Solar & DC

PV Combiner Box Rating Calculator

Aggregate the string currents and voltage into the reference ratings a PV array combiner box needs: the per-string input fuse, the combined output current, the output disconnector or breaker, the busbar, and the SPD operating voltage. A system-level starting point — ratings still depend on enclosure temperature and the device datasheets.

Number of strings

String I_sc (A)

String cold V_oc (V)

ⓘEnter the cold-temperature string V_oc (from the string-voltage calculator), not the STC value — it sets the voltage rating for every DC device in the box.

Combined output current

— A

Input fuse per string

—

Output disconnector / breaker

—

Busbar & device U_e

—

SPD U_cpv

—

What the box has to rate for

Output current = N × I_sc × 1.25 Input fuse (each string) : 1.5 × I_sc ≤ In ≤ module max series fuse Voltage rating U_e ≥ string cold V_oc SPD U_cpv ≥ 1.1 × string cold V_oc

Every string feeds the common busbar, so the output current is the sum of all string currents with the 1.25 continuous-duty factor. The output disconnector, breaker, busbar, and cable are sized to that current, while every DC component — fuses, isolator, SPD, busbar clearances — must be rated for the cold-temperature string voltage.

The building blocks

Input string fuses — one gPV fuse per string (per polarity as required) once enough strings are combined to drive a damaging reverse current.
Output disconnector or DC breaker — isolates and protects the combined output; rated for the output current and the string voltage.
Busbar — carries the summed current continuously at the enclosure temperature.
Type 1 or 2 SPD — protects the array and the downstream inverter; U_cpv set above the cold string voltage.

⚠Enclosure temperature derates the ratings. A combiner box in full sun runs hot inside, reducing device and busbar capacity. Apply the manufacturer’s temperature derating to the current ratings before finalising.

Frequently asked questions

Why the 1.25 factor on the output?

PV is a continuous source and can exceed its STC current under high irradiance. The 1.25 factor sizes the busbar and output device for sustained duty above the nominal string current.

Does every string need an input fuse?

Only when the number of parallel strings is large enough that the reverse current from healthy strings could exceed a faulted module’s withstand — commonly three or more strings. Below that, string fuses may be omitted per IEC 62548.

Type 1 or Type 2 SPD in the box?

Type 2 is usual for array and inverter protection. Where the site has an external lightning protection system or high exposure, a Type 1 (or Type 1+2) is used. The U_cpv requirement is the same in both cases.

Build the combiner box

Match the fuses, disconnector, busbar, and SPD to a combiner box rated for the array voltage and current.

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Standards referencedIEC 60364-7-712 (PV systems) · IEC 62548 (PV array design) · IEC 61439-2 (assemblies) · IEC 61643-31 (PV SPD).

System-level estimate for guidance only. Final combiner-box ratings must account for enclosure temperature, device datasheets, and the array topology, verified by a qualified person.