Under the Affordable Care Act (ACA), employers with 50 or more full-time employees (30 or more hours) are assessed a penalty for each employee who qualifies for a federal tax credit upon health care enrollment in one of the state or federal exchanges, or whose health care costs represent more than 9.5% of the employee's W-2 annual gross income.
Employers are not required to provide part-time employees with health care benefits, consequently, the differential charge between part and full time employees can differ by any amount the employer desires.
The fact that your employer is charging temporary employees less than full-time is inexplicable to me. However, as long as the full-time employee charges do not exceed 9.5% of the employee's W-2 annual gross income, the charge is lawful, regardless of what is charged to part-time employees. See IRS Notice 2011-73
Note: The ACA is an incredibly complex and confusing law. No one, as of yet, can call themselves an "expert" on the ACA. There may be dozens of loopholes and odd outcomes, which make no sense. Added to this, is the fact that Congress and the President are unable or unwilling to make any attempt to repair the defects in the law (or, to repeal it entirely, if you prefer) -- consequently, whatever defects exist will likely remain unremedied into the foreseeable future.
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