Current-Limiting Let-Through and Downstream Withstand

ScopeA current-limiting fuse or breaker opens inside the first quarter cycle, before the fault current reaches its prospective peak, and what it lets through is characterized two ways on the manufacturer's curve: PEAK let-through current, which drives the magnetic forces that bend busbars and rip apart terminations, and let-through I-squared-t, which drives the heating that damages insulation. Downstream conductors and equipment have to survive both, so both are compared. The margin is usually enormous, and that is the point -- a current-limiting device converts an unprotectable conductor into a protected one, and the arithmetic shows by how much. A 4 AWG copper conductor at 41,740 circular mils, 75 C initial and a 250 C insulation damage limit, withstands about 10,080,000 A2s -- 31,760 A for 0.01 s -- against a device letting through 25,000 A2s, a margin of about 400 to one. The adiabatic withstand is a fixed I2t, so time is what decides: an unrestricted 25 kA fault cleared in 0.01 s is 6,250,000 A2s, inside the withstand, but the same fault left for three cycles (0.05 s) is 31,250,000 A2s, three times it, and the insulation is destroyed whether or not the breaker eventually opens. That is what a current-limiting device buys. What a let-through curve does NOT do is let a downstream device be applied above its own interrupting rating. That is a series rating, it exists only as a tested combination published by the manufacturer and marked on the equipment, and it cannot be calculated or inferred from a curve. This is the device-curve counterpart to the conductor short-circuit withstand calculation, which works from the raw available fault current instead. A screen; the manufacturer's published let-through curves, the equipment's marked ratings, and the engineer of record govern.

Formula or tablewithstand_a = conductor_cmil x sqrt(0.0297 x log10((damage_temp_c + 234) / (initial_temp_c + 234)) / duration_s) (ICEA P-32-382; 0.0297 inside the root since 2026-09-19); withstand_i2t = withstand_a^2 x duration_s; margin = withstand_i2t / device let-through I2t; and separately, device peak let-through against equipment peak withstand.

Source and editionConductor thermal withstand by the public-domain ICEA / Onderdonk adiabatic relation (copper constants 0.0297 and 234), compared against the current-limiting device's published let-through I-squared-t and peak current, by name. The let-through values are read off the manufacturer's curve and entered. A series rating is a TESTED, listed, marked combination per NEC 240.86 and cannot be calculated from a let-through curve. The manufacturer's curves, the equipment's marked ratings, and the engineer of record govern.

Free accessThe Onderdonk relation is public domain; the let-through values come from the device manufacturer's published curve and are entered by the user.

What governsEstimate. AHJ and licensed professional govern.

API field namesconductor_cmil, initial_temp_c, damage_temp_c, duration_s, let_through_i2t, let_through_peak_a, equipment_peak_withstand_a, withstand_a, withstand_i2t, margin

• Two characterizations peak let-through drives magnetic force, let-through I2t drives heating; both must be survivedcurrent-limiting device practice
• Onderdonk constants copper 0.0297 and 234 in the adiabatic relationICEA / Onderdonk
• Series rating cannot be calculated it is a tested, listed, marked combination per NEC 240.86, never inferred from a curveNEC 240.86