Transformer Inrush Coordination Point
Why a code-legal primary device still trips on energization: a device meeting the NEC 450.3 percentage limits can still nuisance-trip on the magnetizing inrush. Energized at an unfavorable point on the wave, the core saturates and draws 8-12x FLA (up to ~25x in the first sub-cycle), decaying over a few cycles. FLA = kVA x 1000 / (sqrt(3) x V); inrush point = multiple x FLA at the stated time. A 75 kVA, 480 V transformer draws 90.2 A FLA, so the 12x inrush point is 1,083 A at 0.1 s (and 2,255 A at 0.01 s) -- the primary device's curve must sit RIGHT of that point and LEFT of the damage curve, or it trips every energization. A design aid; the manufacturer's inrush data and a coordination study govern.
Formula and source
FLA = kVA x 1000 / (sqrt(3) x V) three-phase (kVA x 1000 / V single-phase); inrush_point = inrush_multiple x FLA at the stated duration.
Transformer energization-inrush coordination point (IEEE C57.109 through-fault / energization; NEC 450.3 context), first-principles, by name; the manufacturer's inrush data and a coordination study govern.
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