Fuse Sizing Calculator
Estimate load current and a recommended fuse rating for DC, single-phase AC, or three-phase AC circuits.
Note: This is an estimation tool. Always verify with local electrical code, fuse curves, inrush current behavior, and equipment manufacturer recommendations.
What this fuse calculator does
A fuse protects wiring and equipment by opening the circuit when current exceeds a safe value. This calculator helps you quickly estimate a practical fuse size by calculating current from power and voltage (or using known current directly), then applying a sizing factor.
Core formula used
DC and single-phase AC
Current (A) = Power (W) / (Voltage (V) × Power Factor)
For DC loads, power factor is treated as 1.0.
Three-phase AC
Current (A) = Power (W) / (√3 × Voltage (V) × Power Factor)
Recommended fuse current
After finding load current, the calculator multiplies by your sizing factor (default 125%, a common continuous-load starting point):
Fuse target (A) = Load current × (Sizing factor / 100)
The result is rounded up to the next common fuse rating.
How to use it correctly
- Select whether you know power or current.
- Choose DC, single-phase AC, or three-phase AC.
- Enter voltage and power/current values.
- For AC power-based calculations, set a realistic power factor.
- Apply a sizing factor suitable for your design rules.
Practical fuse selection guidance
1) Match the fuse to wire ampacity
The fuse must protect the conductor first. If your wire is rated for less current than the calculated fuse, downsize the fuse or upsize the conductor.
2) Consider inrush and startup current
Motors, transformers, and some power supplies can draw a short surge current when starting. In those cases, a time-delay (slow-blow) fuse may work better than a fast-acting fuse.
3) Check interrupt rating and voltage rating
A fuse must be rated to safely clear the maximum available fault current and at the circuit voltage. Current value alone is not enough.
4) Follow local code and equipment instructions
NEC, IEC, or local standards can require specific overcurrent protection rules. Manufacturer documentation always takes priority over generic estimates.
Example calculations
Example A: 12V DC load, 120W
- Current = 120 / 12 = 10 A
- At 125% sizing: 10 × 1.25 = 12.5 A
- Nearest standard fuse above target: 15 A
Example B: 230V single-phase AC heater, 2000W, PF 1.0
- Current = 2000 / (230 × 1.0) ≈ 8.70 A
- At 125% sizing: ≈ 10.88 A
- Nearest standard fuse above target: 12 A or 15 A depending on available series and code constraints
Common mistakes to avoid
- Using an undersized fuse because only normal running current was considered.
- Ignoring power factor for AC equipment.
- Choosing fuse current without checking cable size.
- Treating all loads as identical despite different surge behavior.
- Skipping datasheet and code requirements.
Bottom line
This fuse calculator is a fast planning tool for electricians, makers, engineers, and students. Use it to estimate current and shortlist fuse ratings, then finalize your design using conductor limits, protection coordination, local code, and device specifications.