Formula used
I_design = I_load × quantity × continuous factor × margin × future factor I_adjusted = I_design ÷ (ambient factor × grouping factor) Selected fuse = next available standard rating at or above I_adjusted
- DC current: I = P ÷ (V × efficiency)
- Single-phase AC: I = P ÷ (V × PF × efficiency)
- Three-phase AC: I = P ÷ (√3 × V × PF × efficiency)
- Apparent power: I = VA ÷ V, or VA ÷ (√3 × V)
How to use this calculator
- Select the current or power calculation mode.
- Choose AC, DC, and the applicable phase.
- Enter load, derating, inrush, and protection details.
- Review the recommended rating and every warning.
- Confirm the final choice against applicable requirements.
Example data
| Application | Typical inputs | Suggested settings |
|---|---|---|
| Lighting circuit | 230 V, 8 A | 125% continuous, fast or general fuse |
| Single-phase motor | 230 V, 2.2 kW, 0.82 PF | Time-delay fuse, six-times inrush |
| Three-phase motor | 400 V, 7.5 kW, 0.86 PF | Three-phase mode, coordinated motor fuse |
| Solar string | 600 V DC, 12 A | DC-rated photovoltaic fuse |
| Automotive load | 12 V DC, 120 W | Automotive blade fuse |
| Electronic supply | 120 V, 500 W | Time-delay protection for capacitor inrush |
Common standard fuse sizes
| Small ratings | 0.1, 0.125, 0.16, 0.2, 0.25, 0.315, 0.4, 0.5, 0.63, 0.8 A |
|---|---|
| General ratings | 1, 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8, 10, 12.5, 16 A |
| Distribution ratings | 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400 A |
| Large ratings | 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6000 A |
Fuse type comparison
| Fuse type | Best suited for | Main consideration |
|---|---|---|
| Fast-acting | Sensitive resistive circuits | Limited inrush tolerance |
| Time-delay | Motors, transformers, power supplies | Allows brief starting current |
| Semiconductor | Power electronics | Very fast energy limitation |
| Photovoltaic DC | Solar strings and arrays | Correct DC rating is essential |
| Automotive blade | Vehicle accessory circuits | Match holder and wire limits |
| High breaking capacity | Industrial fault protection | Verify interrupting capacity |
Safety limitations
This tool provides preliminary engineering estimates only. It cannot replace code studies or manufacturer data. Qualified professionals should verify every final protection selection.
Fuse voltage ratings must meet circuit voltage requirements. Interrupt ratings must exceed available fault current safely. Conductors also need suitable insulation and installation ratings.
Frequently asked questions
Should I always choose the next larger fuse?
Usually, the next standard rating is selected. Conductor and equipment limits can override that choice. Manufacturer instructions always deserve priority during final selection.
What continuous factor should I use?
The factor depends on applicable electrical requirements. Many designs use 125 percent for continuous loads. Confirm the required rule for your exact installation.
Why does ambient temperature matter?
Heat can alter fuse behavior and conductor capacity. Derating reduces allowable loading under hotter operating conditions. Use manufacturer curves for demanding thermal environments whenever available.
Which fuse suits a motor?
Motors commonly create high starting current. Time-delay protection often avoids unnecessary fuse operation. Coordination must still protect conductors and motor equipment.
Can an AC fuse protect DC circuits?
Not unless the fuse has suitable DC ratings. DC arcs are harder to interrupt safely. Always verify voltage and breaking-capacity markings before installation.
What is interrupting capacity?
It is the maximum safely interrupted fault current. Available fault current must remain below that rating. Exceeding it can cause dangerous equipment failure.
Why enter conductor ampacity?
The fuse should protect the connected conductor. Oversized protection may allow dangerous cable heating. Installation methods can further reduce conductor capacity significantly.
How is inrush handled?
The calculator estimates inrush using your multiplier. Startup-tolerant mode adds a limited sizing allowance. Time-current curves remain necessary for precise coordination.
Can I add uncommon fuse sizes?
Yes, enter custom ratings separated by commas. They merge with the included standard rating list. Only use sizes available from your selected manufacturer.
Does this calculator replace electrical design?
No, it supports preliminary fuse-size evaluation only. Protection design also requires fault and coordination studies. Final work should follow qualified professional review.