Some families with costly job-based health coverage may be ineligible for federal subsidies to help them buy less expensive coverage through new online insurance markets, under final rules released Wednesday by the IRS.

The two rules, published by the Treasury Department here and here, uphold earlier proposals outlining what is considered affordable, employer-sponsored coverage.

Under the federal health law, low and moderate-income workers with job-based coverage that is deemed unaffordable can opt out of that and turn to new marketplaces, called exchanges, to buy coverage with government subsidies.

But the rule defines the standard for affordability more narrowly than most consumer groups had hoped — as an amount less than 9.5 percent of household income to cover just that employee’s share of premium costs, not on what he or she must pay to cover their entire family, which is generally more expensive.

Consumer groups had hoped to sway the IRS to base the affordability threshold on the cost of a family plan, saying the rules could prevent some children and spouses from getting coverage. A July report from the U.S. Government Accountability Office estimated that a small percentage of uninsured children — 6.6 percent of the total, or at least 460,000 — may be shut out because of how the government proposed to define affordable coverage.

“It doesn’t make sense to test the affordability of children’s coverage by looking at the cost of covering one person, the employee,” said Joe Touschner, senior health policy analyst at Georgetown University’s Center for Children and Families. More than 100 groups, including the March of Dimes, the American Academy of Family Physicians and the Children’s Defense Fund, had signed a letter to Treasury making that argument.

Touschner acknowledged that if the IRS had used the cost of family coverage as the threshold it would have boosted the cost of the law because more people would become eligible for subsidies.

Supporters of the rule, among them employer groups and insurance brokers, say it closely follows the wording in the law and will be easier to administer.

The final rules also outline exemptions to the federal law’s rule requiring most Americans to carry health insurance coverage, including not being able to afford coverage.

The exchanges and subsidies, as well as the requirement that most Americans carry insurance, go into effect on Jan. 1, 2014.