Tampa Electric Co. v. Nashville Coal Co., 365 U.S. 320 (1961), the Tampa Electric case, was a 1961 decision of the Supreme Court that, together with United States v. Philadelphia National Bank, clarified the legal test for determining whether requirements contracts "may substantially lessen competition" or "tend to create a monopoly" for purposes of section 3 of the Clayton Antitrust Act.

At that point, Nashville Coal advised Tampa Electric that the contract was illegal under the antitrust laws and so would not be performed.

The Court, in a 7-2 opinion written by Justice Tom C. Clark, reversed and turned to the Standard Stations case[9] for guidance: [Standard Stations] held that such contracts are proscribed by § 3 if their practical effect is to prevent lessees or purchasers from using or dealing in the goods, etc., of a competitor or competitors of the lessor or seller, and thereby "competition has been foreclosed in a substantial share of the line of commerce affected."

This combination dictated a finding that "Standard's use of the contracts [created] just such a potential clog on competition as it was the purpose of § 3 to remove" where, as there, the affected proportion of retail sales was substantial.

[I]t clearly appears that the proportionate volume of the total relevant coal product as to which the challenged contract preempted competition, less than 1%, is, conservatively speaking, quite insubstantial.

A more accurate figure, even assuming preemption to the extent of the maximum anticipated total requirements, 2,250,000 tons a year, would be .77%.

sees an annual trade in excess of 250,000,000 tons of coal and over a billion dollars—multiplied by 20 years, it runs into astronomical figures.

nor myriad outlets with substantial sales volume, coupled with an industry-wide practice of relying upon exclusive contracts, as in [Standard Stations].

[Citing Standard Stations][14]The Court rejected the view of Standard Stations that 6.7 percent of the market was enough to make a contract illegal and concluded: In weighing the various factors, we have decided that, in the competitive bituminous coal marketing area involved here, the contract sued upon does not tend to foreclose a substantial volume of competition.

Later court decisions have relied on Tampa Electric to provide guidance on how to determine the relevant market in which competitive impact should be measured.