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Fuse size calculator

Pick the standard fuse rating above your design current and check category, discrimination with the upstream fuse, and cable protection.

Three rules do the work here. The rating is the next standard value at or above the target current, where the target is the design current times the multiplier (1 for a cable, which has no inrush to ride out). Cable protection asks whether In sits at or under Iz with real spare; under ten percent spare the verdict is marginal, not green. Discrimination uses the 1.6 :1 ratio rule of thumb, which is a starting point, not a substitute for the manufacturer's I²t selectivity tables. What this page does not do: breaking-capacity checks against the prospective fault current, motor starting curves, NEC 430.52 exact percentages, or the 1.45 × Iz conventional-fusing proof. The standards and the manufacturer's data are the authority; this page is a first pass, not a coordination study.
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