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Calculate a preliminary fuse size for AC or DC circuits from load current, kW, kVA or voltage, with power factor, design margin, derating, cable ampacity, fuse type, voltage rating and short-circuit breaking-capacity checks.
Inputs
1
1. System and load
Choose the electrical system before entering power or voltage.
Application affects fuse technology and coordination requirements. It does not automatically replace manufacturer or standard requirements.
Use the actual steady-state design/load current where available.
For AC loads, current is calculated using voltage and power factor.
Used for AC kW-to-current calculations. Not used for direct current or DC calculations.
2
2. Design and derating
Enter the design multiplier applicable to your project. 100% means no additional multiplier.
Optional engineering margin applied before selecting the next standard fuse. Do not use this as a substitute for the fuse manufacturer's time-current coordination.
Enter the applicable correction factor for the fuse assembly, enclosure and ambient temperature using manufacturer data.
Use the applicable correction factor for adjacent loaded circuits and installation arrangement.
Enter cable current-carrying capacity after applicable installation correction factors. Use 0 to skip the cable coordination check.
3
3. Fuse selection
Select the intended fuse characteristic. Actual fuse construction, dimensions and application limits are manufacturer-specific.
The fuse voltage rating must be suitable for the circuit voltage and system type. DC interruption requires a suitable DC-rated fuse.
Enter the prospective fault current at the fuse location.
Enter the actual fuse-link breaking capacity from the manufacturer datasheet.
How to use this calculator: Fuse Size Calculator
Calculate a preliminary fuse size for AC or DC circuits from load current, kW, kVA or voltage, with power factor, design margin, derating, cable ampacity, fuse type, voltage rating and short-circuit breaking-capacity checks. The example below is calculated by this page's real engine from the displayed inputs.
1Confirm that the Fuse Size Calculator matches the quantity or design check you need.
2Enter Load current, Real power, and Apparent power using the units printed beside each field.
3Select the applicable Electrical system, Application, and Known load value options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.
Input guide and example values
Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.
Input
Example value
Why it matters
Electrical system
Single-phase AC
Choose the electrical system before entering power or voltage.
Application
General circuit
Application affects fuse technology and coordination requirements. It does not automatically replace manufacturer or standard requirements.
Known load value
Load current (A)
Select the option that matches the real installation or scenario.
Load current
10 A
Use the actual steady-state design/load current where available.
Real power
2 kW
For AC loads, current is calculated using voltage and power factor.
Used for AC kW-to-current calculations. Not used for direct current or DC calculations.
Continuous-load factor
100 %
Enter the design multiplier applicable to your project. 100% means no additional multiplier.
Fuse sizing margin
0 %
Optional engineering margin applied before selecting the next standard fuse. Do not use this as a substitute for the fuse manufacturer's time-current coordination.
Ambient / installation factor
100 %
Enter the applicable correction factor for the fuse assembly, enclosure and ambient temperature using manufacturer data.
Grouping factor
100 %
Use the applicable correction factor for adjacent loaded circuits and installation arrangement.
Corrected cable ampacity
16 A
Enter cable current-carrying capacity after applicable installation correction factors. Use 0 to skip the cable coordination check.
Fuse characteristic
gG — general purpose
Select the intended fuse characteristic. Actual fuse construction, dimensions and application limits are manufacturer-specific.
Fuse rated voltage
500 V
The fuse voltage rating must be suitable for the circuit voltage and system type. DC interruption requires a suitable DC-rated fuse.
Prospective short-circuit current
3 kA
Enter the prospective fault current at the fuse location.
Fuse breaking capacity
10 kA
Enter the actual fuse-link breaking capacity from the manufacturer datasheet.
Formula, derivation and worked example
Fuse sizing is a protection-coordination problem rather than a simple current lookup. The load current is first established from amps, kW or kVA. A user-defined continuous-load factor and design margin are then applied to obtain a sizing current. The calculator selects the next standard fuse rating and checks it against the corrected cable ampacity. It also checks the fuse voltage rating and breaking capacity against the entered circuit voltage and prospective short-circuit current. For PV systems, additional PV-specific inputs are provided because photovoltaic fuse selection depends on string current, parallel-string configuration, module maximum series-fuse rating and the applicable PV protection requirements.
Single-phase AC current: I = P × 1000 / (V × PF)
Three-phase AC current: I = P × 1000 / (√3 × VLL × PF)
DC current: I = P × 1000 / V
kVA single-phase current: I = S × 1000 / V
kVA three-phase current: I = S × 1000 / (√3 × VLL)
Voltage suitability: Fuse rated voltage ≥ circuit voltage
Fault-duty suitability: Fuse breaking capacity ≥ prospective short-circuit current
Substitution steps
1. Calculate operating current
I = entered current
= 10 A
2. Apply continuous-load factor
Ib = I × continuous factor
= 10 A
3. Apply design margin
I_design = Ib × (1 + margin)
= 10 A
4. Select standard fuse
next standard fuse rating ≥ sizing current
= 10 A
5. Correct cable capacity
Iz = Iz,base × k_ambient × k_group
= 16 A
6. Cable / fuse coordination
Ib ≤ In ≤ Iz
= PASS
7. Voltage rating
Fuse rated voltage ≥ system voltage
= PASS
8. Fault-duty check
Breaking capacity ≥ prospective short-circuit current
= PASS
Computed example results
Recommended fuse rating
10 A
gG general-purpose fuse
Operating current
10 A
Design current
10 A
after continuous-load factor
Fuse sizing current
10 A
after optional design margin
Fuse utilization
100%
Corrected cable capacity
16 A
6 A margin to selected fuse
Fuse / cable coordination
✅ PASS
Simplified Ib ≤ In ≤ Iz check
Fuse rated voltage
500 V
Minimum standard voltage class
250 V
Entered fuse voltage is adequate
Voltage rating check
✅ PASS
Entered breaking capacity
10 kA
Recommended minimum breaking class
3 kA
Breaking capacity check
✅ PASS
Fuse characteristic
gG general-purpose fuse
Application
General AC/DC circuit protection.
Calculated real power
2.07 kW
Calculated apparent power
2.3 kVA
PV screening status
Not applicable
PV reverse-current indicator
Not applicable
Screening indicator only; not a substitute for the complete PV protection study
PV module maximum-fuse check
Not applicable
Overall preliminary status
✅ Preliminary fuse selection passes
Understanding the result
Read the main result together with supporting checks, assumptions, limits and intermediate values.
For a manual check, repeat the first equation, confirm the units and change one input at a time.
Common mistakes when using Fuse Size Calculator
Do not mix units for Load current (A), Real power (kW), Apparent power (kVA). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Electrical system on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed Single-phase AC current: I = P × 1000 / (V × PF) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Recommended fuse rating = 10 A from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.
When the Fuse Size Calculator is useful
Fuse Size Calculator is designed for cases where Electrical system, Application, Known load value, Load current are known and you need Recommended fuse rating, Operating current, Design current. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.
Use the calculator for the scope described by its inputs and notes. The displayed method is Single-phase AC current: I = P × 1000 / (V × PF). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.
Electrical system and Application: what changes the answer
The worked example uses Electrical system = Single-phase AC, Application = General circuit, Known load value = Load current (A), Load current = 10 A. With those values, Recommended fuse rating is 10 A. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.
For this calculator, the main input roles are: Electrical system: Choose the electrical system before entering power or voltage. Available choices include Single-phase AC, Three-phase AC, DC. Application: Application affects fuse technology and coordination requirements. It does not automatically replace manufacturer or standard requirements. Available choices include General circuit, Motor circuit, Solar PV / PV string, Battery / DC feeder. Known load value: Select the option that matches the real installation or scenario. Available choices include Load current (A), Real power (kW), Apparent power (kVA). Load current (A): Use the actual steady-state design/load current where available.
How to sanity-check a Fuse Size Calculator result
Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Recommended fuse rating, because it is the first worked-example output shown by the live engine.
Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.
Next logical calculator
Continue with MCB Size Calculator
Useful next check because both tools use Electrical system and Known load value, while MCB Size Calculator answers a different part of the same workflow.
Voltage suitability: Fuse rated voltage ≥ circuit voltage
Fault-duty suitability: Fuse breaking capacity ≥ prospective short-circuit current
Fuse sizing is a protection-coordination problem rather than a simple current lookup. The load current is first established from amps, kW or kVA. A user-defined continuous-load factor and design margin are then applied to obtain a sizing current. The calculator selects the next standard fuse rating and checks it against the corrected cable ampacity. It also checks the fuse voltage rating and breaking capacity against the entered circuit voltage and prospective short-circuit current. For PV systems, additional PV-specific inputs are provided because photovoltaic fuse selection depends on string current, parallel-string configuration, module maximum series-fuse rating and the applicable PV protection requirements.
Formulas explained
I = P × 1000 / (V × PF)
Single-phase AC real power is related to RMS voltage, current and power factor. Rearranging the power equation gives the operating current.
I
AC load current, A
P
Real power, kW
V
RMS voltage, V
PF
Power factor
I = P × 1000 / (√3 × VLL × PF)
Balanced three-phase real power is √3 × VLL × I × PF. The equation is rearranged to calculate line current.
VLL
Three-phase line-to-line voltage
I
Line current, A
I_design = Ib × (1 + margin)
A user-defined design margin can be applied before selecting the next standard fuse rating. This is an engineering input, not a universal fuse-sizing rule.
Ib
Design load current
margin
Additional design margin as a decimal
Iz = Iz,base × k_ambient × k_group
The available cable current capacity can be reduced by installation conditions. The actual correction factors must come from the applicable cable standard and installation method.
Iz
Corrected cable ampacity
k_ambient
Ambient/installation correction factor
k_group
Grouping correction factor
Ib ≤ In ≤ Iz
The protective device should carry the intended design current while remaining suitable for protecting the conductor under the selected design conditions.
Ib
Design current
In
Fuse rated current
Iz
Corrected cable current capacity
Ue(fuse) ≥ U(system)
The fuse's rated voltage must be suitable for the circuit voltage. For DC circuits, the fuse must specifically be suitable for DC interruption at the system voltage.
Ue
Fuse rated voltage
U
Circuit voltage
Breaking capacity ≥ prospective short-circuit current
The fuse must be capable of safely interrupting the prospective fault current at its installation point.
Ibr
Fuse breaking capacity
Isc
Prospective short-circuit current
What is a fuse?
A fuse is an overcurrent protective device containing a fuse-link designed to interrupt excessive current. When the current exceeds the fuse's characteristic, the fuse element melts and interrupts the circuit. Fuse selection depends on current, voltage, fault level, time-current characteristic and application.
How do I calculate fuse size?
Start with the circuit's design current. For AC loads, calculate current from kW or kVA when necessary. Apply the applicable design and continuous-load factors, then select a standard fuse rating that is suitable for the load and does not exceed the protected conductor's corrected ampacity. Finally verify voltage rating, breaking capacity and time-current coordination.
Why is fuse size not simply the next rating above current?
A fuse is part of a protection system. A fuse selected only from load current may fail to protect the cable, may nuisance-open during normal starting current, or may have inadequate voltage or breaking-capacity ratings. The fuse must be coordinated with the conductor and equipment.
What is the difference between fuse rating and breaking capacity?
Fuse rated current, such as 10 A or 32 A, describes the current rating of the fuse-link. Breaking capacity, such as 10 kA or 50 kA, describes the maximum prospective short-circuit current the fuse can safely interrupt under its specified conditions. They are different ratings.
What is a gG fuse?
gG is a general-purpose full-range fuse characteristic used for general overcurrent protection applications. The actual fuse-link application, dimensions, voltage rating and time-current curve must be checked against the manufacturer data.
What is an aM fuse?
aM is a partial-range motor fuse characteristic intended primarily for short-circuit protection in motor circuits and is normally coordinated with separate motor overload protection. Motor starting current and the manufacturer's coordination tables must be checked.
What is a gPV fuse?
gPV is a photovoltaic fuse characteristic specifically developed for PV string and array protection. PV fuse selection must consider the DC voltage, string current, parallel-string arrangement, reverse-current conditions, module maximum series-fuse rating and the applicable PV requirements.
Why does cable ampacity matter when sizing a fuse?
The fuse protects the conductor as well as the equipment. If the fuse rating is higher than the conductor's corrected current-carrying capacity, the conductor may not receive the intended overcurrent protection. Always compare the fuse rating with the corrected cable ampacity.
Why does fuse voltage rating matter?
A fuse must be rated for the circuit voltage. DC interruption is particularly important because a DC arc does not naturally extinguish at a current zero as it does in an AC waveform. A DC circuit therefore needs a fuse specifically suitable for the DC voltage and fault conditions.
What is fuse derating?
Fuse and fuse-holder performance can change with ambient temperature, enclosure conditions, mounting arrangement and other application conditions. Use manufacturer-specific correction factors rather than assuming that the nominal current rating is unchanged in every installation.
Fuse vs MCB
Fuses interrupt current by melting a fuse element, while MCBs use a mechanical switching mechanism with thermal and/or magnetic protection. Fuses can provide very high interrupting capacity and fast current limitation, while MCBs provide resettable protection and convenient switching. The correct device depends on the circuit and coordination requirements.
Why is breaking capacity important?
If the prospective short-circuit current exceeds the fuse's rated breaking capacity, the protective device may not safely interrupt the fault. The fuse breaking capacity therefore needs to be adequate for the maximum prospective fault current at the installation point.
Does cable length affect fuse size?
Cable length does not directly set the fuse's nominal current rating, but it changes cable impedance, voltage drop and fault-current magnitude. A long cable can therefore affect protection performance and must be included in a complete circuit design.
How is a solar PV fuse different?
PV circuits have special DC characteristics and can remain energized whenever irradiance is available. PV fuse-links therefore have application-specific requirements. IEC 60269-6 provides supplementary requirements for fuse-links used to protect PV strings and arrays up to 1,500 V DC.
Worked example
1Example: single-phase AC load = 2.3 kW, system voltage = 230 V, power factor = 0.95.
2Load current = 2,300 / (230 × 0.95) = 10.53 A.
3With a 100% continuous-load factor and 0% additional design margin, the sizing current remains 10.53 A.
4The next standard fuse rating is 12.5 A. If the actual product series does not provide 12.5 A, select the next available standard rating from the manufacturer's range.
5If the corrected cable ampacity is 16 A, the simplified current coordination check passes because the fuse rating is above the design current and below the corrected cable capacity.
6If the fuse breaking capacity is 10 kA and the prospective short-circuit current is 6 kA, the basic fault-duty check passes.
7The final fuse must still be verified against its actual time-current curve, fuse-holder characteristics, voltage rating, installation temperature and manufacturer coordination requirements.
Assumptions
AC calculations use RMS voltage.
Three-phase calculations assume a balanced load.
Three-phase voltage is line-to-line voltage.
Power factor is used only for AC real-power calculations.
DC calculations do not use power factor.
The calculator selects a standard nominal fuse rating based on the entered sizing current.
Ambient and grouping factors are user-entered correction factors and are not universal values.
Cable ampacity should preferably already include all relevant installation correction factors before being entered.
Fuse time-current coordination is not fully represented by a single multiplier.
Motor starting current, transformer inrush, semiconductor I²t and selective coordination require manufacturer-specific checks.
Use actual measured operating current when available.
For kW-based AC calculations, use the actual equipment power factor rather than assuming unity PF for motors.
For three-phase systems, enter line-to-line voltage such as 400 V or 415 V.
Enter the cable's corrected ampacity rather than an uncorrected catalogue ampacity.
Check both fuse rated current and fuse breaking capacity.
For motor circuits, use the manufacturer's fuse and starter/overload coordination data.
For PV circuits, use PV-specific fuse-links and verify the complete string/array protection design.
For long cable runs, check voltage drop and fault-current level separately.
Warnings
Do not select a fuse from load current alone.
Do not use a generic AC fuse on a DC circuit unless the fuse is specifically rated for the required DC voltage and interruption duty.
The fuse breaking capacity must be adequate for the prospective short-circuit current at the installation point.
For motors, transformers and capacitive loads, starting or inrush current can significantly affect fuse selection.
For semiconductor protection, verify I²t and the complete semiconductor-fuse coordination with the equipment manufacturer.
For PV systems, verify module maximum series-fuse rating, string/array configuration, reverse-current conditions, DC voltage rating and PV-specific fuse requirements.
Final protection design must consider the applicable national wiring rules, manufacturer data, conductor protection, disconnection requirements and coordination with upstream/downstream devices.
Standards & references
IEC 60269-1:2024 — Low-voltage fuses — Part 1: General requirements
IEC 60269-2 — Supplementary requirements for fuses mainly used in industrial applications
IEC 60269-3:2024 — Supplementary requirements for fuses for operation by unskilled persons; gG systems
IEC 60269-6:2010+A1:2021 — Fuse-links for protection of solar photovoltaic energy systems
IEC TR 60269-5 — Guidance for application of low-voltage fuses
Start with the design current of the circuit and select a suitable standard fuse rating while ensuring the fuse rating is compatible with the corrected cable ampacity. The basic coordination relationship is Ib ≤ In ≤ Iz, subject to the applicable protection rules and fuse characteristics.
How do I calculate fuse size from kW?+
For single-phase AC, I = P × 1000 / (V × PF). For balanced three-phase AC, I = P × 1000 / (√3 × VLL × PF). For DC, I = P × 1000 / V. After calculating current, the fuse must be checked against the cable, voltage and fault-duty requirements.
What fuse size do I need for a 10 amp load?+
A fuse should not be selected from the 10 A value alone. A suitable fuse depends on the continuous load, starting current, cable ampacity, fuse characteristic, voltage rating, breaking capacity and applicable protection requirements. The next standard fuse rating may be appropriate only after those checks.
What size fuse should I use for a 230 V load?+
The fuse size depends on the load current, not voltage alone. For a known single-phase real-power load, calculate I = P × 1000 / (230 × PF), then select and coordinate the fuse with the cable and fault level.
What is the single-phase fuse size formula?+
For an AC load with known real power, the current is I = P × 1000 / (V × PF). Fuse size is then selected from the standard fuse ratings after checking the cable ampacity, fuse characteristic, voltage and fault duty.
What is the three-phase fuse size formula?+
For a balanced three-phase load, I = P × 1000 / (√3 × VLL × PF). The resulting design current is then used as one input to fuse selection and protection coordination.
Can I use the same fuse size for AC and DC?+
Not automatically. DC interruption is different from AC interruption, so the fuse must have a suitable DC voltage and interruption rating for the circuit. Always check the fuse manufacturer's DC rating.
What is a gG fuse?+
A gG fuse is a general-purpose full-range fuse characteristic commonly used for general overcurrent protection. The exact fuse-link rating and application must be verified from the manufacturer.