Solar PV Cable Sizing
The cable downstream of a combiner box has to be sized for the combined current of every string feeding into it, not any single string.
A combiner box aggregates multiple parallel PV strings into a single downstream conductor — and that downstream conductor has to be sized for the combined current of every string feeding into it, not any individual string's current.
It's an easy mental slip to size the combiner's output cable using the same per-string current figure used for each individual string cable feeding into the box — but the output cable is carrying the sum of all those strings' currents simultaneously, a meaningfully larger design current.
Combined design current = (Number of parallel strings) × (Per-string Isc) × Safety multiplier
Apply the applicable safety multiplier (see the 1.25×Isc article) to the combined current figure, not to each individual string's current separately before summing — the sequence matters for getting the correct final result.
On larger arrays with multiple combiner boxes feeding into a further downstream combining point, the same "recalculate from combined current" discipline applies at every single stage, not just the first one — a common source of undersizing on more complex, multi-stage array topologies.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks