Nelson, who also won the event last Saturday at Carleton, ran a 48.82 and crossed the line just .06 before the second place finisher.
Tyson Molitor (So., Belgrade, Minn.) finished third in the same event in 49.12.
In the 400-meter hurdles, Brant Luehman (Sr., Amery, Wis.) finished fifth overall but was the third-highest finishing Division III competitor with a time of 56.06. He shaved nearly two seconds off of his seed time in the finals.
In the throws, first-year Brennan McKeag (Lakeville, Minn.) continued to impress in the shot put, finishing fourth behind three University of Minnesota competitors with a throw of 14.27 meters (46-10).
Josh Blaschko (So., Lakeville, Minn.) was sixth overall and the third-highest Division III finisher in the shot put at 13.94 meters (45-9).
Also in the throws, Andy Norman (So., Winona, Minn.) and Aaron Wilkerson (Jr., Roseville, Minn.) finished fifth and sixth, respectively, in the hammer throw. Norman threw 48.98 meters (160-8) and Wilkerson threw 48.11 (157-10). There was only one Division III competitor ahead of them in the final standings.