Understanding your parallel string current
Why current adds up differently than voltage
Panels wired in series add their voltages while current stays the same throughout the string. Panels wired in parallel do the opposite: voltage stays the same while current adds together. When you combine two or more strings onto a single MPPT input, you're adding their currents, which is why a single MPPT input has its own current rating separate from the voltage limits covered elsewhere in string design, planned alongside your total panel count with the string length vs panel count calculator.
Why the NEC applies a 125% continuous current factor
NEC 690.8 treats solar output as a continuous current source, meaning it can sustain near-peak output for hours at a time rather than just a brief spike. Equipment and conductor ratings need to account for that sustained load, so the code requires sizing for 125% of the calculated maximum current. This factor exists specifically to prevent equipment from running right at its rated limit during normal peak sun conditions, the same reasoning behind the DC cable loss calculator's conductor sizing approach.
Why this limit is separate from string voltage limits
Voltage and current limits protect against different failure modes. Exceeding a voltage limit risks insulation breakdown or exceeding a component's rated withstand voltage. Exceeding a current limit risks overheating in wiring, connectors, and the inverter's internal current-sensing and switching components. A string design needs to pass both checks independently, and passing one doesn't guarantee the other. This is also why mismatched strings, covered by the string mismatch loss calculator, cause a real output loss even when every individual string passes its own current and voltage checks fine.
What to do when parallel current exceeds the limit
The most common fix is reducing the number of strings sharing a single MPPT input, either by moving one string to a different input if the inverter has more available, or by choosing an inverter model with a higher-rated MPPT input current. Some installers also consider fuses or combiner boxes rated appropriately for the design, which the combiner box sizing calculator covers for larger arrays with many parallel strings.
Downstream wiring needs the same current check
Once your MPPT input current checks out, the conductors carrying that combined current also need a current rating that accounts for the same 125% factor, along with any further adjustments from NEC Table 310.16 for conduit fill and ambient temperature. The wire gauge sizing calculator picks up from this exact combined current figure, and NREL's PVWatts calculator can help confirm the array-level production behind whichever final configuration you land on.