Guide to ERISA 401(k) Non-Discrimination Testing
To qualify for preferential federal tax treatment, employer-sponsored 401(k) plans must prove they are accessible to, and benefit, all ranks of workers. The Internal Revenue Service (IRS) and the Department of Labor (DOL) enforce strict guidelines under the Employee Retirement Income Security Act (ERISA) to ensure that plan architectures do not disproportionately enrich highly compensated employees or business owners.
HCE (Highly Compensated Employee) vs. NHCE (Non-Highly Compensated Employee)
For testing purposes, workers are divided into two distinct groups. An employee is classified as a Highly Compensated Employee (HCE) if they satisfy either of the following criteria:
- Ownership Test: They owned more than 5% of the sponsoring company at any time during the current plan year or the preceding year, regardless of their compensation.
- Compensation Test: Their compensation from the employer in the preceding year exceeded the statutory IRS threshold (for 2026/2025 plan years, this is based on the prior year's limits, set at $155,000). Sponsoring employers can elect to apply the "Top-Paid Group Election" which limits HCE classification to only the top 20% of workers by salary.
Any eligible employee who does not meet the HCE requirements is classified as a Non-Highly Compensated Employee (NHCE).
The Statutory ADP and ACP Testing Rules
Plans must perform two primary mathematical checks annually:
- Actual Deferral Percentage (ADP) Test: Compares the average pre-tax and Roth elective deferral rates (expressed as a percentage of IRS capped compensation) of HCEs against NHCEs.
- Actual Contribution Percentage (ACP) Test: Compares the average employer matching contributions and employee voluntary after-tax contributions of HCEs against NHCEs.
To pass, the HCE group's average rate cannot exceed the permitted limit, calculated relative to the NHCE group's average rate via the Basic Test (1.25 Rule) or the Alternative Test (2x / 2% Spread Rule):
| NHCE Group Average Rate | Maximum Allowed HCE Group Average Rate |
|---|---|
| 2% or less | 2 times the NHCE Rate (e.g., if NHCE is 1.5%, HCE can be up to 3.0%) |
| Between 2% and 8% | NHCE Rate plus 2% (e.g., if NHCE is 4.0%, HCE can be up to 6.0%) |
| Greater than 8% | 1.25 times the NHCE Rate (e.g., if NHCE is 10.0%, HCE can be up to 12.5%) |
The Safe Harbor Exemption Option
Because non-discrimination testing can be mathematically volatile and administratively burdensome, many employers adopt a Safe Harbor 401(k) Plan Design. Safe harbor plans are legally exempt from the annual ADP and ACP tests, meaning business owners and highly compensated executives can maximize their personal elective deferrals up to the full IRS individual limit without worrying about general employee participation.
To qualify for Safe Harbor exemption, the employer must commit to making fully vested (100% immediate) contributions to all participating employees, using one of two standard configurations:
- Basic Matching: Sponsoring companies match 100% of employee elective deferrals on the first 3% of eligible compensation, plus a 50% match on the next 2% of compensation (total potential match of 4% of compensation).
- Non-Elective Safe Harbor: Sponsoring companies contribute a flat, mandatory 3% (or more) of compensation to all eligible employees' accounts, regardless of whether those employees personally elect to contribute.
Top-Heavy Status and SECURE 2.0 Compliance Auditing
A plan is classified as Top-Heavy if the cumulative account balances of Key Employees (officers and significant shareholders) exceed 60% of the total assets in the plan. When top-heavy, the employer is legally mandated to make a minimum contribution (typically 3% of compensation) to all Non-Key employees.
Under the SECURE Act 2.0, new plan setups are required to employ automatic enrollment and escalation features, which structurally boosts lower-paid worker (NHCE) participation. Additionally, participant audits for Form 5500 filings have been streamlined, focusing solely on accounts with actual cash balances at the beginning of the plan year rather than all eligible participants, shielding smaller growing businesses from expensive external CPA auditing requirements.