تحجيم صمامات التيار المستمر: دليل خطوة بخطوة مع الآلة الحاسبة والأمثلة

Introduction: DC Fuse Sizing Is More Than One Formula

DC fuse sizing is not simply a matter of multiplying current by a safety factor and choosing the next fuse size. A correct selection must also satisfy the circuit’s DC voltage, conductor ampacity, equipment limits, temperature conditions and available fault current.

The method is also application-dependent. A photovoltaic string, a battery/inverter circuit and a general DC load do not necessarily use the same sizing rule. This guide explains the main checks and provides a practical calculator for preliminary fuse selection.

هام: The calculator below is a pre-selection tool, not a compliance determination. Always verify the adopted electrical code, equipment manufacturer’s instructions and the actual fuse datasheet before installation.

How to Choose the Right DC Fuse Size: Quick Answer

إذا كنت تريد أن تعرف how to choose the right fuse size أو how to calculate fuse rating for a DC circuit, start with the design current required by the application. For the common NEC PV source-circuit method, the familiar calculation is Isc × 1.5625. For battery/inverter and other DC circuits, use the manufacturer-specified or project-required design current and multiplier rather than assuming the same factor applies everywhere.

After the current calculation, adjust for the fuse manufacturer’s current rerating factor and select the next suitable available fuse size. Then verify four limits before final selection: equipment maximum fuse/OCPD rating, conductor ampacity, maximum DC voltage, and DC breaking capacity.

Quick formula for the common NEC PV method: Minimum current = Isc × 1.5625, then Required fuse nameplate current = Minimum current ÷ manufacturer rerating factor. The result is only a starting point; the selected fuse must still pass all equipment, conductor, voltage and fault-current checks.

Why DC Fuse Selection Requires More Than an Ampere Rating

Unlike AC, DC current does not naturally pass through zero every half-cycle. This makes interruption of a DC arc more demanding. A fuse used in a DC circuit therefore needs an appropriate DC voltage rating, breaking capacity and fuse category for the application.

Do not use a fuse in a DC circuit unless the fuse has a documented DC rating suitable for that circuit. Some fuse products carry both AC and DC ratings, while others are intended only for a specific application.

Four Ratings You Must Check

• Current rating (A): The fuse current rating is defined under specified test conditions. Actual application capability can be affected by ambient temperature, fuse holder design and manufacturer rerating requirements.
• DC voltage rating (VDC): The fuse DC voltage rating must be at least the maximum circuit voltage. For PV strings, this means the maximum cold-corrected string voltage, not simply the module nameplate Voc.
• Breaking capacity / interrupting rating: The documented DC breaking capacity at the actual system voltage must be adequate for the prospective short-circuit current.
• Fuse category and time-current characteristic: The fuse must be appropriate for the protected equipment. For example, PV strings commonly use gPV fuse-links designed for photovoltaic protection.

Fuse Rating Formula: Why There Is No Single Formula for Every DC Circuit

There is no single universal multiplier that correctly sizes every DC fuse. The first step is to identify the application and the design rule that applies to that circuit.

The Common NEC 1.5625 Method for PV Source Circuits

A commonly used NEC-based method for PV source-circuit fuse sizing applies two 125% factors. The first establishes a maximum circuit current from module Isc, and the second is used when sizing the overcurrent protective device under the applicable method.

Common calculation:

Minimum fuse current = Isc × 1.25 × 1.25 = Isc × 1.5625

This is the familiar “1.56 × Isc” rule used in many PV fuse-sizing guides. However, it should not be treated as a substitute for checking the NEC edition adopted by the Authority Having Jurisdiction (AHJ), the module listing and the actual project design.

After calculating the minimum current rating, the selected fuse must still satisfy the module maximum series fuse rating, conductor protection, temperature rerating, DC voltage and breaking-capacity requirements.

IEC PV Fuse Sizing: Use the Current Standard and Module Requirements

For IEC-based PV projects, avoid applying one fixed multiplier without checking the current design standard and the module/fuse manufacturer’s instructions. IEC 62548-1:2023 + AMD1:2025 is the current IEC reference for PV array design requirements, while IEC 60269-6:2010 + AMD1:2021 contains supplementary requirements for fuse-links used to protect photovoltaic strings and arrays.

The practical point is simple: determine the required design current using the applicable project method, then verify that the selected fuse is suitable for the module, conductor, maximum DC voltage and PV application. Do not hard-code an old IEC current range into a calculator and assume it applies to every project.

For a broader system-level explanation of PV array protection, voltage, strings and overcurrent design, see our IEC 62548 practical guide.

Step-by-Step DC Fuse Selection Process

Step 1: Determine the Correct Design Current

Use the current value required by the application and governing design method. For PV this may be based on module Isc. For battery/inverter circuits, prefer the manufacturer’s maximum continuous DC current or specified fuse/OCPD information rather than relying on a lower back-calculated value.

Step 2: Apply the Required Sizing Method

Apply the multiplier, continuous-load rule or design method required by the adopted code, equipment instructions or engineered design. For the common NEC PV method, this may be Isc × 1.5625. Other applications can use different rules.

Step 3: Apply Fuse Temperature Rerating

Fuse current-carrying capability changes with ambient temperature. Use the rerating curve or factor supplied by the actual fuse manufacturer. Do not assume one universal temperature factor applies to all fuse series.

If the required current is 20A and the manufacturer’s current rerating factor at the actual ambient condition is 0.92, the required fuse nameplate current would be:

20A ÷ 0.92 = 21.74A

The next appropriate standard fuse size would then need to be checked against the equipment and conductor limits.

Step 4: Check the Equipment Maximum Fuse Rating

The selected fuse must not exceed the equipment manufacturer’s maximum permitted fuse or OCPD rating. For a PV module, this is commonly shown as the Maximum Series Fuse Rating on the module datasheet.

Step 5: Verify Conductor Protection

Verify the selected fuse against the conductor’s final allowable ampacity after all required correction and adjustment factors. Do not compare the fuse only with a simplified cable-size table.

Step 6: Check the Maximum DC Voltage

The fuse voltage rating must be at least the maximum circuit voltage. In a PV string, calculate the maximum string voltage at the minimum design temperature using the module data and the applicable design method. A fuse should not be labelled “1000VDC suitable” for a design unless the calculated string voltage has actually been checked.

Step 7: Verify DC Breaking Capacity

The fuse’s documented DC breaking capacity at the actual system voltage must be sufficient for the prospective short-circuit current at the fuse location. This is particularly important in battery systems, where fault current can be very high.

For battery systems, use the battery manufacturer’s specified short-circuit current or a properly engineered fault-current calculation. Do not select fuse breaking capacity from a simple nominal-voltage calculation using an estimated loop resistance.

Worked Example 1: Solar PV String Fuse Sizing

Example data:

• Module Isc: 12.8A
• Module maximum series fuse rating: 25A
• Verified conductor ampacity after applicable corrections: 40A
• Fuse manufacturer’s current rerating factor at the installation temperature: 0.92
• Calculated maximum string voltage at the minimum design temperature: 920VDC

Step 1 – Common NEC PV current calculation:

12.8A × 1.5625 = 20.00A

Step 2 – Apply the fuse manufacturer’s current rerating factor:

20.00A ÷ 0.92 = 21.74A

The next suitable standard current rating in this example is 25A.

• Equipment check: 25A does not exceed the module’s 25A maximum series fuse rating. ✓
• Conductor check: 25A is below the verified 40A conductor ampacity. ✓
• Voltage check: The calculated maximum string voltage is 920VDC, so a fuse with a documented rating of at least 920VDC is required. A 1000VDC-rated gPV fuse can satisfy this voltage check if all other ratings are also suitable. ✓
• Breaking-capacity check: Verify the selected gPV fuse’s documented DC breaking capacity against the available fault current for the actual array design.

Preliminary result: 25A gPV fuse, with a DC voltage rating of at least 920VDC and adequate documented DC breaking capacity. The final product selection still depends on the module instructions, fuse datasheet and adopted code requirements.

When Does a PV String Need Overcurrent Protection?

With multiple parallel PV strings, healthy strings can feed reverse current into a faulted string. This is why string fusing becomes important as the number of parallel current sources increases.

However, avoid using “three strings always require a fuse” as a universal rule. The final requirement depends on the available reverse/fault current, conductor ampacity, module maximum overcurrent protection rating and the applicable electrical code.

If you are choosing the actual gPV fuse type, voltage class and form factor after completing the current calculation, see our solar fuse selection guide.

Worked Example 2: Battery / Inverter Fuse Pre-Selection

Battery fuse sizing requires special attention because the available short-circuit current can be much higher than the normal operating current.

Illustrative project data:

• Inverter manufacturer maximum continuous DC current: 125A
• Project-required sizing factor for this example: 1.25
• Equipment maximum permitted fuse/OCPD rating: 175A
• Verified conductor ampacity: 200A
• Battery manufacturer / engineered prospective short-circuit current at the fuse location: 18kA
• Maximum DC system voltage: 58VDC

Current pre-selection:

125A × 1.25 = 156.25A

The next standard current size used in this example is 175A.

• Equipment check: 175A does not exceed the entered 175A equipment maximum. ✓
• Conductor check: 175A is below the verified 200A conductor ampacity. ✓
• Voltage check: The fuse must have a documented DC voltage rating of at least 58VDC.
• Breaking-capacity check: The fuse must have a documented DC breaking capacity not less than the 18kA prospective short-circuit current at the relevant DC voltage.

هام: This example does not mean every 125A inverter circuit should use a 175A fuse. The actual fuse rating and type must follow the inverter, battery, BMS, conductor and applicable installation requirements.

DC Fuse Voltage Rating: Do Not Skip This Check

A fuse’s ampere rating does not tell you whether it can safely interrupt a fault at the system voltage. The DC voltage rating must be equal to or greater than the maximum voltage the fuse can experience.

For PV systems, the maximum string voltage can be substantially higher than the module’s STC Voc when the modules are cold. Calculate the maximum string voltage using the module temperature data and the project design method before choosing a 600VDC, 1000VDC or 1500VDC fuse.

DC Breaking Capacity: Especially Important for Batteries

Breaking capacity, also called interrupting rating, is the maximum prospective fault current that the fuse can safely interrupt under its stated conditions. Always compare DC breaking capacity at the relevant system voltage, not an AC value taken from the same product family.

If a battery manufacturer specifies an 18kA prospective short-circuit current, a fuse with only a 10kA documented DC breaking capacity at the system voltage would be inadequate. Select a product with a sufficient margin and verify the actual fuse datasheet.

Common DC Fuse Sizing Mistakes

If a correctly selected fuse is already opening repeatedly, do not simply increase the ampere rating. Use our blown fuse troubleshooting guide to identify overload, short-circuit, connection and temperature-related causes first.

Relevant Standards for PV Fuse Selection

For a practical manufacturer reference on the commonly used NEC PV method and fuse temperature rerating, see the Littelfuse photovoltaic fuse sizing application guide.

الأسئلة الشائعة

1. How do I choose the right fuse size?

Start with the correct design current, calculate the minimum fuse rating using the method required for the application, and then select a standard fuse size that still protects the conductor and equipment. The final fuse must also have a suitable DC voltage rating, breaking capacity and time-current characteristic.

2. How do I calculate DC fuse rating?

There is no single formula for every DC circuit. For the common NEC PV method, the starting calculation is Isc × 1.5625. Battery/inverter and general DC circuits should use the design current and sizing method required by the equipment manufacturer, applicable code or project specification.

3. Is DC fuse size always current × 1.25?

No. The required method depends on the application and governing rules. A PV source circuit, a battery/inverter circuit and a general DC load should not automatically use the same multiplier.

4. What is the 1.56 rule for solar fuses?

The familiar 1.56 value comes from two 125% factors: 1.25 × 1.25 = 1.5625. It is a common NEC-based PV source-circuit sizing method, but the final design must still follow the adopted NEC edition, equipment listing and AHJ requirements.

5. Can I use an AC fuse in a DC circuit?

Only if that specific fuse also has a suitable documented DC voltage and interrupting rating for the application. Do not assume an AC-only rating is valid for DC.

6. Does the fuse voltage rating need to match the system voltage?

The fuse DC voltage rating must be at least the maximum circuit voltage. For PV, use the maximum cold-corrected string voltage rather than the nominal system voltage alone.

7. How do I choose battery fuse breaking capacity?

Use the battery manufacturer’s prospective short-circuit current or a properly engineered fault-current calculation, then select a fuse whose documented DC breaking capacity at the actual system voltage is not less than the available fault current.

8. What is a gPV fuse?

A gPV fuse-link is designed for photovoltaic protection. IEC 60269-6 provides supplementary requirements for fuse-links used to protect PV strings and arrays. The fuse must still be selected for the actual current, maximum DC voltage and fault conditions of the installation.

Final DC Fuse Sizing Checklist

• Correct design current identified for the application
• Applicable sizing method or multiplier verified
• Fuse manufacturer temperature rerating applied
• Equipment maximum fuse/OCPD rating checked
• Final conductor ampacity checked
• Maximum DC circuit voltage calculated
• Fuse DC voltage rating verified
• Prospective short-circuit current determined
• Documented DC breaking capacity verified
• Correct fuse category / time-current characteristic selected

About KUANGYA

KUANGYA supplies DC fuse and fuse-holder solutions for photovoltaic and other DC protection applications. Explore our DC fuse products, أو اتصل بنا for model-specific ratings, available certifications, OEM options and application support.

Disclaimer: This article and calculator are for educational and preliminary selection purposes only. Electrical protection design must be verified by a qualified professional using the applicable electrical code, local requirements, equipment instructions and actual fuse datasheets.