Fuses in the PV array are utilized to protect from line-line, line-ground, and mismatch faults. When a fuse is “tripped,” or the current surpasses its ampacity rating,
the circuit will break, and thus the current will completely halt at the fuse. The primary purposes of fuses are to prevent fire and provide safety when diagnosing a faulted circuit that has underwent overcurrent. The overcurrent protection requirements for photovoltaic systems are defined in Article 690.9(A) of the NEC.
A few things to first understand; per NEC, the conductor ampacity rating should be 1.56 x Isc of the PV string, where Isc is the solar panel’s short-circuit current. Also, the maximum current a panel can output is defined as 1.25 x Isc. The series fuse ampacity rating of a PV string is 1.56 x Isc, the same as the conductor rating. Note that the temperature de-rating coefficient is not considered here.
When the combined fault currents can exceed the rating of the installed wiring size (1.56 x Isc), fusing is required. Thus, when three or more strings are in parallel, fusing is required because the combined fault current of two other strings would exceed the described wire ratings.
According to Article 690.9(A) exception a and b, fusing would not be required for one or two strings so long as the short-circuit currents from all sources do not exceed the ampacity of the conductors or maximum overcurrent protective device size specified on the PV module nameplate. This is also only true if no external sources are connected. In both scenarios, the combined fault current would not exceed the conductor ampacity (1.56 x Isc), and thus fusing would not be required.
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