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Fuse Sizing & I²t

Size a fuse above the continuous current (typically ×1.25) but below what the wiring can survive. For fast faults what matters is I²t — the energy the fuse lets through before it opens.

Continuous current (A)

Sizing factor

Fuse I²t rating (A²·s)

Fault current (A)

Inrush current (A)

Inrush duration (ms)

Calculated rating

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Next standard fuse

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Melt time at fault

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Inrush I²t

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Survives inrush?

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Simplified: real fuses have a time-current curve, and slow-blow vs fast-acting behave very differently on inrush. Check the curve for anything critical.

A helper, not a substitute for engineering judgment. Verify against the governing standard.