Fuse Size Calculator

Load	Phase	Voltage	Operating Current	Load Factor	Recommended Fuse
Lighting circuit	Single	230 V	8 A	1.00	10 A
5 HP motor	Three	415 V	8.6 A	1.50	20 A
Transformer primary	Three	400 V	22 A	1.25	35 A
Heater bank	Single	230 V	16 A	1.10	25 A

Formula Used

This calculator starts with operating current. If power is entered, current is calculated from electrical power, voltage, power factor, efficiency, and phase type.

Single phase current: I = P / (V × PF × η)

Three phase current: I = P / (√3 × V × PF × η)

Fuse sizing model: Fuse Current = Load Current × Continuous Factor × Load Factor × Starting Multiplier × Safety Factor ÷ Ambient Factor

After the corrected value is found, the tool rounds upward to the next standard fuse rating. This produces a practical selection rather than a theoretical decimal value.

How to Use This Calculator

Choose whether you know current directly or need current calculated from power.
Select single phase or three phase supply and enter the operating voltage.
Enter current or power, then add power factor and efficiency when needed.
Pick the correct load type to reflect starting behavior and inrush conditions.
Set continuous load percentage, ambient derating factor, and extra safety margin.
Add cable ampacity to check whether the proposed fuse remains coordinated.
Press calculate to show the result above the form and export the result as CSV or PDF.

Frequently Asked Questions

1. What does this fuse size calculator estimate?

It estimates a practical fuse rating using operating current, load behavior, safety margin, and derating inputs. The result helps with preliminary electrical protection selection.

2. Can I size a fuse using power instead of current?

Yes. Enter power, voltage, phase type, power factor, and efficiency. The calculator converts power into current before selecting the next suitable standard fuse size.

3. Why does motor protection need a larger fuse?

Motors can draw high inrush current during startup. A larger fuse often avoids nuisance blowing while still providing short circuit protection when coordinated properly.

4. What is the ambient derating factor?

It adjusts the fuse recommendation for hotter operating conditions or enclosure effects. Lower effective cooling may require a different protection choice than normal room conditions.

5. Does the calculator replace code compliance checks?

No. It supports early design and review. Final protection selection should still follow applicable standards, manufacturer data, fault levels, and local installation rules.

6. Why compare fuse size with cable ampacity?

A fuse larger than conductor ampacity may fail to protect the cable adequately. Coordination between overcurrent protection and conductor rating is essential for safety.

7. What is the difference between IEC and UL series?

They use different preferred rating steps. The calculator lets you round to a common standard family that better matches your equipment market and project practice.

8. Should I choose the exact recommended value every time?

Use it as a strong starting point. You should still review available interrupting ratings, time current curves, downstream coordination, and equipment manufacturer recommendations.