6.2 DOT Specification Cargo Tanks, Nameplates & Retest Markings
Key Takeaways
- Under 49 CFR 180.407, most specification cargo tanks need an external visual inspection and a leakage test every year, plus an internal visual inspection and pressure test every 5 years.
- Cargo tanks carrying lading corrosive to the tank need yearly internal visual inspections, and unlined tanks in that service need a thickness test every 2 years.
- Test markings show the month, year, and test type, such as "10-99 P, V, L," in letters at least 1.25 inches high near the specification plate or on the front head.
- A specification cargo tank with a test or inspection past due may not be filled and offered for transportation until it passes, unless it was filled before the due date.
- DOT-406 tanks have an MAWP of 2.65 to 4 psig, DOT-407 tanks at least 25 psig, DOT-412 tanks at least 5 psig, and MC-331 tanks 100 to 500 psig.
49 CFR 383.119(h) adds a requirement that applies only to some tank drivers: for drivers of DOT specification tank vehicles, retest and marking requirements. Specification cargo tanks carry hazardous materials, and their markings tell the driver, shipper, and inspector whether the tank is built for the product and currently qualified.
Main Cargo Tank Specifications
| Specification | Typical service | Key design facts |
|---|---|---|
| MC-306 / DOT-406 | Gasoline, diesel, other petroleum and low-pressure liquids | DOT-406 MAWP 2.65 to 4 psig (49 CFR 178.346-1); usually oval, often multi-compartment aluminum |
| MC-307 / DOT-407 | Chemicals and other liquids needing higher pressure | DOT-407 MAWP at least 25 psig with a circular cross-section (178.347-1); often insulated and jacketed |
| MC-312 / DOT-412 | Corrosive liquids such as acids | DOT-412 MAWP at least 5 psig; tanks over 15 psig must be circular (178.348-1); often smaller in capacity and sometimes lined |
| MC-331 | Liquefied compressed gases such as propane and anhydrous ammonia | Design pressure 100 to 500 psig (178.337-1); round with rounded heads |
| MC-338 | Cryogenic liquids such as liquid nitrogen or oxygen | Insulated, vacuum-jacketed tank |
MAWP is maximum allowable working pressure. The MC-306, MC-307, and MC-312 specifications were replaced by DOT-406, DOT-407, and DOT-412: no cargo tank may be certified to those older MC specifications after August 31, 1995, except under recertification rules for existing tanks. Older MC tanks can stay in service when they keep passing the required tests.
Uninsulated MC-331 tanks must be painted white, aluminum, or a similar reflecting color on the upper two-thirds, unless covered with a bright metal jacket (178.337-1(d)). The reflective finish limits heating from the sun, which raises pressure in a liquefied gas tank.
Nameplate and Specification Plate (49 CFR 178.345-14)
DOT-406, -407, and -412 tanks carry corrosion-resistant metal plates on the left side of the vehicle near the front of the cargo tank (or the frontmost tank), in characters at least 3/16 inch high. The two plates may be combined.
| Nameplate (tank) | Specification plate (vehicle) |
|---|---|
| DOT specification number | Cargo tank motor vehicle manufacturer and certification date |
| Original test date | Cargo tank manufacturer and date of manufacture |
| Tank MAWP and test pressure | Maximum weight of lading (maximum payload) |
| Design temperature range | Maximum loading rate and maximum unloading rate |
| Nominal (water) capacity | Lining material and heating system ratings, if applicable |
| Maximum design density of lading | |
| Shell and head materials, minimum thicknesses |
Two plate items matter every day. Maximum lading density and maximum payload limit what you may load (Section 1.3). The maximum loading and unloading rate may not be exceeded (Section 3.3).
Periodic Tests and Inspections (49 CFR 180.407(c))
Tests must be performed or witnessed by a Registered Inspector registered with FMCSA who meets the qualification rules (180.409). The schedule depends on the test, the tank design, and the service:
| Test or inspection | Marking | Interval |
|---|---|---|
| External visual inspection | V | 1 year for most tanks (6 months for vacuum-loaded tanks with full-opening rear heads) |
| Leakage test | K | 1 year for most tanks (2 years for MC-330/331 in chlorine service) |
| Internal visual inspection | I | 5 years for most tanks; 1 year for insulated tanks (except MC-330, 331, 338) and tanks carrying lading corrosive to the tank |
| Lining inspection | L | 1 year for lined tanks carrying lading corrosive to the tank |
| Pressure test (hydrostatic or pneumatic) | P | 5 years for most tanks; 1 year for insulated tanks with no manhole or insulated and lined; 2 years for vacuum-loaded tanks with full-opening rear heads |
| Thickness test | T | 2 years for unlined tanks carrying material corrosive to the tank |
Some narrow categories get different intervals, such as certain small MC-331 tanks in dedicated propane service, which can go 10 years between internal visual inspections and pressure tests. Always follow the markings and the carrier's records for your specific tank.
What some of the tests involve
- Leakage test: at no less than 80% of the MAWP on the specification plate (with exceptions), held for at least 5 minutes, with piping, valves, and accessories in place.
- Hydrostatic pressure test: the tank is filled with water or a similar liquid and held at test pressure for at least 10 minutes while inspected for leaks, bulging, or other defects.
- External visual inspection: includes operating the emergency valves, replacing missing bolts, nuts, and fusible elements, and confirming required markings are legible. For tanks carrying corrosive lading, areas covered by the upper coupler (fifth wheel) assembly must be inspected at least once every 2 years with the upper coupler removed.
Reading Test Markings (49 CFR 180.415)
After a tank passes a test, it must be marked with the month and year and the type of test:
- Letters and numbers at least 1.25 inches high, near the specification plate or anywhere on the front head.
- Abbreviations: V external visual, I internal visual, P pressure test, L lining inspection, T thickness test, K leakage test, K-EPA27 leakage test under EPA Method 27.
Examples from the regulation
- "10-99 P, V, L" means that in October 1999 the tank passed the pressure test, external visual inspection and test, and lining inspection.
- "2-00 K-EPA27" means that in February 2000 the tank passed the leakage test under EPA Method 27.
For a vehicle with several tanks built to different specifications and tested at different intervals, the markings must appear in order from front to rear.
Working out the next due date
A tank marked "06-25 V, K" passed its external visual inspection and leakage test in June 2025. Both are yearly tests for most tanks, so they are due again in June 2026. The same tank also needs its 5-year internal visual inspection and pressure test tracked from their own markings.
The Past-Due Rule
- A specification cargo tank for which a test or inspection has become due may not be filled and offered for transportation or transported until the test or inspection is successfully completed. This does not apply to a tank filled before the due date (49 CFR 180.407(a)(1)).
- 49 CFR 173.33(a)(3) likewise prohibits filling and offering a specification cargo tank motor vehicle with a past-due retest, with an exception for a cargo tank supplied by a motor carrier other than the person offering the hazmat.
- A tank that fails a test must be repaired and retested, or removed from hazardous materials service with its specification plate removed, obliterated, or covered (180.407(a)(6)).
- Regardless of schedule, a tank that shows dents, cuts, gouges, corrosion, leakage, or any condition that might make it unsafe must be tested and inspected before further use (180.407(b)).
Common Test Traps
- "Which tests are yearly for most tanks?" External visual (V) and leakage (K).
- "Which are every 5 years?" Internal visual (I) and pressure test (P) for most tanks.
- "Is the thickness test every 5 years?" No. It is required every 2 years for unlined tanks carrying material corrosive to the tank.
- "What does 'P, V, L' mean?" Pressure test, external visual inspection, and lining inspection.
For most specification cargo tanks, how often does 49 CFR 180.407 require the external visual inspection (V) and the pressure test (P)?
A cargo tank is marked "10-99 P, V, L." What does the marking mean?
A DOT-407 cargo tank's yearly leakage test became due last week, and the tank is empty. What does 49 CFR 180.407 say?