Short answer: It varies by state and carrier. Many carriers require the employer to pay at least 50% of the employee-only premium, though some states set no statutory minimum and leave it to the carrier (applied uniformly). The requirement is waived during the year-end open-enrollment window.
There’s no single national rule. Most carriers require employers to contribute at least 50% of the employee-only premium, but the floor differs; Illinois sets 25%, while Texas sets no statutory percentage and lets the carrier decide (applied uniformly to all small employers). Whatever the requirement, it’s waived for groups that enroll during the annual special open-enrollment window (around Nov 15 to Dec 15) for a January 1 effective date.