When do I need an oversize permit?
When any dimension of your load exceeds the federal default — 8'6" wide, 13'6" tall, 53' long (or 28' on each trailer of a double-trailer combination) — or when GVW exceeds 80,000 lbs (which separately triggers an overweight permit). The federal defaults come from 23 CFR §658; some states allow slightly higher dimensions on specific routes without a permit.
The oversize permit threshold is dimensional. Width above 8'6", height above 13'6", length above 53' single-trailer or 28'+28' on doubles, or any combination of those — each requires a per-trip oversize permit in every state of travel.
Some states extend specific dimensions on specific routes. Texas allows up to 14' wide on selected oversize-network routes; Wyoming and Montana allow longer doubles on the interstate system. Each exception is route-specific and codified in the state DOT's permit-network maps.
Overweight (GVW above 80,000 lbs) is a parallel permit — typically issued by the same state DOT process as the oversize but on a separate fee schedule based on weight and miles. A load that is both oversize and overweight needs both permits in each state.
For occasional movements, per-trip permits are the standard tool. Repeat carriers running consistent routes sometimes use annual permits where states offer them — Texas, Pennsylvania, Indiana, and Ohio have established annual permit programs for repeat oversize haulers.