Combiner Box Sizing Chart
PV combiner box sizing by string count and fuse rating, using the NEC 690.8 Isc rule, with the string math that decides it.
A PV combiner box merges parallel strings into one home-run, and its fuse ratings follow the module short-circuit current. The rule: fuse at 1.56 × Isc (NEC 690.8), and match the box to the string count.
| Box (strings × A) | Fuse per string | Typical array | Output |
|---|---|---|---|
| 2 × 15 A | 15 A | 2 parallel strings | 30 A max |
| 3 × 15 A | 15 A | 3 strings residential | 45 A max |
| 4 × 20 A | 20 A | 4 strings, 580 W class | 80 A max |
| 6 × 15 A | 15 A | 6 strings commercial | 90 A max |
| 12 × 20 A | 20 A | Utility ground-mount | 240 A max |
Fusing protects the string conductors from reverse current. If your inverter or charge controller limits backfeed (most modern ones do), you may still need fuses to protect the modules — check the module datasheet max series fuse rating.
Series string voltage must fit the inverter window (Voc at record low, times the inverter max), and string current is what flows through each combiner input. Two strings in parallel double the current into the home-run conductor — size that conductor to 1.56 × Isc too, per NEC 690.8(B). For the full PV math see our solar panel output chart and inverter sizing guidance.
Do I need a combiner box for one string?
No — one string goes straight to the inverter or charge controller. Combiners earn their cost from two parallel strings up.
Inside or outside?
Outside, next to the array, NEMA 3R or better. The disconnect goes at the combiner or at the inverter depending on the AHJ.
Building a PV system?
Combiner boxes, fuses, and disconnects in stock alongside panels and inverters — contractor pricing and system sizing help from a real desk.
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