According to NEC 690.9, the overcurrent protection device for a PV source circuit should be rated at 125% of the module’s short-circuit current (Isc)
NEC 690.8(B)(1) – Overcurrent Protection for PV Source Circuits
This section outlines the requirements for overcurrent protection for PV source circuits, which are the circuits that run from the photovoltaic modules to the inverter. Specifically, 690.8(B)(1) deals with the overcurrent protection device (OCPD) size and how it relates to the maximum current that can be expected in the PV source circuits.
Key Requirements of NEC 690.8(B)(1)
- OCPD Sizing for PV Source Circuits:
- The OCPD for PV source circuits must be rated at least 125% of the maximum current calculated for the source circuit, according to the method in 690.8(A).
- The maximum current in the PV source circuit is determined based on the module short-circuit current (Isc) multiplied by 1.25.
- The OCPD for PV source circuits must be rated at least 125% of the maximum current calculated for the source circuit, according to the method in 690.8(A).
- Maximum Source Circuit Current:
- The maximum current of the PV source circuit is calculated by the formula: Maximum PV Source Circuit Current=Module Isc×1.25\text{Maximum PV Source Circuit Current} = \text{Module Isc} \times 1.25Maximum PV Source Circuit Current=Module Isc×1.25
- The Isc value is provided by the module manufacturer and reflects the current produced by the module under short-circuit conditions.
- Protection Device:
- The OCPD should protect the circuit from overloads and fault conditions.
- The OCPD rating should not exceed the current-carrying capacity of the conductor, considering the conductor ampacity.
Example of Sizing
- Assumed Isc for Module: 9.0 A (from the manufacturer datasheet).
- Calculation of Maximum Source Circuit Current:
- 9.0 A × 1.25 = 11.25 A
- Required OCPD Size:
The OCPD for this circuit must be rated for at least 11.25 A, rounded up to the next standard size (typically 15 A).
Why This Matters:
- Correct OCPD sizing ensures protection against overcurrent situations while complying with NEC code.
- Overprotection or underprotection can lead to either unnecessary tripping or inadequate protection of the PV system, potentially causing hazards or damage.