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If the employee chooses to max out his Roth 401(k) contributions he would have the following:

19,500 pre-tax Roth 401(k) contribution
47,500 after tax 401(k) contributions
note: I updated this for the 2021 limit of $58k

A 47,500 In Plan Roth 401(k) conversion of after tax 401(k) contributions

Net Roth 401(k) Contributions for the year: 58K

You seem to be confused. The $19,500 salary deferral is either pre-tax (Traditional) or Roth. There is no such thing as a "pre-tax Roth 401(k) contribution". If it's a Roth contribution, you pay taxes on it. If it's a pre-tax contribution, then it doesn't count toward the 'net Roth' contribution, since it's in the pre-tax account, not the Roth account.

Additionally, you seem not be accounting for company matching contributions. Personally, I've never seen a company that allowed for after-tax 401(k) plan contributions that didn't also have at least the minimum match to make the plan a safe harbor. (And because of the highly compensated employee rules, it's unlikely that an employee would be able to make that level of contribution without the plan being a safe harbor plan and, thus, having a match.) The employer match is always pre-tax and counts toward the $58k maximum. So it's very unlikely that even if the employee choose a Roth salary deferral instead of a pre-tax deferral, that they will be able to make a $58k Roth contribution. If they only have a 50% match on their salary deferral, that's $9,750 So, their maximum after tax contribution would be $58,000 minus $19,500 minus $9,750, or $28,750 - not $47,500 Even after immediately converting all after-tax contributions, that would only allow for $28,750 + $19,500, or $48,250 as contributions to the Roth account.

You also seem to be discounting the fact that the employee needs to make enough money to pay the taxes (SS, Medicare and income) on the contribution, plus be able to make enough money for living expenses to actually support themself, plus any dependents (spouse, kids, parents, etc.) that they happen to have. So, I don't think your scenario is realistic. Therefore, I will answer the next question for the general audience, too.

So, for tax purposes there is no difference between contributions under the salary deferral limit and the after tax contriebutions....correct?

Sorry, not correct. For tax purposes there actually is a very large difference. Salary deferral contributions have the choice of being either pre-tax (Traditional) or after-tax (Roth). Pre-tax contributions can be used to decrease income for reasons like - qualifying to make regular Roth IRA contributions, qualifying for ACA subsidies, qualifying for tax credits, keeping investment income from being subject to the Net Investment Income surtax, etc. So deferral contributions offer significantly more flexibility than after-tax contributions.

Depending on the plan's rules, after-tax contributions may or may not be immediately convertible to a Roth IRA or a Roth sub-account within the plan. If they aren't immediately convertible to a Roth, then a significant part of the benefit - generating tax-free growth - isn't available. That growth will, instead, be subject to ordinary income taxes. That generally makes investing in taxable accounts more attractive than making after tax contributions that can't be immediately converted to a Roth, since, at least under current tax law, LTCG and qualified dividends are taxed at lower rates than ordinary income.

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