Conductor Ampacity and String Overcurrent Protection
A 10 kW DC array has two parallel strings of twelve 415 W modules. Each string's Isc is 13.5 A, Imp is 12.8 A, and the module maximum series fuse rating is 25 A. The strings are combined at a DC combiner near the array, and a single output circuit runs in EMT on the roof to a string inverter. The roof is in a hot climate: design ambient is 40°C, and the conduit sits 4 in. above the roof, so the added temperature reaches 62°C. The output circuit has four current-carrying conductors in one conduit. The installer wants to use 10 AWG THWN-2 copper for the strings and 8 AWG for the combined output. Terminals are rated 75°C. The building is a dwelling, and the inverter is not module-level.
1. Calculate the minimum conductor ampacity required for each string and for the combined DC output, using the common 125% × 125% approach. Show the arithmetic.
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String Isc = 13.5 A. Per 690.8(A), the maximum current is 1.25 × 13.5 = 16.9 A. Per 690.8(B), conductor ampacity must be at least 125% of that value: 1.25 × 16.9 = 21.1 A per string. For two strings combined, the maximum current is 2 × 13.5 = 27.0 A; 690.8(A) gives 1.25 × 27.0 = 33.8 A, and 690.8(B) gives 1.25 × 33.8 = 42.2 A for the combined output conductor before adjustment. These are the minimums before ambient and conduit-fill correction.
2. Does each string require an overcurrent device? The module maximum series fuse rating is 25 A. Explain.
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No string OCPD is required for two parallel strings in this case. If one string develops a short, the other string can backfeed it with at most one string's Isc, about 13.5 A, which is below the 25 A maximum series fuse rating. A third parallel string would change the answer: a faulted string could receive 2 × 13.5 = 27 A, exceeding 25 A, so string OCPDs would be needed. The combined output still needs the appropriate overcurrent protection and disconnect per the inverter and NEC requirements.
3. The array is on a dwelling roof. What rapid-shutdown requirement applies, and what is a compliant approach?
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NEC 690.12 requires rapid shutdown of PV system conductors. The exact voltage limit depends on the adopted edition: the 2020 NEC requires controlled conductors outside the array boundary to be reduced to 80 V or less within 30 seconds, while the 2023 NEC reduces that limit to 30 V. In both cases, controlled conductors must be limited to 1 ft (305 mm) from the array boundary. A compliant approach is module-level electronics, such as microinverters or DC optimizers, that shut down the module output, plus a clearly labeled rapid-shutdown switch at the service panel. A single string inverter alone does not satisfy the 1 ft/voltage requirement because the DC conductors remain energized between the array and the inverter.