Excavation Protective System and Egress Triggers (OSHA Subpart P)
trench-slope and excavation-bench-plan both assume you already know a protective system is required. This answers the prior question -- and the two triggers sit at DIFFERENT depths, in the order people get backwards. Egress (1926.651(c)(2)) bites at 4 FT or more, within 25 ft of LATERAL travel. Cave-in protection (1926.652(a)(1)) bites at 5 FT, unless the dig is entirely in stable rock. So a 4.5 ft trench needs a ladder and may need no shoring at all. The other trap: that under-5-ft exception has TWO conditions, not one -- under 5 ft AND examined by a competent person with no indication of a cave-in. Depth alone exempts nothing. Also counts egress points over a run, since 25 ft each way means one ladder covers 50 ft. OSHA is public domain, so the text is quoted.
Formula and source
a means of egress is required where depth >= 4 ft, within 25 ft of lateral travel, so with each point serving 25 ft in each direction the count is max(1, ceil(trench length / 50)); cave-in protection is required unless the excavation is entirely in stable rock OR depth < 5 ft AND a competent person has examined the ground with no indication of a potential cave-in.
OSHA 29 CFR 1926.651(c)(2) and 1926.652(a)(1), Subpart P - Excavations. US federal regulations in the public domain, quoted directly.
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