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Transformer Inrush Current & Fuse Sizing Calculator electrical
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Transformer Inrush Current & Fuse Sizing Calculator

Compute transformer magnetizing inrush current peaks (8x to 14x FLA at 0.1s) and size primary and secondary overcurrent protection per NEC Table 450.3.

Transformer Nameplate

NEC 450 Protection Architecture

Currents & Protection Results

Peak Magnetizing Inrush Current (at 0.1 sec)
-- Amps
from -- A primary full load current
Primary Fuse Size
-- A
NEC 450.3 maximum
Secondary FLA
-- A
-- A secondary main (125%)
Secondary Available Short Circuit: -- kA Isc
Inrush Coordination Margin: Fuse clears inrush point
Inrush Duration Benchmark: 0.10 seconds (6 cycles)
Primary fuse time-current curve (TCC) must pass to the right of the inrush point (12x FLA @ 0.1s) while remaining to the left of the ANSI/IEEE C57.109 transformer thermal damage curve.

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Frequently Asked Questions

What causes transformer magnetizing inrush current?

When a transformer is energized at a point in the AC voltage cycle when voltage is near zero, or when residual magnetism remains in the iron core, the required magnetic flux peaks at more than twice its steady-state maximum. The iron core drives deep into saturation, dropping coil permeability to that of air and drawing huge inrush current spikes of 8x to 14x rated FLA for several cycles.

What is the standard 0.1-second inrush point on a Time-Current Curve (TCC)?

IEEE Standard C57.109 specifies that for coordination studies, transformer magnetizing inrush is plotted as a single point at 0.10 seconds (6 cycles on 60 Hz) equal to 12 times rated primary full-load current (for medium transformers). The fuse or breaker minimum-melt curve must pass to the right of this point.

Why does NEC 450.3 allow higher primary fuse ratings when secondary protection is installed?

When an approved secondary overcurrent device is set to trip at 125% of secondary FLA, it reliably handles all downstream operational overloads. This frees the primary fuse to function strictly as short-circuit and inrush protection, permitting it to be sized up to 250% or 300% of primary FLA without nuisance blowing.