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We have examined the upper critical field of a large and representative setof present multi-filamentary Nb3Sn wires and one bulk sample over a temperaturerange from 1.4 K up to the zero field critical temperature. Since all presentwires use a solid-state diffusion reaction to form the A15 layers,inhomogeneities with respect to Sn content are inevitable, in contrast to somepreviously studied homogeneous samples. Our study emphasizes the effects thatthese inevitable inhomogeneities have on the field-temperature phase boundary.The property inhomogeneities are extracted from field-dependent resistivetransitions which we find broaden with increasing inhomogeneity. The upper90-99 % of the transitions clearly separates alloyed and binary wires but apure, Cu-free binary bulk sample also exhibits a zero temperature criticalfield that is comparable to the ternary wires. The highest mu0Hc2 detected inthe ternary wires are remarkably constant: The highest zero temperature uppercritical fields and zero field critical temperatures fall within 29.5 +/- 0.3 Tand 17.8 +/- 0.3 K respectively, independent of the wire layout. The completefield-temperature phase boundary can be described very well with the relativelysimple Maki-DeGennes model using a two parameter fit, independent ofcomposition, strain state, sample layout or applied critical state criterion.Comment: Accepted Journal of Applied Physics Few changes to shorten document,  replaced eq. 7-

The upper critical field of filamentary Nb3Sn conductors