Example: A panel has a Voc of 40V, and a temp coefficient of -0.30%/°C. The site's record cold is -10°C. The temperature difference from 25°C is -35°C. Calculation: -35 x -0.30% = +10.5% voltage increase. Max Voltage = 40V x 1.105 = 44.2V. If your inverter has a strict limit of 500V, the absolute maximum number of panels you can string in series is 11 (44.2V x 11 = 486.2V).
Calculating String Ampacity for Wiring
While voltage defines the length of a series string, current (ampacity) defines the thickness of the wiring and the size of the fuses or breakers. Solar panels can briefly produce more current than their rated Short Circuit Current (Isc) due to cloud-edge effects or reflection.
NEC 690.8 requires a continuous current multiplier. You take the panel's Isc and multiply it by 1.25 to account for solar enhancement. Then, because the circuit is continuous, you multiply by an additional 1.25 to size the overcurrent protection device (OCPD).
The 1.56 Ampacity Rule
The standard shorthand for calculating the minimum wire ampacity and fuse size for a solar string is multiplying the Isc by 1.56 (which is simply 1.25 x 1.25).
For example, if a panel has an Isc of 10 Amps. The required circuit ampacity and OCPD sizing must be at least 10A x 1.56 = 15.6 Amps. The installer would select wire rated for at least 15.6A and typically a standard 15A or 20A fuse depending on the panel's max series fuse rating.
Frequently asked questions
What happens if my PV string exceeds the inverter voltage limit?
Over-voltaging an inverter, even for a few seconds on a cold morning, will blow the internal capacitors and permanently destroy the unit. This damage is detectable by manufacturers and is never covered by warranty.
Where do I find the record cold temperature for my area?
Installers should use the ASHRAE extreme annual mean minimum design temperature data, which is widely available for all US airports and municipalities.
Why is the temperature coefficient a negative number?
The coefficient is negative because voltage holds an inverse relationship with temperature. As temperature goes up, voltage goes down. As temperature goes down (negative change), the voltage goes up.
Do microinverters require string voltage calculations?
No, because microinverters are attached to individual panels on the roof and output standard AC voltage directly. The DC voltage is limited strictly to the single panel attached to it.