Most buried fiber sits between 18 and 48 inches down. Drops to a house run shallowest, street mains deeper, and highway or railroad crossings deepest of all — the exact number is set by state rules, the road owner, frost depth, and what else is already underground.
The typical numbers, and who sets them
Rules of thumb across the industry: residential drops 12–18 inches, distribution mains 24–36 inches, and highway right-of-way commonly 36–48 inches (TxDOT, for example, generally wants 42 inches in state ROW). Railroad crossings run deeper still — often 5 feet or more below the rails, in steel casing.
There is no single national law: each state DOT, municipality, and railroad sets standards for its own ground, and utility owners layer their own specs on top. It's one of the things our state-by-state guides document.
Why depth varies: frost, farming, and neighbors
In northern states, cable goes below the frost line so freeze-thaw cycles can't heave it — designs run 42–60 inches in places like Minnesota and Montana, while Gulf Coast states barely freeze and bury shallower. Farm country often goes deeper than suburbia because tillage reaches down. And in crowded urban corridors, the real driver is what's already there: fiber threads between gas, water, and power at whatever depth the conflicts allow.
Depth is not protection — locates are
A backhoe digs past four feet without noticing, which is why no burial depth substitutes for the 811 locate system: every state requires calling before digging, and marked lines carry a hand-dig tolerance zone. Buried fiber also gets tracer wire or marker tape, and long rural runs get warning posts — layers of protection that all assume the digger asked first.