How to Select the Right PV Fuse for Your Solar String Array

Key point: PV fuse selection is a four-parameter calculation — not a guess, not a rule of thumb. This guide walks through every step with a worked example so you can confirm the right current rating for any solar string array, every time.

Why PV Fuse Selection Gets It Wrong So Often

Of all the specification decisions in a solar PV system, PV fuse selection has the highest rate of error — and the most serious consequences when it goes wrong.

Undersized fuses nuisance-trip during high irradiance periods, silently removing strings from generation for hours or days before anyone notices. Oversized fuses fail to protect the string cabling and panels during reverse current faults, leaving the installation exposed to cable overheating, insulation damage, and fire risk. Neither outcome is acceptable.

The good news is that correct solar fuse selection is not difficult — it follows a defined, repeatable process set out in IEC 62548 (PV array design requirements) and the relevant fuse standard IEC 60269-6. This guide walks through that process step by step, with a complete worked example and a selection checklist you can use on every project.

Before continuing, if you are not yet familiar with what makes a gPV fuse different from a standard DC fuse, read our guide: What is a gPV Fuse and Why Does Your Solar System Need One? — and if you have not yet confirmed your system voltage (1000V DC or 1500V DC), read: 1000V DC vs 1500V DC Solar Fuse — What’s the Difference? Those two articles cover the foundational decisions. This guide covers the current rating selection that follows.

The Four Selection Parameters

Correct PV fuse selection requires confirming four parameters, in this order:

1. Voltage rating — Does the fuse match the system DC voltage?
2. Utilisation category — Is it a certified gPV fuse to IEC 60269-6?
3. Physical format — Does it fit the installed fuse holder?
4. Current rating — Is the rating within the required window?

Parameters 1, 2, and 3 are covered in the companion articles referenced above. This guide focuses on parameter 4 — calculating the correct PV fuse current rating — which is where the majority of selection decisions and errors are made.

The Current Rating Window

The correct solar fuse current rating is not a single value — it is a window defined by two constraints. The fuse rating must fall above a lower bound (to avoid nuisance tripping) and below an upper bound (to ensure it protects the cable and panels):

Lower bound: In ≥ 1.25 × Isc (STC)

The fuse must not operate under peak operating conditions. Solar panel output current peaks at maximum irradiance — typically modelled at the standard test condition (STC) short-circuit current (Isc). Under real-world conditions, slightly elevated irradiance can push the actual Isc above the STC value. The 1.25 multiplier provides the required safety margin to prevent nuisance trips at peak irradiance.

Upper bound: In ≤ minimum of (Irev max, Iz)

The fuse must protect both the string cable and the solar panels from overcurrent damage:

• Irev max — the maximum reverse current rating of the solar panel, specified on the panel manufacturer’s datasheet under “maximum series fuse rating” or “max reverse current.” This is the maximum current the panel can tolerate flowing backwards through it without damage.
• Iz — the current-carrying capacity of the string cable (positive conductor), determined by the cable cross-section, installation method, and ambient temperature.

The fuse rating must not exceed the lower of these two values.

The selection window:

1.25 × Isc ≤ In ≤ min(Irev max, Iz)

Choose the standard rated current that falls within this window. If the window is very narrow, or if no standard rating falls within it, review the string design — it may indicate a mismatch between panel Isc, cable sizing, and panel reverse current rating that needs to be resolved at design level rather than by fuse selection alone.

Step-by-Step PV Fuse Current Rating Calculation

Step 1 — Find the Panel Short-Circuit Current (Isc)

Locate the Isc at STC on the solar panel manufacturer’s datasheet. This is the short-circuit current at 1000 W/m² irradiance, 25°C cell temperature, and AM1.5 spectrum — the standard test conditions. This figure is always clearly labelled on the panel’s electrical characteristics table.

Example: Panel Isc (STC) = 10.2A

Step 2 — Apply the Irradiance Correction Factor

Real-world irradiance can briefly exceed 1000 W/m² — particularly in climates with reflective cloud edges or in high-altitude installations. IEC 62548 and most national installation standards require a correction factor of 1.25 to be applied to Isc to account for this.

Corrected Isc = Isc (STC) × 1.25

Example: 10.2A × 1.25 = 12.75A

This corrected value is the lower bound for the fuse current rating. The fuse must be rated at or above 12.75A to avoid nuisance operation under peak irradiance conditions.

Step 3 — Find the Panel Maximum Reverse Current Rating (Irev max)

Find the maximum series fuse rating on the panel datasheet — this value is also commonly labelled as “max reverse current,” “maximum overcurrent protection,” or “maximum series fuse.” It represents the highest current the panel can tolerate flowing backwards through the cell string without causing module damage or fire risk.

This figure defines the upper limit of the fuse current rating — the fuse must protect the panel from currents above this value.

Example: Panel maximum series fuse rating = 20A

Step 4 — Confirm the String Cable Current-Carrying Capacity (Iz)

The string cable (positive conductor) also defines an upper bound for the fuse rating — the fuse must protect the cable from overcurrent damage. The cable’s current-carrying capacity (Iz) depends on its cross-section, conductor material, insulation type, and installation method.

For PV string cabling in the UK and most of Europe, solar DC cable to EN 50618 in 4mm² or 6mm² cross-section is most commonly used. Typical Iz values for PV cable are:

Always use the specific cable manufacturer’s data for the actual installation conditions rather than generic tables. Apply derating factors for elevated ambient temperature, cable bundling, and conduit installation as applicable.

Example: 4mm² EN 50618 PV cable, Iz = 40A

Step 5 — Determine the Selection Window and Choose the Rating

Apply the selection formula:

1.25 × Isc (STC) ≤ In ≤ min(Irev max, Iz)

Example:

• Lower bound: 12.75A (Step 2)
• Upper bound: min(20A, 40A) = 20A (Step 3 and 4)
• Selection window: 12.75A ≤ In ≤ 20A

From the available gPV fuse current ratings (1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 25, 32A for the Lawson LFPV-32), the ratings that fall within the 12.75A to 20A window are: 15A and 20A.

Both are technically valid. The choice between them involves a judgement about protection sensitivity:

• A 15A fuse provides tighter protection — it will operate at a lower overcurrent above the panel Isc, giving faster response to developing reverse current faults. It sits closer to the lower bound (12.75A) and may be more susceptible to nuisance tripping under unusual high-irradiance conditions.
• A 20A fuse provides a wider margin against nuisance tripping and sits exactly at the upper bound. It still fully protects the panel (whose Irev max is 20A) and the cable.

For most standard installations, 15A is the preferred choice — it provides meaningful protection sensitivity while remaining well above the corrected Isc. For installations in locations with known occasional very high irradiance (high-altitude, low-latitude sites), 20A may be preferred to eliminate any nuisance trip risk.

Selected PV fuse: Lawson LFPV-32, 15A, 1000V DC, gPV

Worked Example — Summary Table

Multi-String Systems — Does the Number of Strings Affect Fuse Selection?

This is a common point of confusion. The number of parallel strings in the array does affect the maximum reverse current that a faulted string can experience — but it does not directly change the PV fuse current rating for each string.

Here is why: the fuse in each string protects that string’s cable and panels. The maximum current it will ever carry is the current produced by that string — approximately 1.25 × Isc during normal operation, and the reverse current fed by all other strings during a fault. The reverse current in a fault scenario will always be below the panel’s Irev max rating if the panel has been correctly certified (which IEC 61730 requires). The fuse selection window remains the same regardless of how many parallel strings are connected.

Where the number of strings becomes relevant is in confirming that the maximum reverse current that a single faulted string will experience — the sum of Isc from all other parallel strings — does not exceed the panel’s Irev max. If it does, the system design needs review: either the number of parallel strings must be reduced, or a panel with a higher Irev max specified.

For the vast majority of commercial and utility-scale installations, this condition is met by design — but it is worth confirming during the protection coordination review.

Common PV Fuse Selection Mistakes

Mistake 1: Selecting the Fuse to Match the Cable Rating, Not the Panel Isc

A 4mm² PV cable rated at 40A does not mean a 40A fuse is correct. The upper bound for fuse selection is the minimum of Irev max and Iz — in the example above, Irev max (20A) is the binding constraint, not the cable (40A). Fitting a 40A fuse would leave the panels completely unprotected during a reverse current fault.

Mistake 2: Forgetting the 1.25 × Isc Lower Bound

Selecting a fuse at exactly the STC Isc — say, 10A for a panel with Isc of 10.2A — risks nuisance tripping during high irradiance periods. The 1.25 correction factor is mandatory; it is not a conservative rule of thumb.

Mistake 3: Using the Same Fuse Rating for All Panel Types

Different panel models have different Isc values and different Irev max ratings. A fuse correctly selected for one panel model is not automatically correct for a different model, even of nominally similar power output. Always perform the calculation for each panel type used in the project.

Mistake 4: Not Checking the Datasheet for Irev max

Some engineers assume Irev max is simply 2 × Isc. This is a reasonable approximation but not universally correct — panel manufacturers specify their own Irev max values, and using an assumed value rather than the actual datasheet figure is not compliant with IEC 62548. Always use the manufacturer’s stated value.

Mistake 5: Ignoring Cable Derating

String cable installed in conduit, or grouped with other cables, or operating in elevated ambient temperatures carries less current than the open-air single-cable rating. If derating reduces Iz below Irev max, Iz becomes the binding upper constraint for fuse selection. Always apply derating before determining the upper bound.

PV Fuse Selection Checklist

Use this checklist to confirm every solar fuse selection before ordering:

System voltage

• System maximum DC voltage (Voc at minimum temperature) confirmed
• Fuse voltage rating matches: 1000V DC (LFPV-32) or 1500V DC (LFPV-35)

Utilisation category

• Fuse is certified gPV to IEC 60269-1 and IEC 60269-6
• gPV marking confirmed on product datasheet

Physical format

• Fuse format (10×38mm or 10×85mm) confirmed against installed fuse holder
• Fuse holder rated for same DC voltage as fuse

Current rating

• Panel Isc (STC) obtained from manufacturer datasheet
• Corrected Isc calculated: Isc × 1.25
• Lower bound confirmed: In ≥ corrected Isc
• Panel Irev max (max series fuse rating) obtained from manufacturer datasheet
• Cable Iz calculated for actual installation conditions including all derating factors
• Upper bound confirmed: In ≤ min(Irev max, Iz)
• Selected current rating falls within the window
• Selection reviewed for nuisance trip risk in high-irradiance conditions

Compliance

• Number of parallel strings × Isc does not exceed panel Irev max (string design check)
• Selection documented in project protection schedule

Frequently Asked Questions

Q: What if no standard gPV fuse rating falls within my selection window?

A very narrow or impossible selection window usually indicates a mismatch in the system design — often between panel Isc, panel Irev max, and cable sizing. Review the string design: check whether the cable can be upsized to raise Iz, whether a panel with a higher Irev max can be specified, or whether the number of parallel strings needs to be reduced. Do not try to solve a design problem with an out-of-window fuse selection.

Q: My panel datasheet lists “max series fuse: 20A” and my corrected Isc is 18.75A. Is there any valid fuse for this string?

The selection window here is 18.75A to 20A — only the 20A rating falls within it. This is a valid selection. The window is narrow, but a 20A gPV fuse provides compliant string protection for this configuration. Document the selection clearly in the project schedule and note that any panel substitution during the project must trigger a re-check of the fuse rating.

Q: Does the fuse need to be re-selected if I change the panel model mid-project?

Yes — always. Different panel models have different Isc and Irev max values. A fuse correctly selected for the original panel specification may be outside the correct selection window for a different panel. This is one of the most common protection compliance issues on commercial solar projects where panel substitutions occur during procurement.

Q: Should I use the same current rating for all strings in a mixed-orientation array?

If all strings use the same panel model, yes — the selection window is the same for all strings regardless of orientation, because the window is defined by the panel’s Isc and Irev max rather than the actual operating current. Strings facing different orientations will produce different currents during normal operation, but the fuse is sized for worst-case conditions, not average output.

Q: What is the correct fuse for bifacial panels?

Bifacial panels can produce higher Isc than monofacial panels of equivalent nameplate power under high albedo conditions (snow, light-coloured ground surfaces). Some manufacturers specify a bifacial gain factor of up to 30% above the standard Isc. Use the manufacturer’s specified maximum Isc (including bifacial gain) rather than the standard STC Isc when calculating the lower bound for fuse selection. The 1.25 correction factor is applied to this maximum Isc value.

Summary

Correct PV fuse selection follows a defined four-step calculation that is consistent, repeatable, and fully compliant with IEC 62548 and IEC 60269-6:

1. Find the panel Isc (STC) from the manufacturer datasheet
2. Multiply by 1.25 — this is the minimum fuse current rating
3. Find the panel Irev max and cable Iz — the lower of these is the maximum fuse current rating
4. Select the standard gPV fuse rating that falls within this window

Avoid the five common mistakes — cable rating as upper bound, ignoring the 1.25 correction, using one rating for all panels, assuming Irev max, and neglecting cable derating.

Use the checklist before every project order. And always specify a certified gPV fuse to IEC 60269-6 — not a standard DC fuse, not an uncertified “solar compatible” product, and not the same voltage rating across 1000V DC and 1500V DC systems.