Renewable Energy calculator
Solar Combiner Sizing Calculator
This PV combiner calculator checks source-circuit current, continuous current, string OCPD review points, combiner output current, and optional inverter input-current warnings from module Isc and parallel-string count.
Updated July 10, 2026
Four PV strings at 13.8 A module Isc screen at 69 A combiner output current, 86.25 A continuous output current, a 25 A string OCPD review point, and a 90 A output OCPD review point.
Source-circuit current = Isc x 1.25 | Continuous source current = Isc x 1.25 x 1.25 | Combiner output current = source-circuit current x parallel strings.
Enter module Isc, parallel strings, module max series fuse rating, and optional inverter input-current limit for a PV combiner screen
Example Calculations
How to Use
Solar combiner sizing uses module Isc and parallel strings
PV combiner and string protection decisions depend on short-circuit current, string count, equipment ratings, and the adopted code path. This calculator keeps the first-pass current basis visible so a designer can carry the result into datasheet, combiner, disconnect, conductor, inverter, utility, and AHJ review.
For the wider solar workflow, pair this page with the inverter sizing calculator, the solar system sizing calculator, the solar combiner sizing chart, and the solar planning hub.
Recommended inputs
| Input | Purpose |
|---|---|
| Module Isc | Short-circuit current from the PV module datasheet. |
| Parallel strings | Number of source circuits that combine at the combiner or inverter input. |
| Module max series fuse rating | Datasheet ceiling used to screen whether the OCPD review point exceeds the module listing. |
| Inverter max input current | Optional equipment input-current screen for the combined output. |
Formula path
Source-circuit current = module Isc x 1.25
Continuous source-circuit current = source-circuit current x 1.25
Combiner output current = source-circuit current x parallel strings
Continuous output current = combiner output current x 1.25
Backfeed current estimate = source-circuit current x (parallel strings - 1)
Worked example
A four-string array uses modules with 13.8 A Isc and a 25 A maximum series fuse rating. The first source-circuit current screen is 13.8 x 1.25 = 17.25 A. The continuous source-circuit current is 17.25 x 1.25 = 21.56 A, so the first standard string OCPD review point is 25 A.
The combiner output current estimate is 17.25 x 4 = 69 A. The continuous output current is 69 x 1.25 = 86.25 A, so the first standard output OCPD review point is 90 A. The backfeed current estimate is 17.25 x 3 = 51.75 A, which triggers string OCPD review.
What this page does not claim
- It is not a finished conductor, disconnect, combiner, or overcurrent-device design.
- It does not replace module, inverter, combiner, fuse holder, or rapid-shutdown equipment instructions.
- It does not determine available fault current, interrupting rating, wire ampacity, voltage drop, or terminal temperature by itself.
- It does not replace the adopted NEC, utility interconnection, AHJ review, or installer design.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
When does a PV string normally need its own fuse?
The calculator flags string OCPD review when there are three or more parallel strings or when the remaining strings could backfeed more current than the module series-fuse rating. The final decision still depends on the adopted NEC edition and equipment listings.
Why does the calculator show both source-circuit current and continuous current?
The first 125% screen is the PV source-circuit current basis. The additional 125% continuous-current screen is useful for OCPD review points and for carrying assumptions into conductor and equipment review.
Can I use the selected OCPD value as the final fuse size?
No. It is a review point, not a final approval. Verify module maximum series fuse rating, fuse family, combiner listing, conductor ampacity, terminal temperature, available fault current, utility/AHJ requirements, and manufacturer instructions.
How does this relate to inverter sizing?
Use the inverter sizing calculator for string voltage, MPPT range, and DC/AC ratio. Use this combiner page for parallel-string current, string OCPD review, and combined output-current documentation.
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