How to Size a PV String Fuse: Current, Voltage and Temperature Derating

A PV string fuse is a small device with a large job: it must disconnect a faulted string without nuisance operation during normal solar production. The correct rating is not chosen from the array’s nameplate power alone. It must match the module’s short-circuit current, the system voltage, the installation environment and the maximum series-fuse rating stated by the module manufacturer.

This guide gives a practical selection sequence for DC PV string fuses. It is intended for preliminary engineering and purchasing discussions; the final design must follow the applicable electrical code, the inverter and module instructions, and the project’s approved drawings.

Start with the purpose of string fusing

In a single-string array, a fault current may be too limited for a string fuse to serve a useful purpose. In parallel PV arrays, however, healthy strings can feed reverse current into a faulted string. That reverse current can exceed the conductor or module’s safe limit. String fuses are used to interrupt that contribution before damage occurs.

Fuses therefore need to be considered together with the number of parallel strings, the combiner-box layout, cable size and the module data sheet. Do not assume that a fuse is automatically required or that one selected for AC service is suitable for PV DC duty.

Step 1: Collect the four essential data points

• Module short-circuit current (Isc): use the value from the module data sheet, including the stated test conditions.
• Maximum series fuse rating: this is a hard upper limit from the module manufacturer. A larger fuse can fail to protect the module conductors.
• Maximum string voltage: calculate the cold-weather open-circuit voltage for the complete series string.
• Available reverse current: assess how many parallel strings can contribute current to a faulted string.

Step 2: Establish the current basis

Start from the module Isc rather than the operating current at the maximum power point. PV current rises with irradiance and can also vary with real site conditions. Design rules commonly apply a continuous-current margin to Isc; the exact multiplier depends on the governing code and project standard. Use that calculated design current as the minimum continuous current that the selected fuse must carry without nuisance operation.

Next, choose the nearest suitable gPV fuse rating above that design current, while staying at or below the module’s maximum series fuse rating. A gPV fuse has a time-current characteristic designed for photovoltaic circuits and can clear DC fault currents under the conditions specified by its manufacturer.

Illustrative only: if a module’s Isc is 13 A and the project calculation produces a design current of 16.25 A, a 20 A gPV fuse may be a candidate. It is acceptable only if the module’s maximum series fuse rating is at least 20 A, the holder accepts the fuse, and the remaining voltage, temperature and fault-duty checks are satisfied.

Step 3: Check DC voltage, not only current

DC arcs are harder to interrupt than AC arcs because current does not naturally cross zero. The fuse voltage rating must meet or exceed the highest possible DC voltage of the string, calculated at the lowest expected cell temperature. A fuse marked for 1000 V DC must not be used in a 1500 V string simply because its ampere rating looks correct.

Check the fuse, the fuse holder, the combiner enclosure and all adjacent components as a system. Their DC ratings must be compatible with the same maximum system voltage.

Step 4: Apply temperature derating

Fuse current ratings are specified at a reference ambient temperature. A combiner box in direct sun can operate far above that condition. Higher ambient temperature reduces the current a fuse can carry continuously, while a tightly packed enclosure can increase heat further.

Use the fuse manufacturer’s derating curve for the actual expected ambient temperature and mounting arrangement. Also check whether the fuse holder has its own derating table. If the derated continuous-current capability falls below the design current, select a different rating or improve the thermal design—provided that the module’s maximum series-fuse limit is still respected.

Step 5: Verify interrupting rating and coordination

The fuse must be able to interrupt the prospective DC fault current at the system voltage. In a PV combiner, that value is influenced by the number of parallel strings and by any connected sources that can backfeed the circuit. Verify the manufacturer’s DC breaking-capacity rating; never assume an AC breaking rating applies to a DC circuit.

Then review coordination with upstream and downstream protective devices. A string fuse should protect its string conductor and module circuit without causing the entire combiner or inverter feeder to trip unnecessarily for a localized fault.

Common sizing mistakes

• Choosing from array watts instead of Isc and the module data sheet.
• Using a fuse with an adequate current rating but insufficient DC voltage rating.
• Ignoring the module maximum series-fuse rating.
• Installing a general-purpose fuse instead of a PV-rated gPV fuse.
• Skipping hot-enclosure derating and fuse-holder limits.
• Assuming more fuse amps always means better reliability.

Buying checklist

• gPV classification and DC voltage rating matched to the string.
• Current rating supported by the project calculation and temperature derating.
• Rating no higher than the module’s maximum series fuse rating.
• DC breaking capacity appropriate for the available fault current.
• Fuse holder, terminals and enclosure rated for the same DC voltage and ambient conditions.
• Traceable documentation, markings and test data from the supplier.

Final takeaway

A correctly sized PV string fuse balances two limits: it must carry normal solar current under the hottest expected conditions, and it must clear reverse-fault current before the module or conductor is damaged. Begin with the module documentation, calculate the site-specific current and cold-voltage conditions, apply the manufacturer’s derating data, and verify the complete DC protection path.

For related selection guidance, see our solar PV combiner box buyer’s guide and our comparison of cylindrical and square-body PV fuses.