690.7 Maximum Voltage.
Code Change Summary: Changes were made to the requirements for calculating maximum voltage of a PV system.
In the 2014 NEC®, to calculate the maximum PV system voltage for DC source circuits, the open circuit voltage (VOC) of each crystalline or multicrystalline silicon PV module wired in series was corrected for cold weather expectations by using adjustment factors in Table 690.7 and then added together for each module in the series string. If listed modules were provided with temperature coefficients by the manufacturer, then those values would be used instead of Table 690.7. The final values were used to determine the voltage rating of cables, disconnects, and overcurrent devices.
In the 2017 NEC®, the rules have been revised into a list of three options which creates more flexibility for PV system design:
Option 1. Follow the instructions using the open-circuit voltage temperature coefficients included in the listing or labeling of the PV module.
Option 2. Use the correction factors provided in Table 690.7(A) for crystalline and multicrystalline silicon modules.
Option 3. For PV systems with a generating capacity of 100 kW or greater, a documented and stamped PV system design provided by a licensed professional electrical engineer, using an industry standard method is permitted.
In addition, the 2017 NEC®, also added special provisions in 690.7(B) for calculating maximum voltage of DC-to-DC Converter Source and Output Circuits.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2014 Code Language:
690.7 Maximum Voltage.
(A) Maximum Photovoltaic System Voltage. In a dc PV source circuit or output circuit, the maximum PV system voltage for that circuit shall be calculated as the sum of the rated open-circuit voltage of the series-connected PV modules corrected for the lowest expected ambient temperature. For crystalline and multicrystalline silicon modules, the rated open-circuit voltage shall be multiplied by the correction factor provided in Table 690.7. This voltage shall be used to determine the voltage rating of cables, disconnects, overcurrent devices, and other equipment. Where the lowest expected ambient temperature is below −40°C (−40°F), or where other than crystalline or multicrystalline silicon PV modules are used, the system voltage adjustment shall be made in accordance with the manufacturer’s instructions.
When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum PV system voltage as required by 110.3(B) instead of using Table 690.7.
(See NEC® for remainder of section)
2017 Code Language:
690.7 Maximum Voltage. The maximum voltage of PV system dc circuits shall be the highest voltage between any two circuit conductors or any conductor and ground. PV system dc circuits on or in one- and two-family dwellings shall be permitted to have a maximum voltage of 600 volts or less. PV system dc circuits on or in other types of buildings shall be permitted to have a maximum voltage of 1000 volts or less. Where not located on or in buildings, listed dc PV equipment, rated at a maximum voltage of 1500 volts or less, shall not be required to comply with Parts II and III of Article 490.
(A) Photovoltaic Source and Output Circuits. In a dc PV source circuit or output circuit, the maximum PV system voltage for that circuit shall be calculated in accordance with one of the following methods:
(1) Instructions in listing or labeling of the module: The sum of the PV module–rated open-circuit voltage of the series connected modules corrected for the lowest expected ambient temperature using the open-circuit voltage temperature coefficients in accordance with the instructions included in the listing or labeling of the module
(2) Crystalline and multicrystalline modules: For crystalline and multicrystalline silicon modules, the sum of the PV module–rated open-circuit voltage of the series connected modules corrected for the lowest expected ambient temperature using the correction factor provided in Table 690.7(A)
(3) PV systems of 100 kW or larger: For PV systems with a generating capacity of 100 kW or greater, a documented and stamped PV system design, using an industry standard method and provided by a licensed professional electrical engineer, shall be permitted.
(See NEC® for remainder of section)