Mega back door with multiple 401k

[Mike S.]

Regarding the non deductible amount that you can contribute to a 401k, is the amount limited by your earned income in the sponsoring business or the total of your earned income? Lets say that you earn 100k in business A and 10k in business B. Business A 401k does not allow non deductible contribution while business B 401k allow supplemental non deductible contributions. The total of your contributions can not exceed $19,500 pre tax or ROTH, and the total of your non deductible contribution can not exceed $37,000. And the total of both can not be above your earned income. I understand that the pre-tax or ROTH contribution in each 401k can not be higher than the earned income in each business. So the pre-tax or ROTH contribution in 401k B could not be higher than $10,000 in that example. However what is the requirement for supplemental non-deductible contributions? Does the earned income limit is also independently assessed per sponsoring business? Could one contribute the $37,000 in 401k B?