27 C.F.R. § 70.98
(a) General rule. If any person is required by the provisions of 26 U.S.C. enforced and administered by the Bureau or regulations prescribed thereunder to deposit any tax in a government depository that is authorized under 26 U.S.C. 6302(c) to receive the deposit, and fails to deposit the tax within the time prescribed therefor, a penalty shall be imposed on such person unless the failure is shown to be due to reasonable cause and not due to willful neglect. The penalty shall be:
(3) For deposits of taxes required to be made after December 31, 1989.
(iv) 15 percent of the amount of the underpayment if the tax is not deposited before the earlier of:
(B) The day on which notice and demand for immediate payment is given under 26 U.S.C. 6862 or the last sentence of 26 U.S.C. 6331(a).
For purposes of this section, the term “underpayment” means the amount of tax required to be deposited less the amount, if any, that was deposited on or before the date prescribed therefor. Section 7502(e) of the Internal Revenue Code applies in determining the date a deposit is made.
(b) Assertion of reasonable cause. To show that the underpayment was due to reasonable cause and not due to willful neglect, a taxpayer must make an affirmative showing of all facts alleged as a reasonable cause in a written statement containing a declaration that it is made under the penalties of perjury. The statement must be filed with the appropriate TTB officer. If the appropriate TTB officer determines that the underpayment was due to reasonable cause and not due to willful neglect, the penalty will not be imposed.
(26 U.S.C. 6656)
[T.D. ATF-251, 52 FR 19314, May 22, 1987, as amended by T.D. ATF-301, 55 FR 47615, Nov. 14, 1990; T.D. ATF-353, 59 FR 2523, Jan. 18, 1994]