ANSWER: The answer is provided in 15 USCS § 1673.  Although most creditors can only take 25% of your “disposable earnings,” child support is divided into three categories:

 1.   If you are supporting others, no more than 50% of your income.

2.   If you are not supporting others, no more than 60% of your “disposable income,” unless arrears are more than 12 weeks, which it can go up to 65% of your “disposable  income.”

“Disposable income” certainly includes income only after taxes and social security are removed.

The law is unclear about child support.

15 USCS § 1673 (2005)

§ 1673.  Restriction on garnishment

(a) Maximum allowable garnishment. Except as provided in subsection (b) and in section 305 [15 USCS § 1675], the maximum part of the aggregate disposable earnings of an individual for any workweek which is subject to garnishment may not exceed
   (1) 25 per centum of his disposable earnings for that week, or
   (2) the amount by which his disposable earnings for that week exceed thirty times the Federal minimum hourly wage prescribed by section 6(a)(1) of the Fair Labor Standards Act of 1938 [29 USCS § 206(a)(1)] in effect at the time the earnings are payable,
 
whichever is less. In the case of earnings for any pay period other than a week, the Secretary of Labor shall by regulation prescribe a multiple of the Federal minimum hourly wage equivalent in effect to that set forth in paragraph (2).
 
(b) Exceptions.
   (1) The restrictions of subsection (a) do not apply in the case of--
      (A) any order for the support of any person issued by a court of competent jurisdiction or in accordance with an administrative procedure, which is established by State law, which affords substantial due process, and which is subject to judicial review.
      (B) any order of any court of the United States having jurisdiction over cases under chapter 13 of title 11 of the United States Code [11 USCS §§ 1301 et seq.]
      (C) any debt due for any State or Federal tax.
   (2) The maximum part of the aggregate disposable earnings of an individual for any workweek which is subject to garnishment to enforce any order for the support of any person shall not exceed--
      (A) where such individual is supporting his spouse or dependent child (other than a spouse or child with respect to whose support such order is used), 50 per centum of such individual's disposable earnings for that week; and
      (B) where such individual is not supporting such a spouse or dependent child described in clause (A), 60 per centum of such individual's disposable earnings for that week;
      except that, with respect to the disposable earnings of any individual for any workweek, the 50 per centum specified in clause (A) shall be deemed to be 55 per centum and the 60 per centum specified in clause (B) shall be deemed to be 65 per centum, if and to the extent that such earnings are subject to garnishment to enforce a support order with respect to a period which is prior to the twelve-week period which ends with the beginning of such workweek.
 
(c) Execution or enforcement of garnishment order or process prohibited. No court of the United States or any State, and no State (or officer or agency thereof), may make, execute, or enforce any order or process in violation of this section.

15 USCS § 1673

The term disposable income is defined in 15 USCS § 1672:

15 USCS § 1672 (2005)

§ 1672.  Definitions

For the purposes of this title [15 USCS §§ 1671 et seq.]:
   (a) The term "earnings" means compensation paid or payable for personal services, whether denominated as wages, salary, commission, bonus, or otherwise, and includes periodic payments pursuant to a pension or retirement program.
   (b) The term "disposable earnings" means that part of the earnings of any individual remaining after the deduction from those earnings of any amounts required by law to be withheld.

   (c) The term "garnishment" means any legal or equitable procedure through which the earnings of any individual are required to be withheld for payment of any debt.

15 USCS § 1672

One court has interpreted “disposable income” as meaning income after taxes, less one’s support obligation.

“As we construe title III of the Consumer Credit Protection Act (US Code, tit 15, §§ 1671-1677), the maximum amount which may be garnished from the earnings of an individual for any workweek is 25% of his "disposable earnings" (i.e., after-tax earnings [see US Code, tit 15, § 1672, subd [b]), except in the case of an order for support, in which event the maximum is 50% of disposable earnings, and up to 60% where the individual receiving support is a spouse or dependent child (US Code, tit 15, § 1673).”  General Motors Acceptance Corp. v. Metropolitan Opera Asso., 98 Misc. 2d 307, 308 (N.Y. Misc. 1978)

The court also held:

The above view of the Federal legislation is espoused by the Secretary of Labor, who is charged with the enforcement of its provisions (US Code, tit 15, § 1676), and whose interpretation, if not irrational or unreasonable, is to be accorded great weight ( Matter of Howard v Wyman, 28 NY2d 434, 438; Brennan v Kroger Co., 513 F2d 961). Petitioner's contention that payroll deductions required under a support order should not be included when computing the percentage reduction of a  [*309]  debtor's disposable earnings is not an accurate reading of the language of the statute. HN4 The term "garnishment" is not restricted but includes any legal or equitable procedure through which the earnings of an individual are required to be withheld for payment of any debt -- thus encompassing orders of support as well as ordinary creditor-debtor garnishments (see US Code, tit 15, § 1672, subd [c]). The cases relied upon by [***5]  petitioner (e.g., Costa v Chevrolet-Tonawanda Div. of Gen. Motors Corp., 53 Misc 2d 252, affd 24 AD2d 732) antedate the passage of title III of the act, and are not controlling. This court is bound to give precedence to the provisions of the Federal statute (US Code, tit 15, § 1673, subd [c]), and we decide the case in a manner which will best further the manifest congressional purpose of maximizing a debtor's share of his own earnings.

General Motors Acceptance Corp. v. Metropolitan Opera Asso., 98 Misc. 2d 307, 308-309 (N.Y. Misc. 1978)

Other courts appear to disagree.

Alimony and child support payments, even if ordered by court, are not "amounts required by law to be withheld" and therefore must be included in debtor's "disposable earnings" within meaning of § 302(b) of the Consumer Credit Protection Act (15 USCS § 1672(b)). First Nat'l Bank v Hasty (1976, ED Mich) 415 F Supp 170, affd without op (1977, CA6 Mich) 573 F2d 1310 and affd without op (1977, CA6 Mich) 573 F2d 1310.

Biweekly wage assignment of $130 for child support previously ordered in dissolution of marriage action is part of wage earner's disposable earnings within meaning of 15 USCS § 1672(b), since amounts withheld pursuant to court order are not "required by law to be withheld," which means deductions for federal, state and local withholding taxes, and social security taxes. Koethe v Johnson (1982, Iowa) 328 NW2d 293.