Hazmat Cargo Tank Rules
Updated October 27, 2026
Cargo tanks carry most bulk hazmat on the highway, and the hazmat test has its own part on them. This page covers markings, outage, who must watch a tank during loading and unloading, and how a liquid load handles. The rules come from 49 CFR 172.326 to 172.336, 177.834, 177.837 and 177.840 and the CDL manual.
Cargo tanks and portable tanks
- A cargo tank is bulk packaging permanently attached to the vehicle (or loaded and unloaded without removing it). It stays on the vehicle while it is loaded and unloaded.
- A portable tank is bulk packaging that is not permanently attached. It is usually filled or emptied off the vehicle and then loaded for transport.
- The most common cargo tanks are the MC306 for liquids and the MC331 for gases.
Markings
| Marking | Rule |
|---|---|
| ID number | The material’s four-digit number from column 4 of the Hazardous Materials Table, in black characters 3.9 inches (100 mm) high, on orange panels, on the placards, or on a white square-on-point display when no placard is required |
| Where on the tank | Each side and each end if the tank holds 1,000 gallons or more; two opposing sides if it holds less |
| Portable tanks | Also the owner or lessee’s name and the shipping name on two opposing sides; if the ID numbers can’t be seen on the vehicle, they go on both sides and both ends of the vehicle too |
| Specification tanks | The dates and types of their periodic retests and inspections |
| Placards | Required for any amount of hazmat, including residue, until the tank is cleaned and purged |
Outage
Liquids expand as they warm. Outage is the empty space you leave in a tank so the load has room to expand. Different liquids expand by different amounts, so the required outage depends on the product. Know it before you load.
Attending a tank during loading and unloading
A cargo tank must be attended by a qualified person at all times while it is being loaded, and the carrier must make sure it is attended during unloading. The person responsible for loading is also responsible for making sure it is attended. The person watching must:
- be alert;
- be within 25 feet of the tank, with a clear view of the tank and delivery hose as far as practical;
- know the hazards of the material;
- know what to do in an emergency; and
- be authorized to move the tank and able to do so.
Propane and anhydrous ammonia deliveries have additional attendance rules.
Valves, grounding and engines
- Close all manholes and valves before moving a tank of hazmat, no matter how little is in it or how short the move.
- Flammable liquids: turn off the engine before loading or unloading unless it runs the pump. Ground the tank before opening the filling hole and keep it grounded until the hole is closed.
- Compressed gas: keep liquid discharge valves closed except while loading and unloading. If the engine runs the transfer pump, turn it off after the transfer and before unhooking the hose. Chock an uncoupled trailer.
Liquid surge and center of gravity
- Surge: in a partly filled tank, the liquid moves forward when you brake and can push the truck ahead. Brake early and smoothly.
- Baffles are bulkheads with holes that slow front-to-back surge, but they do not stop side-to-side surge, which can still cause a rollover.
- Smooth bore tanks have no baffles, so forward-and-back surge is strong.
- High center of gravity: tankers can roll over at the posted speed limit on curves and ramps. Take curves well below the posted speed.
Test yourself
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While a tank of gasoline is being loaded, you walk 40 feet away into the terminal office, where you cannot see the tank. Does this meet the attending rule?Answer: AThe person attending a cargo tank during loading must be alert, within 25 feet and have a clear view of the tank and hose. The 100-foot distance is for attending a parked vehicle, not a tank being loaded.
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Who is responsible for making sure a cargo tank is attended while it is being loaded?Answer: BUnder 49 CFR 177.834(i), the person responsible for loading the cargo tank is also responsible for making sure it is attended at all times during loading.
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Which cargo tank specification is most common for gases such as propane?Answer: CThe CDL manual names the MC331 as the most common cargo tank for gases and the MC306 as the most common for liquids such as gasoline.
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Where must a 9,000-gallon cargo tank display the identification number of its contents?Answer: DBulk packaging of 1,000 gallons or more, such as this cargo tank, must show the ID number on each side and each end, on the placards, orange panels or white square-on-point displays.
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Why must you leave outage when loading a tank with liquid?Answer: AOutage is the space left for the liquid to expand as it warms. Different liquids expand by different amounts, so they need different outage.
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What do baffles inside a liquid tank do?Answer: BBaffles slow the forward and backward movement of liquid. They do not control side-to-side surge, which can still contribute to a rollover.
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Why do loaded tank vehicles need extra care on curves and ramps?Answer: CMuch of the load sits high off the road, so tankers can roll over on curves and ramps at the posted speed limit. Take them well below the posted speed.
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On a compressed gas cargo tank, when should the liquid discharge valves be open?Answer: DKeep liquid discharge valves on a compressed gas tank closed except when loading and unloading.
See also: hazmat placards and hazmat vs. tanker endorsement.
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