SMD footprint and integration
The 04513.15MRSN is a Littelfuse NANO²® 451 series SMD fuse in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm (0.240" x 0.106" x 0.106"). The surface-mount footprint is a standard two-pad layout; the square end caps take reflow solder without a separate clip or holder.
Rated current, voltage, and the so-what on interrupting capacity
Rated 3.15 A at 125 VAC with a 50 A interrupting capacity at rated voltage. The 50 A breaking capacity is the maximum fault current the fuse can safely clear without rupturing the package — a figure that matters when the secondary-side fault current from a 150 VA transformer can exceed 10 A. The fast-blow response time means it opens quickly on overcurrents above roughly 200% of rating, which suits circuits where the inrush is low (logic supplies, sensor rails) and a slow-blow element would let a sustained fault damage the board trace.
Melting I²t and cold resistance — the pre-arcing energy budget
The melting I²t is 1.92 A²s, a figure the PCB designer uses to size the copper trace that feeds the fuse pad: a trace carrying 3.15 A for the fuse's pre-arcing time must not vaporise before the fuse element opens. The 0.0215 Ohm cold resistance is the starting value; resistance rises with temperature as the element heats toward the melting point, so the voltage drop under a sustained overload is higher than the 68 mV calc at rated current.