Hello, Where do I enter SEP IRA if I already have a retirement plan for another employer?

[BMcCalpin]

You enter the SEP contributions in the Business tab of TurboTax under "Less Common Business Situations" at the line called "Self-employed retirement". Note that this description is from TurboTax Home & Business (desktop); TurboTax Self-Employed may be laid out a little differently.

If you are self-employed and have an SEP-IRA, then the retirement plan at your employer does not affect your SEP-IRA contribution.

If you are an employee and you have both a 401(k) and SEP IRA through your employer, your deferral contribution to the 401(k) cannot exceed $18,000 ($24,000 if over 50), and the total contributions to both accounts cannot exceed $53,000 ($59,000 including 401(k) catch-up contributions).

[dmertz]

$18,000 was the 2017 401(k) regular elective deferral limit.  The corresponding limit for 2018 is $18,500.  But as mentioned, this has nothing to do with the limit on SEP contributions since SEP contributions are employer contributions, not employee contributions.  The SEP contribution limit would only be affected if the two businesses were a controlled or affiliated group that is required to be treated as a single employer for the purpose of a retirement plan, which doesn't seem to be the case here.