For the first time ever, interest rates for jumbo mortgages have dropped below the average rates for conforming mortgages, lenders say. Jumbo mortgages are those that exceed the $417,000 limit to qualify for backing by mortgage giants Freddie Mac and Fannie Mae. The average interest rate for a 30-year fixed-rate conforming mortgage last week was 4.73 percent while the average for a jumbo 30-year fixed-rate mortgage was 4.71 percent.