MC 306/DOT 406 Tank Requirements, Inspections & Penalties

A DOT 406 cargo tank must be built to the low-pressure specification in 49 CFR 178.346, carry a legible specification plate, keep its emergency discharge and rollover protection intact, and pass a recurring cycle of external, internal, leakage, thickness, and pressure tests under 49 CFR 180.407. The DOT 406 tank requirements below cover design, authorized cargo, testing intervals, who can sign off on the work, and what PHMSA charges when any of it slips.

What a DOT 406 Tank Is

DOT 406 is the current federal specification for non-pressure cargo tanks used to haul bulk liquids such as gasoline, diesel, jet fuel, heating oil, and ethanol on public highways. It replaced the older MC 306 specification on August 31, 1995. After that date, no new tank could be built or certified to MC 306.¹

Existing MC 306 tanks did not become illegal. One can stay in service as long as it passes every required inspection on schedule. Owners can also recertify a tank to its original MC 306 specification if the build records support it, or upgrade it to DOT 406. Either path requires verification by a Design Certifying Engineer or Registered Inspector.¹ The boundary that trips people up is repairs: any structural work on an MC 306 tank performed after August 31, 1995, must meet the DOT 406 specification, from welding procedures to material grades to accident-damage protection.²

Design and Construction Rules

DOT 406 tanks are built for liquids that do not need pressure to stay liquid at ambient temperatures. The maximum allowable working pressure is generally 3 psig, with certain designs permitted up to 4 psig. That low-pressure rating is what separates a DOT 406 from higher-pressure specifications like DOT 407 or DOT 412.

Approved Materials

Federal regulations allow four material categories for the shell and heads: mild steel, high-strength low-alloy steel, austenitic stainless steel, and aluminum. Aluminum dominates petroleum service because it keeps tare weight down and payload up. Minimum thickness is specified separately for heads (including bulkheads and baffles used as reinforcement) and for the shell, and separately for each material.³

Safety Features

Rollover protection is built into the top of every DOT 406 tank so that manholes, vents, and other fittings survive an overturn without rupturing. Emergency venting systems release pressure in a controlled way during a fire. All product piping and valves used for loading and unloading must have accident-damage protection to keep cargo contained during a crash.

Internal baffles or bulkheads divide many DOT 406 tanks into compartments, which allows delivery of different products on the same load and controls liquid surge during braking and turning. Not every DOT 406 is compartmented, but tanks hauling multiple products or large volumes almost always are.

Emergency Discharge Controls

Every loading and unloading outlet must have an internal self-closing stop valve, or an external valve mounted as close to the tank wall as possible. A self-closing system must shut all outlets within 30 seconds of activation.⁴

The remote-actuation control must be positioned more than 10 feet from the outlet whenever the vehicle is long enough to allow it. For flammable, poisonous, oxidizing, or pyrophoric liquids, the system must also include a thermal trigger that activates automatically at no more than 250 °F.⁴ If the external actuating mechanism gets sheared off in a collision, the outlet must remain closed and retain cargo. A malfunctioning emergency valve is a common reason tanks get placed out of service at roadside inspections.

Specification Plate

Every DOT 406 tank carries a corrosion-resistant metal specification plate permanently stamped in characters at least 3/16 inch high. The plate records the manufacturer, date of manufacture, maximum payload in pounds, maximum loading and unloading rates in gallons per minute, lining material if any, and heating system design pressure and temperature where applicable.⁵ If the plate is missing, illegible, or contradicts the tank’s actual configuration, the tank cannot pass inspection.

Authorized Cargo and Operating Limits

Common loads are petroleum products: gasoline, diesel, jet fuel, heating oil, ethanol, and similar flammable or combustible liquids. Part 173 of Title 49 ties cargo authorization to commodity characteristics like flash point and boiling point, so not every liquid fits in every tank configuration.⁶

The MAWP on the specification plate governs what the tank can hold under any conditions. If a liquid’s vapor pressure would exceed the tank’s rated pressure at the highest expected operating temperature, that liquid cannot be loaded. Maximum payload markings and outage (ullage) requirements leave room for thermal expansion.

Mandatory Inspections and Testing

Keeping a DOT 406 tank on the road requires the recurring cycle spelled out in 49 CFR 180.407. Intervals below are for a typical DOT 406 tank carrying non-corrosive liquids. Tanks hauling corrosive materials, insulated tanks, and vacuum-loaded tanks with full-opening rear heads face shorter intervals on several of these tests.⁷

External Visual Inspection — Every Year

The inspector examines the shell, piping, emergency devices, and structural attachments for corrosion, dents, leaks, and damage. On trailer-mounted tanks, the annual inspection includes a visual check of the upper coupler (fifth wheel) assembly without removing it.⁷

Leakage Test — Every Year

The tank is pressurized to at least 80 percent of the MAWP on the specification plate, and every seam, fitting, and closure is checked for leaks.⁷ This is the test most likely to catch deteriorating gaskets, worn valve seats, and hairline cracks before they fail on the road.

Internal Visual Inspection — Every Five Years

An inspector enters the tank, or uses remote viewing equipment, to look for internal corrosion, pitting, cracks, and structural defects invisible from outside. Tanks hauling corrosive materials must be inspected internally every year.⁷

Thickness Test — Every Five Years

Ultrasonic gauges measure remaining wall thickness at multiple points on the shell and heads and compare readings against the minimums on the specification plate. If any reading falls below the design minimum, the tank comes out of service until it is repaired or retired. Unlined tanks carrying corrosive materials face thickness testing every two years.⁷

Pressure Test — Every Five Years

The tank is filled with water or a liquid of similar viscosity at no more than 100 °F, then pressurized to the test pressure specified for the design. It must hold that pressure for at least 10 minutes while the inspector checks for leaks, bulging, or other defects. For DOT 406, the test pressure is 5 psig or 1.5 times the MAWP, whichever is greater. During the pressure test, the upper coupler assembly on trailer-mounted tanks must be removed so the inspector can examine the area underneath for hidden corrosion. Tanks carrying corrosive liquids require this removal every two years.⁷

Who Can Perform the Inspections

All inspections and tests under 49 CFR 180.407 must be performed or witnessed by a Registered Inspector, who must be registered with the Federal Motor Carrier Safety Administration, trained and experienced with DOT-specification cargo tanks, and competent with the required testing equipment.⁸

One narrow exception: an owner or carrier operating a cargo tank with a capacity under 3,500 gallons, used exclusively for flammable liquid petroleum fuels, may perform the annual external visual inspection and leakage test without meeting the full education and experience requirements of the Registered Inspector definition.⁸ Every five-year test still requires a credentialed inspector.

An employee who is not a Registered Inspector may perform the hydrostatic or pneumatic portion of the pressure retest, but external and internal visual inspections must be done by the Registered Inspector personally.⁸

Repairs and Modifications

Structural repairs on DOT specification cargo tanks are not open work. The facility must hold a valid National Board Certificate of Authorization for use of the National Board “R” stamp, follow the edition of the National Board Inspection Code in effect at the time of the repair, and be registered under 49 CFR Part 107, Subpart F.²

For non-ASME-stamped tanks, which includes most DOT 406 and MC 306 tanks, the work can alternatively be performed by a cargo tank manufacturer holding a valid ASME “U” stamp certificate, using its own quality control procedures.²

Modifications, stretching, and rebarrelling carry more. A Design Certifying Engineer must certify in writing that the modified tank meets the structural integrity and accident-damage protection requirements of the applicable specification, all new materials must meet the specification in effect at the time of the work, and a Registered Inspector issues a supplemental certificate of compliance when the job is complete.²

Records You Have to Keep

Both the tank owner and the motor carrier, when they are different, must keep a copy of every inspection and test report. The retention period runs until the next test of the same type is successfully completed, at which point the older report can be discarded.⁹ A motor carrier leasing a tank for fewer than 30 days is exempt from the retention requirement, but the owner’s obligation does not change.

In practice, each tank needs an organized file with the most recent annual external visual and leakage test reports, plus the most recent five-year internal visual, thickness, and pressure test reports. PHMSA treats missing inspection documentation as its own violation, with baseline penalties running from $1,200 for incomplete records to $5,000 for having no records at all.¹⁰

Penalties for Non-Compliance

PHMSA and the Federal Motor Carrier Safety Administration share enforcement authority over cargo tank operations. For violations that do not directly or substantially affect safety, the agency may issue a ticket. More serious problems start with a Notice of Probable Violation, which triggers formal proceedings that can lead to civil penalties or compliance orders.¹⁰

The maximum civil penalty for a knowing violation of the Hazardous Materials Regulations is $102,348 per offense. If the violation causes death, serious injury, or substantial property destruction, the ceiling rises to $238,809 per offense. A continuing violation counts as a separate offense each day it persists.¹⁰

Cargo tank-specific violations have baseline penalty guidelines:

Failure to have a tank tested or inspected on schedule: $8,000 and up, with a 25 percent increase for each additional instance.
Missing or incomplete test records: $1,200 to $3,700 for incomplete records, or $5,000 for no records at all.
Missing test and inspection markings on the tank: $600 per item.
Failure to conduct monthly discharge system inspections: $2,500.

Repeat offenders pay more. Each prior enforcement case adds a 25 percent increase to the baseline, and if PHMSA cites you for the same violation within the previous six years, the baseline doubles. Once a prior violation is on record for the same issue, corrective-action credits normally worth up to 25 percent are no longer available.¹⁰