An employer announces 20 layoffs in January, 25 more in February, and 30 in March — all at the same plant. No single round looks big enough to trigger the federal WARN Act. Added together, though, the total is 75 workers in under 90 days, and that can change everything.

That is the point of the WARN Act's 90-day aggregation rule: an employer cannot avoid giving 60 days' notice simply by splitting one layoff into smaller pieces.

The thresholds it protects

The federal WARN Act generally requires 60 days' written notice before two kinds of events at a covered employer — one with 100 or more full-time employees:

A plant closing : the permanent or temporary shutdown of a single site of employment that causes employment loss for 50 or more full-time employees during any 30-day period.

A mass layoff : an employment loss during any 30-day period for either 500 or more full-time employees, or 50 to 499 full-time employees who make up at least 33 percent of the full-time workforce at that site.

Those 30-day…