MPPT Input Sizing — Strings per Tracker and Current Limits

After string length, the second half of array layout is deciding how many strings feed each MPPT — set by current, not voltage.

The current limits

Every MPPT input has two ratings:

• Maximum operating input current (I_mppt,max) — the most the tracker will draw at the maximum power point.
• Maximum short-circuit current (I_sc,max) — the most it can safely withstand; parallel string I_sc must stay below this.

A string’s current equals one panel’s current (series connection), so I_mp,string = panel I_mp (~10–14 A for modern modules).

Worked example

Inverter with 2 MPPTs, each I_mppt,max = 26 A, panel I_mp = 13.2 A. 26 / 13.2 = 1.97 → 2 strings per MPPT, 4 strings total across the inverter.

Balance the trackers

Put equal string counts and, ideally, equal string lengths on each MPPT. If the roof forces unequal orientations (east and west), assign each orientation its own MPPT so one does not drag down the other.

When to use a combiner vs separate MPPTs

• Separate MPPTs — different orientations, tilts, or shading between sub-arrays; small residential systems.
• String combiner box — many parallel strings of identical orientation feeding one large MPPT (commercial ground mounts); needs string fuses once you parallel 3+ strings.