((a)) ** Civil penalty** If a claim for refund or credit with respect to income or employment tax is made for an excessive amount, unless it is shown that the claim for such excessive amount is due to reasonable cause, the person making such claim shall be liable for a penalty in an amount equal to 20 percent of the excessive amount.

((b)) ** Excessive amount** For purposes of this section, the term “excessive amount” means in the case of any person the amount by which the amount of the claim for refund or credit for any taxable year exceeds the amount of such claim allowable under this title for such taxable year.

((c)) ** Noneconomic substance transactions treated as lacking reasonable cause** For purposes of this section, any excessive amount which is attributable to any transaction described in section 6662(b)(6) shall not be treated as due to reasonable cause.

((d)) ** Coordination with other penalties** This section shall not apply to any portion of the excessive amount of a claim for refund or credit which is subject to a penalty imposed under part II of subchapter A of chapter 68.