Fuse Sizing Calculator Formula
Understand the math behind the fuse sizing calculator. Each variable explained with a worked example.
Formulas Used
Minimum Fuse Rating
min_fuse = operating_current * safety_factorVariables
| Variable | Description | Default |
|---|---|---|
operating_current | Operating Current(A) | 5 |
safety_factor | Safety Factor | 1.25 |
How It Works
Fuse Sizing
Fuse >= I_operating x Safety_Factor
Standard practice: 1.25 (125%) for continuous loads. Round up to next standard fuse value.
Worked Example
Circuit draws 5 A normally.
operating_current = 5safety_factor = 1.25
- 01Min fuse = 5 x 1.25 = 6.25 A
- 02Use a 7 A or 7.5 A standard fuse.
Frequently Asked Questions
What safety factor?
125% for continuous loads per NEC. 110-115% for intermittent loads.
Fast-blow vs slow-blow?
Fast-blow opens instantly. Slow-blow tolerates brief surges for motors/transformers.
Protect wire or device?
Fuse must be rated below wire ampacity to protect wiring.
Ready to run the numbers?
Open Fuse Sizing Calculator