Electronic submission is routinely confused with posting, and the thresholds are routinely applied to the whole company instead of to each establishment.
Group one: 20–249, designated industries, 300A. If an establishment had 20 to 249 employees at any time during the previous calendar year and is classified in an industry listed in Appendix A to Subpart E, it must electronically submit information from the 300A (29 CFR 1904.41(a)(1)(i)). Appendix A is the list of higher-hazard industries — construction, manufacturing, warehousing, nursing care and many others.
Group two: 250 or more, any recordkeeping industry, 300A. If an establishment had 250 or more employees at any time during the previous year and Part 1904 requires it to keep records, it submits the 300A (29 CFR 1904.41(a)(1)(ii)).
Group three: 100 or more, Appendix B industries, everything. If an establishment had 100 or more employees and is in an industry listed in Appendix B, it submits information from the 300 Log and the 301 Incident Reports in addition to the 300A (29 CFR 1904.41(a)(2)). Every covered establishment also provides its Employer Identification Number ((a)(4)).
Everyone else. Only those three categories must routinely submit; others submit only if OSHA notifies them (29 CFR 1904.41(b)(1), (a)(3)). Part-time, seasonal and temporary workers all count toward the employee thresholds ((b)(2)). Submissions are due by March 2 of the year after the year the forms cover (29 CFR 1904.41(c)).
Posting the 300A on the wall by February 1 and submitting it to OSHA by March 2 are different duties with different triggers. Many small employers must do the first and are exempt from the second.