690.8(A)(1) Photovoltaic Source Circuit Currents.
Code Change Summary: Revisions were made to circuit sizing for PV source circuits.
In previous editions of the NEC®, the maximum circuit current for PV source circuits (conductors between PV modules and from modules to the common connection point of the dc system) was calculated by taking the short circuit current (ISC) from the module nameplate and multiplying it by 125%.
Example: A PV module with a marked ISC of 5.61 amps.
5.61 X 125% = 7.01 amps. This is considered the maximum circuit current.
Determining the maximum source circuit current is important because when adding several strings of PV modules in parallel, the current is additive. The maximum circuit current must be determined in order to calculate the minimum conductor size that has sufficient ampacity to carry the PV system DC current continuously.
In the 2017 NEC®, another calculation method was added to recognize the unique circumstances related to larger PV systems where local irradiance levels may differ and specific elevation and orientation characteristics may be present in the design of an engineered PV system.
Now, one of the following methods may be used to calculate the maximum DC source circuit current for a PV system:
(1) The sum of parallel-connected PV module–rated short-circuit currents multiplied by 125%.
(2) 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. The calculated maximum current value shall be based on the highest 3-hour current average resulting from the simulated local irradiance on the PV array accounting for elevation and orientation. The current value used by this method shall not be less than 70 percent of the value calculated using 690.8(A)(1)(1).
A new informational note was also added to provide insight on what is considered an “industry standard method”.
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:
690.8 Circuit Sizing and Current.
(A) Calculation of Maximum Circuit Current. The maximum current for the specific circuit shall be calculated in accordance with 690.8(A)(1) through (A)(5).
(1) Photovoltaic Source Circuit Currents. The maximum current shall be the sum of parallel module rated short-circuit currents multiplied by 125 percent.
2017 Code Language:
690.8 Circuit Sizing and Current.
(A) Calculation of Maximum Circuit Current. The maximum current for the specific circuit shall be calculated in accordance with 690.8(A)(1) through (A)(6).
(1) Photovoltaic Source Circuit Currents. The maximum current shall be calculated by one of the following methods:
(1) The sum of parallel-connected PV module–rated short-circuit currents multiplied by 125 percent
(2) 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. The calculated maximum current value shall be based on the highest 3-hour current average resulting from the simulated local irradiance on the PV array accounting for elevation and orientation. The current value used by this method shall not be less than 70 percent of the value calculated using 690.8(A)(1)(1).
Informational Note: One industry standard method for calculating maximum current of a PV system is available from Sandia National Laboratories, reference SAND 2004-3535, Photovoltaic Array Performance Model. This model is used by the System Advisor Model simulation program provided by the National Renewable Energy Laboratory.