16.4 Why ADP and ACP Testing Exists & Which Plans Are Statutorily Exempt
Key Takeaways
- The ADP and ACP tests exist because a cash or deferred arrangement lets each employee choose their own contribution rate, so IRC §401(k)(3) and §401(m)(2) police the outcome of those choices rather than the plan's written formula.
- A safe harbor 401(k), a QACA, a SIMPLE 401(k) and a plan with no HCEs eligible to defer are all exempt from ADP testing; a plan is exempt from ACP testing only as to contributions that themselves satisfy an ACP safe harbor.
- Safe harbor status exempts a plan from ADP testing but not from coverage under IRC §410(b), from top-heavy testing where other employer money is present, or from the IRC §415 limits.
- A plan whose only HCE defers nothing still runs the test; the exemption applies when no HCE is eligible, not when no HCE defers.
The Problem Congress Was Solving
A traditional profit-sharing plan has one allocation formula written into the document, and IRC §401(a)(4) can be applied to that formula on paper. A cash or deferred arrangement is different: the document says only "you may defer up to the legal limit," and each employee chooses their own rate. Two employees under identical plan language can end up with wildly different benefits.
That creates an obvious risk. Owners and executives have the disposable income to defer the full §402(g) limit; rank-and-file employees living closer to their paychecks often defer little or nothing. A plan could be perfectly nondiscriminatory in form and thoroughly discriminatory in operation.
IRC §401(k)(3) (the ADP test) and IRC §401(m)(2) (the ACP test) answer that by testing outcomes. They compare what HCEs actually contributed, as a percentage of pay, against what NHCEs actually contributed. If the gap is too wide, the plan must fix it.
The Sponsor Conversation
When a sponsor asks why the plan cannot simply let the owner defer the maximum, the accurate explanation has four parts:
- "Your plan gets a tax subsidy." Deferrals are pre-tax and the earnings grow tax-free; Congress conditions that subsidy on the benefits being broadly shared.
- "Because each person picks their own rate, we test the results, not the formula."
- "The test measures the average rate of your highly compensated group against the average rate of everyone else."
- "If your employees do not participate, the owners are capped — which is why we care so much about NHCE participation, and why safe harbor designs exist."
That last sentence reframes an employee-communication problem as an owner-compensation problem, which is usually what moves a sponsor to fund a safe harbor or improve enrollment.
Which Plans Escape the ADP Test
| Plan or situation | Exempt from ADP? | Authority / condition |
|---|---|---|
| Traditional safe harbor 401(k) | Yes | IRC §401(k)(12) — 3% nonelective or qualifying match, notice, vesting |
| QACA safe harbor | Yes | IRC §401(k)(13) — automatic enrollment safe harbor |
| SIMPLE 401(k) | Yes | IRC §401(k)(11) — mandatory 3% nonelective or 100% match on 3% |
| Starter 401(k) (SECURE 2.0 §121) | Yes | Deferral-only plan, no employer contributions, capped deferrals |
| No HCE is eligible to make elective deferrals | Yes | With no HCE in the test, there is nothing to compare |
| 403(b) plan | Yes — no ADP test | 403(b) deferrals are subject to universal availability, not ADP |
| Plan where the only HCE chose not to defer | No | The HCE ADP is simply 0.00%; the test still runs |
| Governmental and church plans | Yes | Statutorily exempt from §401(k)(3) |
The distinction in the last two rows is heavily tested. The exemption turns on whether an HCE is eligible, not on whether an HCE deferred. A plan with one HCE who deferred nothing runs the test with an HCE ADP of 0.00% — and passes trivially, but it runs.
Which Contributions Escape the ACP Test
The ACP exemption is narrower and is best thought of contribution by contribution rather than plan by plan:
| Contribution | Exempt from ACP? |
|---|---|
| Traditional safe harbor matching contribution | Yes |
| QACA safe harbor matching contribution | Yes |
| Safe harbor nonelective contribution | Not applicable — nonelective contributions are not in the ACP test at all |
| Additional discretionary match within the ACP safe harbor limits (not on deferrals above 6% of compensation, not exceeding 4% of compensation, match rate not increasing) | Yes |
| Additional discretionary match outside those limits | No — ACP test required |
| Voluntary after-tax employee contributions | No — always ACP tested, even in a safe harbor plan |
| QMACs used in the ADP test | Excluded from ACP (cannot be double-counted) |
The trap that catches everyone: a sponsor adopts a safe harbor plan, assumes "we are exempt from testing," and then adds voluntary after-tax contributions so the owner can pursue a mega-backdoor Roth. After-tax contributions are never covered by an ACP safe harbor. The plan must run the ACP test on them, and because the owner is typically the only person making them, it fails almost every time.
What an Exemption Does Not Buy
Safe harbor status is narrow relief. These obligations survive it:
| Requirement | Still applies to a safe harbor plan? |
|---|---|
| IRC §410(b) minimum coverage | Yes — always |
| IRC §401(a)(4) general nondiscrimination on any non-safe-harbor allocation | Yes |
| IRC §415(c) annual additions limit | Yes |
| IRC §402(g) deferral limit | Yes |
| IRC §416 top-heavy | Exempt only if the plan holds nothing but deferrals and safe harbor contributions |
| ACP test on after-tax contributions | Yes |
| Form 5500, participant disclosures, fidelity bond | Yes |
Aggregation and Disaggregation Ground Rules
Before running either test, the administrator must fix the testing population:
- Mandatory disaggregation separates collectively bargained employees, employees of qualified separate lines of business, and the ESOP portion of a plan.
- Otherwise excludable employees may be disaggregated at the plan's election under IRC §410(b)(4)(B).
- Long-term part-time employees eligible solely under the SECURE 2.0 500-hour rule are excluded from ADP/ACP testing, coverage, and top-heavy — a statutory carve-out that materially helps plans with large part-time populations.
- If a plan is permissively aggregated for §410(b), it must be aggregated for ADP/ACP as well. Coverage and testing must travel together.
Common ASPPA QKA Exam Traps
- Trap 1 — "No HCE deferred, so no test." The exemption is for no HCE being eligible.
- Trap 2 — Believing a safe harbor plan runs no tests at all. Coverage, §415, §402(g), and often top-heavy still apply.
- Trap 3 — Assuming after-tax contributions ride the ACP safe harbor. They never do.
- Trap 4 — Treating the safe harbor nonelective as ACP-exempt. Nonelective contributions are not ACP-tested in the first place.
- Trap 5 — Aggregating for coverage but not for ADP/ACP. Permissive aggregation must be applied consistently across both.
- Trap 6 — Including LTPT-only participants in the ADP test. They are statutorily excluded.
A 401(k) plan has one HCE, the owner, who elected to defer nothing for the plan year. Is the plan exempt from the ADP test?
A traditional safe harbor 401(k) using the basic match adds voluntary after-tax employee contributions so the owner can pursue a mega-backdoor Roth strategy. What testing applies?
Which obligation does a traditional safe harbor 401(k) plan still have to satisfy?