California Code, Family Code - FAM § 4722

(a) Any person with a court order for child support, the payments on which are more than 30 days in arrears, may file and then serve a notice of delinquency, as described in this chapter.

(b) Except as provided in Section 4726, and subject to Section 4727, any amount of child support specified in a notice of delinquency that remains unpaid for more than 30 days after the notice of delinquency has been filed and served shall incur a penalty of 6 percent of the delinquent payment for each month that it remains unpaid, up to a maximum of 72 percent of the unpaid balance due.