Akash Hirpara String current is the current flowing through a series-connected PV string, which is the same through every module in that string. Under NEC 2023 690.8(A)(1), maximum circuit current is 1.25 times the summed module short-circuit current (Isc), and designers use it to size conductors, fuses, and inverter MPPT inputs.
Key Takeaways
- String current = module-level current.
- Imp at MPP, Isc at short circuit.
- Per-MPPT current = parallel strings × Imp.
- Wire sized to 1.25 × Isc × 1.25.
Why String Current Stays Constant Across a String
Series wiring forms a single electrical loop, and a loop only has one current flowing through it at any given moment, so whatever current the first module in the string produces is the same current every other module in that string carries, no matter how many modules are added.
This is the opposite of what happens to voltage, which stacks up module by module as more units are added in series.
Because of that distinction, a designer checking string current only ever needs the module’s datasheet Imp (at maximum power) or Isc (at short circuit), string length is irrelevant to the current value, even though it’s central to the voltage calculation used for MPPT window compliance.
Worked Example
Take a module rated 13.0 A Isc and roughly 12.0 A Imp at STC (Imp is typically about 0.92 × Isc). Whether the string has 18 modules or 26 modules, the string current at maximum power stays 12.0 A and the worst-case short-circuit current stays 13.5 A, series wiring doesn’t add current, only voltage.
Where this matters is downstream: if four of those strings land in parallel at a combiner box or a single inverter MPPT, the combined current at that junction is 4 × 12.0 A = 48.0 A, and it’s that combined figure, not the single-string current, that the busbar, fuse, and feeder conductor all have to be sized to carry.
Related Reading
String current is the number that everything downstream in the DC electrical design has to accommodate, so it’s worth reading alongside wire sizing, which walks through the NEC 690.8 conductor calculation that starts with Isc, and conductor ampacity, which covers how that required ampacity gets matched to an actual AWG size after temperature and conduit-fill derating.
Our guide to Solar SLD software shows how modern NEC 2023 tools carry string current through to the auto-generated conductor and OCPD schedule on the single-line diagram, and the PV yield simulation software roundup covers how bifacial current uplift gets modeled in a bankable yield report.
For a closer look at how string current feeds into overcurrent protection sizing on Indian installations specifically, QbitsEnergy’s guide on string sizing and OCP is a useful companion reference.
Disclosure: Qbits Energy is a Heaven Designs group company.
Frequently Asked Questions
3 commonly searched questions about String Current.
Why is current the same in every module of a string?
Does a string need fuses?
How does bifacial gain affect string current?
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