Solar Combiner Sizing Chart

Use this worksheet after the calculator result to record PV source-circuit current, string OCPD review points, combiner output current, equipment input-current check, and AHJ follow-up.

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Quick reference table

A solar combiner sizing chart is a calculator-led PV planning worksheet. It keeps Isc, string count, OCPD review points, output current, and equipment limits visible before conductor, disconnect, utility, and AHJ review.

Solar combiner sizing worksheet

Solar combiner sizing worksheet
Item	Record from calculator	Follow-up
Module current	Module Isc and series fuse rating	Verify module datasheet and listing
String count	Parallel strings and backfeed check	Check whether string OCPD review is triggered
Output current	Combiner output and continuous output current	Review conductor, disconnect, and combiner rating
Equipment limit	Inverter input-current status	Compare with datasheet and utility/AHJ review

PV protection review

PV protection review
Review item	Record on worksheet	Why it controls the next step
String fuse review	Source current, continuous source current, selected review point	OCPD cannot exceed module and holder limits
Combiner output	Total source current, continuous output current, output OCPD review point	Output conductors and disconnects follow this lane
Equipment listing	Combiner, fuse holder, inverter, rapid shutdown	Listed equipment instructions control final acceptance
Review path	Adopted NEC, utility, AHJ, installer	The worksheet is not a permit package by itself

Formula basis

Source-circuit current = module Isc x 1.25 | Continuous output current = source-circuit current x parallel strings x 1.25.

Module Isc comes from the PV module datasheet.
Parallel strings are the source circuits combined at the combiner or inverter input.
Module maximum series fuse rating is the datasheet ceiling for string OCPD review.
Equipment input-current limit comes from the inverter or charge-controller datasheet.

Worked examples

Four-string rooftop combiner recordRecord module Isc, four parallel strings, source-circuit current, continuous current, string OCPD review point, output OCPD review point, backfeed current, and inverter input status.

Two-string direct inverter recordDocument why the backfeed check does not trigger string OCPD review, then keep output current and equipment input-current notes visible for datasheet review.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

This worksheet is a PV current and OCPD review record, not a final conductor or overcurrent-device design.
Final decisions depend on the adopted NEC edition, equipment listings, manufacturer instructions, available fault current, conductor ampacity, voltage drop, utility requirements, AHJ expectations, and review by a qualified installer.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Verify adopted NEC 690.8 and 690.9, module maximum series fuse rating, combiner and fuse-holder listings, conductor ampacity, disconnect ratings, rapid-shutdown equipment, utility interconnection rules, AHJ expectations, and manufacturer instructions before installation.

How to use this chart

1Capture module dataRecord module Isc, maximum series fuse rating, and source-circuit current basis from the datasheet.

2Capture combiner currentDocument parallel strings, backfeed current, combiner output current, and continuous output current.

3Route final reviewCarry OCPD review points into conductor, disconnect, combiner, inverter, rapid-shutdown, utility, AHJ, and qualified-installer coordination.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Capture inputsWrite module Isc, parallel strings, module series fuse rating, and inverter or controller input-current limit.
Capture outputsWrite source current, continuous current, output current, OCPD review points, status, and warnings.
Capture review itemsList module, combiner, fuse holder, conductor, disconnect, inverter, rapid-shutdown, utility, AHJ, and installer actions.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Treating a calculator review point as the final fuse size without checking listings and the adopted NEC.
Checking string voltage but forgetting parallel-string current and backfeed current.
Comparing output current with inverter limits before applying the continuous-current check.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Why is this worksheet marked high risk?

PV combiner and OCPD decisions affect listed equipment, conductors, disconnects, utility interconnection, and adopted NEC review. The worksheet preserves the math but does not approve the installation.

Does the output OCPD review point select the final breaker or fuse?

No. It is the first standard review point from the screened current. Final selection still needs equipment ratings, conductor ampacity, terminal temperature, interrupting rating, and AHJ review.