Financial whiz kid Cortes W. Randell yesterday was sentenced to seven years in prison for his fraud conviction and banned from interstate communication with anyone but his family or his lawyers.

Former Labor Department official John B. Mumford, Randell's business partner, was sentenced to six months in jail for his part in the fraud that led to the 1976 collapse of a Virginia real estate firm.

In addition, U.S. District Court Judge Albert V. Bryan Jr., sitting in Alexandria, ordered Randell to sever all business connections.

Bryan also placed both men on five years probation after their prison terms are completed. Bryan sentenced Mumford to three years in prison but suspended all but six months.

Lawyers for both men said they will appeal their convictions. Bryan allowed them to remain on personal recognizance bond pending their appeal.

An Alexandria federal jury deliberated two days last month before convicting the men on all 17 counts of an indictment charging them with mail and stock fraud, interstate transportation of stolen funds and submitting a false loan application to the Veterans Administration.

Although both men faced a maximum penalty of 105 years in prison and an $82,000 fine, U.S. Attorney William B. Cummings said after the sentencing that Randell received "a relatively strong sentence" particularly for what is considered white-collar crimes.

Following the sentencing last month Assistant U.S. Attorney Theodore S. Greenberg said that federal prosecutors are investigating Randell on "another criminal matter." Published reports have said that prosecutors are investigating Randell in connection with another of his ventures, the Washington Trade Exchange.

The exchange is an elaborate barter enterprise that allows businesses in the Washington area to trade their merchandise and services with one an other through the use of special credits.

Randell resigned this month from the Trade Exchange, according to his attorney.

Randell's attorney said Randell would begin working with a community Bible study group. Mumford testified during the trial that he and Randell met in a Bible study class.

The indictment alleged that the two men diverted funds of the National Commercial Credit Corp. to themselves and intentionally concealed from investors and financial institutions in 1974 and 1975 the unstable financial condition of NCCC which invested in residential properties in the Washington area. As a result, about 125 investors lost more than $1.5 million when the firm filed for bankruptcy in 1976, prosecutors alleged.

NCCC, formerly called Potomac Valley Homes Inc., would buy homes with assumable first mortgages, repair the homes and resell them to persons who could not afford the normal down payment.

NCCC would realize its profit by issuing second trust notes which it took back from the home buyers. These notes then would be sold by NCCC at a discount for cash or pledged on lines of credit with banks or private lenders.