A PV module datasheet’s maximum series fuse rating is not the fuse size to order. It is one upper boundary in a design that also depends on the exact module short-circuit current, parallel-string count, reverse-current sources, string cable, cold-condition voltage, fuse-holder environment and governing rules.
This guide shows buyers which records make a string-fuse proposal reviewable. It is not a protection design or a substitute for the responsible engineer, current code, module instructions or fuse manufacturer. Use the gPV versus gG guide for application-category identity; here the question is how the current and voltage inputs support an exact string-fuse selection.
Start with the exact module and power bin
Request the module manufacturer, full model code and power bin. Record front-side Isc and Voc at the stated reference condition, their tolerances and temperature coefficients, maximum system voltage, maximum series fuse rating, and any manufacturer-specific overcurrent instruction. For bifacial modules, also request the approved rear-side current or bifacial-gain method used by the project designer.
Do not replace Isc with Imp. Imp describes current near the maximum-power operating point. Fuse review needs the short-circuit and adjusted maximum-current inputs required by the governing method. Likewise, watts alone do not define current: two modules with the same nominal power may have different Isc, Voc and maximum-series-fuse values.
A current JinkoSolar datasheet illustrates the document problem. Its 495–520 W bifacial family lists front-side STC Isc values from 15.58 to 15.83 A, higher BNPI Isc values from 17.15 to 17.43 A, and a 35 A maximum series fuse rating. Those figures are model-family data, not a fuse recommendation. They show why the RFQ must identify the exact variant and the current basis used in the calculation. JinkoSolar 495–520 W bifacial module datasheet.
Treat the module maximum series fuse rating as a ceiling associated with the module, not a target. The selected fuse must also carry the expected operating current after applicable corrections, protect the conductor and module within the design limits, withstand the system voltage and interrupt the prospective DC fault current.
Count every source that can feed a faulted string
Parallel strings normally deliver current toward the common DC bus. If one string develops a fault, the other connected strings can feed current back into that path. Batteries, converters or other equipment may add sources depending on the architecture. The RFQ should therefore state the number of parallel strings at the relevant connection point and identify all possible backfeed sources.
Ask for a one-line diagram that shows module count per string, parallel-string count, combiner arrangement, fuse location, inverter inputs, isolation points and any storage connection. The calculation should distinguish normal forward current from the reverse or fault current that the protective device is intended to clear.
Eaton’s current Bussmann photovoltaic application guide defines Np as the number of strings in parallel. In its named worked example, it estimates maximum reverse current into a faulted string as (Np − 1) × 1.25 × Isc. That is useful, visible engineering evidence for the guide’s stated assumptions. It should not be presented as a universal equation for every jurisdiction or architecture. Eaton Bussmann photovoltaic fuse application guide, string-protection section.
Eaton treats systems with three or fewer parallel strings differently from those with more than three and expressly notes that local rules or other connected sources can still require protection. A procurement specification should preserve that boundary: string count is an input to the study, not a stand-alone yes-or-no rule.
Build a documented fuse-rating window
A defensible proposal shows the lower and upper boundaries instead of reporting only the selected ampere value. The lower boundary addresses expected maximum current and environmental derating so normal operation does not cause unwanted opening. The upper boundaries include the module maximum overcurrent or series-fuse rating, corrected string-cable ampacity and any equipment limit.
Within Eaton's Np > 3 string-protection branch, the guide recommends a current rating of at least 1.56 × Isc, a voltage rating of at least 1.20 × Voc × Ns, and a current rating no greater than the module maximum overcurrent protection rating or string-cable rating. It also requires checking current-carrying capability after ambient-temperature derating and addresses altitude. These are Eaton recommendations for that stated branch; require the designer to reconcile them with the current project standard and actual conditions. Eaton Bussmann photovoltaic fuse application guide, selection section.
Do not conceal the arithmetic in a software screenshot. Ask for the source values, units, factors, intermediate results and chosen standard rating. If a next standard ampere size is selected, the record should show that it remains within every upper limit after corrections. If no available rating fits the window, the response should identify the design conflict rather than force a catalogue part into the schedule.
Keep conductor ampacity and voltage drop separate. A cable can have adequate corrected ampacity but excessive voltage loss on a long route. Conversely, a larger cable chosen for voltage loss still needs a fuse selection consistent with its termination, holder and module limits.
Check voltage, breaking capacity and the holder
The current rating is only one line of the fuse specification. Calculate maximum string voltage from the exact series-module count, module Voc, temperature coefficient and minimum design temperature under the applicable method. The cold-weather PV string voltage guide explains the separate source-data package. Require a fuse and holder with a supported DC voltage rating at or above the resulting requirement.
Ask for the prospective fault current at the fuse location and confirm the marked DC breaking or interrupting capacity. The DC fuse voltage and interrupting-capacity guide explains why voltage rating and breaking capacity answer different questions. A high voltage marking does not by itself prove that the device can safely interrupt the available fault.
Freeze the fuse manufacturer, series, full part number, physical size, terminal form, gPV or applicable PV classification, rated current and voltage, breaking capacity, time-current data and temperature-derating information. Do the same for the holder: model, pole arrangement, conductor range, terminal method, rated current and voltage, environmental rating, certificate and installation conditions.
Ambient temperature means the condition at the fuse and holder, not a convenient outdoor weather value. A combiner exposed to sun and internal losses can run hotter than ambient air. Link the enclosure assumption to the DC fuse ambient-temperature guide, and request a revised calculation if enclosure layout or thermal conditions change.
Identify the governing evidence set
The IEC catalogue currently lists IEC 60269-6:2010+A1:2021, Edition 1.1, with a stability date of 2028. Its public scope covers fuse-links protecting PV strings and arrays in circuits up to 1,500 V DC. This identifies the PV fuse-link standard family; it does not select the rating for a particular project. IEC 60269-6 official publication record.
IEC also offers IEC 62548-1:2023+A1:2025 as the current consolidated array-design publication. Its public scope includes DC array wiring and electrical protection devices and specifically recognizes design issues for bifacial modules. State the exact edition used by the project rather than writing only “per IEC.” IEC 62548-1 consolidated publication record.
For a North American evidence path, UL marks UL 248-19 active, last revised November 25, 2024. Its public scope describes PV fuses intended to protect strings, arrays and associated wiring against overloads or short circuits within their marked ratings. UL 248-19 official scope. UL separately marks Edition 2 of UL 4248-19 for PV fuseholders active, also last revised November 25, 2024, so the quotation should identify evidence for both components where applicable. UL 4248-19 official scope.
Compare the complete calculation package
Use one schedule across bidders so a bare fuse price is not compared with an engineered fuse-and-holder response.
On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.
| Design input | Required evidence | Hold point before release |
|---|---|---|
| Module and array current | Exact datasheet, Isc and tolerance, bifacial or irradiance method, parallel-string count and every possible backfeed source | Power class or Imp is supplied instead of exact Isc and configuration |
| Fuse-rating window | Governing calculation showing expected maximum current, reverse current, module maximum-series-fuse limit and corrected cable ampacity | Proposed rating exceeds a module or cable limit, or has no capacity after derating |
| Voltage and interruption duty | Cold-condition string voltage, fuse and holder DC ratings, prospective fault current, breaking capacity, certificates and exact part numbers | AC-only evidence, insufficient DC voltage rating or an undefined fuse-holder combination |
Require a calculation revision, drawing revision and equipment schedule to share the same module and string configuration. Define the review triggered by a module substitution, string-count change, cable change, holder change, new enclosure or revised minimum temperature. Preserve the approved documents with the procurement record and identify the installed fuse and holder by lot or other project-required traceability.
Send a PV string-fuse evidence RFQ
Send the exact module datasheet, modules per string, parallel-string count, one-line diagram, lowest design temperature, expected fuse-enclosure temperature, altitude, cable schedule, possible backfeed sources, jurisdiction and required standard editions. Ask bidders to return the visible calculation and exact fuse-holder combination.
Buyer FAQ
Is the module maximum series fuse rating the fuse we should buy?
No. It is an upper module limit, not an automatic selection. The chosen fuse must also satisfy expected current, reverse-current exposure, cable protection, environmental derating, DC voltage and breaking-capacity requirements under the governing design method.
Can the module Imp value be used instead of Isc?
Not for the cited string-protection screening method. Imp describes current near maximum power, while the source guide starts with module Isc and applies its documented factors. Preserve both values on the datasheet but use the input required by the approved calculation.
Do two parallel strings always need individual fuses?
Do not decide from string count alone. Compare all available source currents with module and conductor limits under the current code, equipment instructions and system architecture. Local rules or another backfeed source can change the conclusion.
Can one fuse selection be copied to another module power bin?
Only after rechecking exact Isc, bifacial assumptions, maximum series fuse rating, string count, cable rating, cold voltage, ambient condition and holder data. Record the new calculation instead of treating similar wattage as equivalence.