Federal law limits wage garnishment to 25% of the weekly disposable earnings OR to the amount that is over 30 times the Federal Minimum hourly wage. It is based on the lower of the two numbers.  You can find the full text of the act on the FDIC website at 15 USCS Section 1673.

As of August 3, 2009, the first $217.50 is exempt.  If the federal minimum hourly wage changes, then this number will change too.

There are exceptions.  The limit does not apply to:

1. court or administrative ordered support;
2. Chapter 13 Bankruptcy wage deduction payments;
3. federal or state tax collection.

The exception for child support allows wage garnishment ranging from  50% to 65% of the disposable earnings, depending on whether the wage earner is supporting a non-garnishing spouse or dependant child, and whether the support order covers a time period that predates twelve weeks before the beginning of the work week pay period.

Some states such as Texas do not allow debt collectors to garnish your paycheck. They can however attempt to garnish your bank account.