61 Pa. Code § 4.8
If a taxpayer has received a notice of allowance of a credit, the taxpayer may apply in writing for a cash refund to the Department under section 1108(b)(4) of the FC (72 P. S. § 1108(b)(4)). A formal petition is not required, but the application shall be in writing, signed by the person entitled to the refund, or his agent. The application shall fully identify the taxpayer, the tax, and the amount claimed. If the Department fails to make a cash refund within 90 days after receipt of a proper application for a cash refund, interest will commence on the credit thereafter, and continue until a date within 30 days of the date of the Commonwealth's refund check.
61 Pa. Code § 4.8
This section cited in 61 Pa. Code § 35.2 (relating to interest, additions, penalties, crimes and offenses).