Hazmat compatibility groups for explosives are the thirteen letters (A through S) that federal regulations assign to every Class 1 material to control what can share a vehicle or storage area. The letter rides alongside a hazard division number (1.1 through 1.6) to form a classification code like 1.4G, where the number tells you the blast severity and the letter tells you the chemistry and initiation risk. Two materials in the same division can still be forbidden from traveling together if their letters clash, which is exactly the problem the group system solves.
How the Classification Code Reads
The division number always comes first. It ranks Class 1 materials by the type of physical hazard they create: 1.1 is mass-detonation, 1.2 is fragment-throwing, 1.3 is intense fire with minor blast, 1.4 is a minor hazard largely confined to the package, 1.5 is very insensitive but technically mass-explosive, and 1.6 is extremely insensitive articles with virtually no chance of accidental initiation.1eCFR. 49 CFR 173.50 – Class 1 Definitions
The letter that follows captures what the material is made of, how it is packaged, and whether it can set itself off or needs an external trigger.2eCFR. 49 CFR 173.52 – Classification Codes and Compatibility Groups of Explosives So a code of 1.3G identifies a fire-hazard explosive in the pyrotechnic group, and a code of 1.4S identifies a minor-hazard article packaged so that any accidental function stays within the package.
What Each of the Thirteen Letters Means
Group A: Primary Explosive Substances
Raw primary explosive substances. These are the most sensitive materials in the entire system and can detonate from friction, impact, or heat alone. Because of that sensitivity, Group A cannot share a vehicle with anything else.3eCFR. 49 CFR 177.848 – Segregation of Hazardous Materials
Group B: Articles Containing a Primary Explosive
Finished articles that contain a primary explosive without two or more effective protective features. Detonators and blasting caps used in mining and construction are placed here even when they do not technically contain a primary explosive substance, because the handling risk is similar.2eCFR. 49 CFR 173.52 – Classification Codes and Compatibility Groups of Explosives
Group C: Propellants and Deflagrating Substances
Propellant explosive substances and other deflagrating explosive substances or articles containing them. No built-in initiator.
Group D: Secondary Detonating Substances and Black Powder
Secondary detonating explosive substances, black powder, and articles containing them, in each case without initiation means or a propelling charge.
Group E: Secondary Detonating Articles With a Propelling Charge
Articles containing a secondary detonating explosive substance and a propelling charge (other than one containing a flammable liquid or gel or hypergolic liquid), still without their own initiator.2eCFR. 49 CFR 173.52 – Classification Codes and Compatibility Groups of Explosives
Group F: Self-Initiated Explosive Articles
Articles that contain a secondary detonating explosive along with their own means of initiation. The built-in trigger makes accidental activation a realistic possibility, so handling and segregation are tighter than for C, D, or E.
Group G: Pyrotechnic Substances and Articles
Fireworks, illumination devices, smoke-producing articles, tear-producing items, and incendiary substances. A Group G item can fall into any division from 1.1 through 1.4, so the letter alone does not tell you the blast severity; you need the full code.
Group H: Explosives With White Phosphorus
Articles that combine an explosive substance with white phosphorus, which ignites on contact with air.
Group J: Explosives With a Flammable Liquid or Gel
Articles that pair an explosive with a flammable liquid or gel.
Group K: Explosives With a Toxic Chemical Agent
Articles combining an explosive with a toxic chemical agent. Groups H, J, and K each carry additional containment and segregation requirements because of the secondary hazard layered on top of the explosive risk.2eCFR. 49 CFR 173.52 – Classification Codes and Compatibility Groups of Explosives
Group L: Special-Risk Explosives Requiring Isolation
Explosives that present a special risk, such as water-activation or the presence of hypergolic liquids, phosphides, or pyrophoric substances that ignite on contact with air. A Group L item can travel only with an identical Group L item. Two different Group L materials can never share a vehicle, because even experts cannot always predict how their chemistry would interact during a breach or spill.3eCFR. 49 CFR 177.848 – Segregation of Hazardous Materials
Group N: Extremely Insensitive Detonating Articles
Articles that predominantly contain extremely insensitive substances and are assigned only the 1.6N classification code. Their probability of accidental initiation is negligible.
Group S: Articles Whose Accidental Function Stays Contained
Items packed or designed so that any accidental functioning does not significantly hinder firefighting or other emergency response in the immediate vicinity of the package. This is the most forgiving designation in the system.2eCFR. 49 CFR 173.52 – Classification Codes and Compatibility Groups of Explosives
You will notice the letters I, M, O, P, Q, and R are not used. The system skips them.
Which Groups Can Share a Vehicle
The compatibility table in 49 CFR 177.848 is the definitive reference. It works like a grid: find one group’s letter on one axis and the other group’s letter on the second, and the intersecting cell tells you whether co-loading is allowed. A blank cell means no restriction. An “X” means the two groups cannot travel together under any circumstances. A number points to a conditional rule, such as special handling when detonators are present or a prohibition on loading Division 1.4S fireworks with Division 1.1 or 1.2 materials.3eCFR. 49 CFR 177.848 – Segregation of Hazardous Materials
The absolute rules that come up most often:
- Group A cannot share transport with anything else. Every cell in the Group A row shows an “X.” Primary explosives are simply too sensitive to risk any proximity to other materials.
- Group L can only travel with an identical Group L material. Different Group L items are prohibited from riding together even though they share the letter.
- Groups C, D, and E can generally load together without restriction, since these materials share similar chemical behavior and none carry their own initiation mechanism.
- Group S can travel alongside Groups B through K and Group N subject to conditional rules (primarily around detonators and fireworks), but Group S is still barred from co-loading with Group A or Group L.
For any combination not covered by these four patterns, the table itself is the authority. Check the intersecting cell before loading.
What a Violation Costs
Civil penalties for a knowing violation of federal hazmat transportation law can reach $102,348 per offense, or $238,809 if the violation causes death, serious injury, or substantial property destruction. Each day of a continuing violation counts as a separate offense. The minimum civil penalty for training-related violations is $617.4eCFR. 49 CFR 107.329 – Maximum Penalties
The Letter Is Only Part of Compliance
The compatibility group answers the loading question. It does not answer every question a carrier has to answer before wheels turn. A shipment of explosives also needs correct division-and-group placards on each side and each end of the vehicle, with an exploding-bomb symbol on Divisions 1.1, 1.2, and 1.3 placards.5eCFR. 49 CFR 172.522 – EXPLOSIVES 1.1, 1.2, and 1.3 Placards It needs a 24-hour emergency response telephone number on the shipping paper, answered by a person, not voicemail.6eCFR. 49 CFR 172.604 – Emergency Response Telephone Number Any quantity of Division 1.1, 1.2, or 1.3 material triggers a written transportation security plan; for Divisions 1.4, 1.5, and 1.6, the plan requirement kicks in only when the shipment is large enough to require placarding.7eCFR. 49 CFR 172.800 – Purpose and Applicability And the driver hauling placarded explosives needs a hazmat endorsement on their CDL, backed by a TSA security threat assessment.8Transportation Security Administration. HAZMAT Endorsement Getting the compatibility letter right keeps two materials from reacting in a trailer; the rest of the framework keeps everyone else safe if something goes wrong anyway.