Tank Vehicle Fundamentals and Endorsement Applicability
When the rules apply
A CMV is a tank vehicle when it hauls liquid or gaseous material in one or more tanks attached to the vehicle or chassis - permanently or temporarily - and each tank is rated at more than 119 gallons while the combined rated capacity is 1,000 gallons or more. Both numbers matter: a single 150-gallon tank is not a tank vehicle; two 600-gallon tanks (1,200 gallons total) are. Portable tanks, drop tanks and swap-body cargo tanks count. Empty storage containers that were not designed for transport do not.
The N endorsement is a knowledge-test endorsement required whenever you drive a tank vehicle, loaded or empty. Hazardous materials authority is separate: the H endorsement (shown as X when combined with N) covers placarded cargo, not tank construction.
Know the tank you drive
Cargo tanks are built and rated for specific products: DOT 406/407-style tanks for petroleum fuels, MC 331 for compressed gases, MC 338 for refrigerated liquids. The specification plate and markings state design pressure and authorized service - never load a product the tank is not rated for.
Trap to expect: questions that imply baffles, a full tank, or an empty tank change whether the endorsement applies. They do not; only the capacity definition and vehicle type do.
Rule of thumb: add up the rated (not actual) capacity of every attached tank. Two 500-gallon tanks = 1,000 gallons = tank vehicle. One 500-gallon tank alone = not a tank vehicle.