Why are there no federal taxes being taken out of my wife's checks?

So my wife works retail and makes minimum wage(7.25/hr). She claims Married with 0 exemptions at her place of employment. It even states Married with 0 exemptions on her paycheck. She works about 65 hrs each paycheck(biweekly pay). Her checks are about $410 after everything gets taken out, but nothing is being taken out by Federal. Why? How much should they be taking out for a check thats about $475 before anything is taken out?