Cal. Pub. Cont. Code § 4109

Subletting or subcontracting of any portion of the work in excess of one-half of 1 percent of the prime contractor’s total bid as to which no subcontractor was designated in the original bid shall only be permitted in cases of public emergency or necessity, and then only after a finding reduced to writing as a public record of the awarding authority setting forth the facts constituting the emergency or necessity.