DG Segregation Chart: Symbols, Steps, and Exceptions

The DG segregation chart in 49 CFR 177.848 tells you which hazardous material classes can share a transport vehicle. You read it by finding one material’s hazard class on the left column and the other’s on the top row, then checking the symbol where they meet: a blank space means no federal restriction, an “X” means the two cannot travel together at all, an “O” means they can share a vehicle only if physically separated so a leak from one cannot reach the other, and an asterisk sends you to a separate compatibility table for explosives.¹

What You Need Before Opening the Chart

For each material in the load, pull two pieces of information: the primary hazard class (or division) and any subsidiary hazard labels. Both appear in Section 14 of the Safety Data Sheet and on the shipping paper (Bill of Lading) that accompanies every hazmat shipment.

Subsidiary labels matter as much as the primary class. Federal rules require packages to carry every applicable hazard label, not just the main one.² A flammable liquid (Class 3) may also carry a corrosive (Class 8) subsidiary label, and you have to run the chart against every label on every package, not just the primary. When results conflict, the stricter one controls.

Pay attention to the division number, not only the class. The chart lists divisions separately — 4.1 (flammable solids) is treated differently from 4.2 (spontaneously combustible), and 5.1 (oxidizers) differently from 5.2 (organic peroxides), because these materials react differently when combined with other cargo.

Reading the Chart Step by Step

The segregation table in 49 CFR 177.848(d) is a symmetrical grid. Hazard classes and divisions run down the left column and across the top row. The symbol at each row-column intersection is the rule for that pair.

A worked example: you have a Class 3 flammable liquid and a Division 5.1 oxidizer. Find Class 3 on the left column. Move across to the Division 5.1 column. If the intersection shows an “X,” those two shipments cannot ride in the same vehicle, and no amount of separation inside the trailer changes that. If the flammable liquid also carries a Class 8 subsidiary label, run a second check — Class 8 against Division 5.1 — and apply whichever result is more restrictive.¹

One useful shortcut: materials within the same class can generally be loaded together without a subsidiary-hazard check, as long as they can’t react dangerously with each other.

What Each Symbol Means

Four symbols appear in the table, and each has a specific meaning under 49 CFR 177.848(e):¹

Blank space — no federal segregation restriction applies. The two materials can be loaded together freely.
“X” — do not load, transport, or store the two materials together at any point during transit. Physical separation inside a single vehicle does not satisfy this restriction.
“O” — the materials may share a vehicle only if they are separated so a leak from one package could not reach the other under normal transport conditions. Barriers, non-hazardous freight between them, or sufficient floor space all qualify. One extra rule applies no matter which separation method you use: Class 8 corrosive liquids may never be loaded above or next to Class 4 flammable or Class 5 oxidizing materials.
“*” — appears only where two Class 1 explosive entries meet. It directs you to the explosives compatibility table in paragraph (f).

There is also a note “A” in the table’s second column. It creates a narrow exception allowing ammonium nitrate (UN1942) and ammonium nitrate fertilizer to be loaded with Division 1.1 or 1.5 explosives, even though an “X” would otherwise apply.¹

When the Chart Sends You to the Explosives Compatibility Table

Class 1 materials get a second table in 49 CFR 177.848(f) because the explosive risk depends on the compatibility group letter assigned to each item, not just the division. There are 13 compatibility groups: A, B, C, D, E, F, G, H, J, K, L, N, and S. Group A includes primary detonating substances; group S covers items that present no significant hazard if accidentally ignited.¹

Whenever the main chart shows an asterisk for two Class 1 entries, switch to the compatibility table and check the row and column for the two group letters. Some combinations are permitted, others are flatly prohibited. Skipping this second lookup is a common error because it’s tempting to treat all explosives as one category.

Separating an “O” Load Inside the Vehicle

When the chart shows an “O,” the regulation asks you to physically prevent commingling if a package fails. In practice that means placing non-hazardous freight between the two hazardous loads, using physical dividers or barriers inside the trailer, or leaving enough floor space that spilled material from one package cannot flow to the other.¹

Incompatible materials should not be stacked on top of each other or placed where gravity could bring them together during a spill. Break-bulk operations, where individual packages are handled separately, raise the risk of accidental contact, and the separation obligation runs from origin to destination. Beyond segregation, every hazmat package in a motor vehicle also has to be blocked and braced against shifting — that rule applies even to blank-space combinations.

Exceptions for Small and Limited Quantities

Not every hazmat shipment triggers full segregation procedures. The small quantity exception at 49 CFR 173.4 covers very small amounts moving by domestic highway or rail — generally no more than 30 mL of liquid or 30 g of solid per inner container, with a completed outer package no heavier than 29 kg (64 pounds).³ Separately, class-specific limited quantity provisions (for example, 49 CFR 173.150 for flammable liquids) exempt qualifying shipments from labeling and placarding requirements, though the packaging rules still apply.⁴ The exact limits vary by class and packing group, so check the subpart for the material you’re shipping.

What Getting the Chart Wrong Costs

Civil penalties reach $102,348 per violation, and $238,809 per violation where the offense results in death, serious illness, severe injury, or substantial property destruction. Each day a continuing violation persists counts as a separate offense.⁵

Criminal prosecution is available for knowing or reckless violations: a fine and up to five years of imprisonment, doubled to ten years when the violation involves a release that causes death or bodily injury.⁶ Criminal fines follow the general federal sentencing rules in Title 18, up to $250,000 for individuals and $500,000 for organizations on a felony conviction.

Enforcement happens at the roadside. A DOT inspector who finds an “X” combination in a single trailer can place the vehicle out of service on the spot while the load is corrected; the fine follows later, but the shipment stops immediately.

1
eCFR. 49 CFR 177.848 – Segregation of Hazardous Materials
2
eCFR. 49 CFR 172.402 – Additional Labeling Requirements
3
eCFR. 49 CFR 173.4 – Small Quantities for Highway and Rail
4
eCFR. 49 CFR 173.150 – Exceptions for Class 3
5
eCFR. 49 CFR 107.329 – Maximum and Minimum Penalties
6
Office of the Law Revision Counsel. 49 USC 5124 – Criminal Penalty