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Explosive placard compatibility group letter


172.504(g) and 172.522(b)

Regulation summary

Section 172.504(g) establishes the general placarding requirements for Class 1 explosive materials, and 172.522(b) establishes the appropriate division number and compatibility group letter on EXPLOSIVE placards.


In this scenario, a shipper inquires if a Class 1 (explosive) compatibility group letter must be displayed on an EXPLOSIVE placard when the material is transported by a motor vehicle under the Hazardous Materials Regulations (HMR).

In this interpretation, PHMSA addresses the display of the compatibility group letter for Division 1.1, 1.2, and 1.3 explosive materials.

Interpretation Summary

In interpretation Ref. No. 08-0049, PHMSA advises that 172.504(g) requires the display of the compatibility group letter on placards or labels for shipments of Class 1 materials transported by aircraft or vessel only.

  • The HMR does not require the compatibility group letter for motor vehicle or rail shipments of Class 1 materials.
  • However, 172.505(c) permits it to be displayed provided the placards otherwise conform to the general placarding requirements.
  • Section 172.401(a)(2) also permits a hazard warning label to be displayed provided the label represents the hazard class of the material in the package.