Legal Question in Business Law in California

There is a limit. It's set by the usury laws, which are in the California Constitution and the Civil Code. Look first at the Cal. Constitution, Article 15, Section 1. It says, in brief and leaving out possibly important details, that the maximum interest rate is 7% unless there is a written agreement, then it's 10% on a consumer loan other than for real property and improvements thereto, and otherwise the limit is the higher of 10% or 5% plus the S.F. Fed's rate on advances to member banks under Sec. 13 and 13a of the Federal Reserve Act.

Since the Fed rates are currently so low, as a practical matter the ceiling is 10% unless the lender comes under one of the numerous exceptions, e.g., a federal bank, a loan arranged by a licensed real-estate broker, etc.