Fuse Sizing Calculator

The first electrical fuses were lead wires bridged across terminal boards, patented by Louis François Clément Breguet in 1847 for telegraph systems and refined by Thomas Edison in the 1880s for his DC lighting distribution. Edison's 1890 patent for a glass-enclosed cartridge fuse — with a fusible element visible through the transparent body — established the form factor still used today in automotive and equipment applications.

The critical concept of "I²t" (melt energy) was formalized in the 1920s as engineers built standard time-current curves for fuse coordination. A fuse's pre-arcing I²t value (A²·s) quantifies the let-through energy before the element melts — essential for specifying fuses that protect semiconductors. SCRs and IGBT modules specify a maximum I²t; the upstream fuse's I²t must be lower, creating the "fuse protects the semiconductor" selection rule that dominates drive and UPS design.

Modern fuse standards diverged geographically: UL 248 (North America, formerly UL 198) and IEC 60127 (global, European-led) define different preferred-value series, dimensions, and test methods. A 3.15 A IEC fuse is common in Europe; a 3 A UL fuse is standard in North America. Slow-blow (T or ᵗT) and fast-blow (F or ᶠF) designations are standardized in both systems but with different time-current tolerance bands.

Enter the nominal load current, inrush multiplier, and fuse type (fast/slow/time-delay). The tool multiplies by the safety factor for that class (1.25× fast, 1.5-2× slow, 2-3× time-delay), then rounds up to the nearest standard value from the IEC 60127 / UL 248 preferred-value series: 0.5, 1, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8, 10, 12.5, 16, 20, 25, 32, 40, 50 A.

Important: fuse rating protects the wire, not the load. Wire ampacity must equal or exceed fuse rating. Interrupt rating (AIC/kA) must exceed prospective short-circuit current at the fuse location. For semiconductor protection (SCR, IGBT), use purpose-built ultra-fast semiconductor fuses rated in I²t rather than standard branch-circuit fuses. Everything runs client-side; no values leave your browser.