the document should contain language such as
(b) Refund of Excess Elective Deferrals. In the event that Elective Deferrals under this Plan when added to a
Participant's other elective deferrals under any other plan or arrangement (whether or not maintained by the Employer) exceed the limit described in the preceding Subsection, the Plan Administrator shall distribute, by April 15 of the following calendar year, the excess amount of Elective Deferrals plus income thereon.
If it is an HCE the excess is included in ADP testing, if NHCE it is not included.
if there is a related match on the excess it would be forfeited.
gains adjustment, etc are treated the same as you would for an ADP failures.
from the Coverage/nondiscrimination answer book 12:14
Excess deferrals should be distributed with earnings to the participant no later than the April 15 following the calendar year in which they were made. The excess deferral is taxed in the year it was deferred, and the earnings are taxed in the year of distribution. Therefore, two 1099-R forms are needed: one reports the excess deferral with a reporting code “P” indicating the distribution is taxable in the prior year; the other reports the earnings (losses), with a reporting code “8” indicating the distribution is taxable in the current year.
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The IRS looks at the W-2 so it knows the excess deferral exists so, famous last words, when looking at the W-2 it will adjust things even if the individual doesn't indicate the excess. What is 'confusing' to the individual is the 1099 won't come until the following year even though they need to indicate the amount on this years form, hence the code P