Partial Plan Termination #'s to use

[Guest Yvonne Bonitatibus]

Our company closed a branch engineering office due to a project that was not getting off the ground. The closure did not have anything to do with downsizing, only canceling a project. With the termination of the project, we also terminated 56 employees. We had a total of 279 employees eligible to participate on the date of the termination. During a recent IRS audit on the 401(k) plan, we were advised that this could constitute a partial plan termination. What numbers should we use to determine if we are over 20%? What other facts should I look at to help make our case? I've been searching on the internet, but am having a difficult time finding any information on what we should look at.