- What the X Code Actually Requires
- Base CDL Eligibility
- Passing Two Separate Knowledge Tests
- Hazardous Materials Domains You Must Master
- Tank Vehicle Domains You Must Master
- TSA Security Threat Assessment
- Entry-Level Driver Training (ELDT) Rules
- What Legally Counts as a "Tank Vehicle"
- Renewal Cycle and Ongoing Eligibility
- Who Hires Drivers With the X Combination
- Sequencing Your Preparation
- Frequently Asked Questions
- X isn't a separate exam - it's the code for holding both H and N endorsements at once.
- You must pass two knowledge tests: hazmat (commonly 30 questions) and tank vehicle (commonly 20 questions).
- Both tests require an 80 percent score under 49 CFR 383.135 - no partial credit averaging between them.
- The H endorsement requires a TSA Security Threat Assessment costing $85.25 (or $41.00 with an accepted TWIC).
What the X Code Actually Requires
A lot of confusion around this credential starts with the letter itself. On a commercial driver's license, "X" is not something you study for or test into directly - it is the combination code that appears on the license when a driver has already earned both the Hazardous Materials (H) endorsement and the Tank Vehicle (N) endorsement. Once a state licensing agency confirms you hold both, the two are merged into the single X designation. If you're wondering exactly what that means for your career path, our breakdown of What Is X? and the related explainer on X Meaning walk through the terminology in more detail.
Because X is a combination rather than a standalone credential, "requirements to get X" really means "requirements to earn H and N simultaneously (or in sequence) and keep both current." Everything below is built around that reality, and it's why preparation for this credential looks different from studying for a single-subject test.
Base CDL Eligibility
Before either endorsement can be added, a candidate needs an active or pending commercial driver's license application, since H and N are both endorsements layered onto a CDL rather than independent credentials. Because endorsement rules and general CDL licensing rules are handled separately, candidates should confirm their state's specific licensing prerequisites directly with their driver licensing agency rather than assuming a nationwide standard applies to every step of the process.
What is standardized nationwide, however, is the content and passing threshold of the knowledge tests themselves, since those come from federal regulation rather than state discretion. That's the part this article focuses on.
Passing Two Separate Knowledge Tests
To hold X, you need to pass two distinct knowledge tests, each governed by its own section of federal code:
- Hazardous Materials knowledge test - content areas defined in 49 CFR 383.121, commonly delivered as 30 questions.
- Tank Vehicle knowledge test - content areas defined in 49 CFR 383.119, commonly delivered as 20 questions.
Both tests carry the same federally mandated passing score: 80 percent correct, per 49 CFR 383.135. There's no averaging the two together - a strong hazmat score doesn't offset a weak tank vehicle score, and vice versa. You must clear the bar on each test independently. If you want the exact math behind that threshold, see X Passing Score 2026: Exactly What You Need to Pass.
Key Takeaway
Treat this as preparing for two exams, not one. Build separate review sessions for hazmat regulations and for tank vehicle handling rather than blending them into a single generic study block.
Hazardous Materials Domains You Must Master
The hazmat side of X draws from four content areas, and it is consistently the heaviest block of material candidates need to absorb because it covers so much regulatory detail - the Hazardous Materials Table, shipping papers, marking, labeling, placarding, packaging, ORM-D handling, accident reporting, and rules around tunnels and railroad crossings.
Domain 1: Hazardous Materials Regulations
Candidates must understand how to classify, document, and communicate the presence of hazardous materials being transported.
- Reading and applying the Hazardous Materials Table correctly
- Shipping paper requirements and where they must be kept in the cab
- Marking, labeling, and placarding rules for different hazard classes
Domain 2: Hazardous Materials Handling
This covers practical handling knowledge that goes beyond paperwork.
- Packaging requirements and ORM-D (other regulated materials) rules
- Loading, segregation, and incompatible materials awareness
- What must be reported after a hazmat-related incident
Domain 3: Operation of Emergency Equipment
Focuses on the equipment a driver must know how to use when transporting hazardous cargo.
- Fire extinguisher types and appropriate use
- Spill and leak response equipment carried on the vehicle
Domain 4: Emergency Response Procedures
Tests judgment under incident conditions, not just memorized rules.
- Initial response actions in a hazmat-related emergency
- Notification and reporting sequence during an incident
- Special considerations near tunnels and railroad crossings
For a full walkthrough of how all ten domains fit together and how much weight each tends to carry, see the X Exam Domains 2026: Complete Guide to All 10 Content Areas.
Tank Vehicle Domains You Must Master
The tank vehicle side is smaller in question count but tests a genuinely different skill set: physics and handling behavior rather than paperwork.
Domain 5: Causes, Prevention, and Effects of Cargo Surge on Vehicle Handling
Liquid movement inside a partially loaded tank changes braking and steering dynamics.
- How surge affects stopping distance and stability
- Techniques to minimize surge-related loss of control
Domain 6: Braking Procedures for Empty, Full, and Partially Full Tanks
Braking behavior changes significantly depending on load state.
- Why partially full tanks are often the hardest to control
- Adjusting following distance and braking technique by load level
Domain 7: Baffled and Compartmented Versus Non-Baffled Tank Handling
Internal tank construction changes how liquid movement is managed.
- How baffles reduce surge compared to non-baffled tanks
- Handling differences for compartmented tanks carrying multiple products
Domain 8: Tank Vehicle Type and Construction Differences
Not all tank vehicles behave the same on the road.
- Differences between vehicle and tank construction types
- How construction affects center of gravity and rollover risk
Domain 9: Effects of Road Grade and Curvature on Tank Vehicle Handling
Terrain interacts with liquid cargo in ways solid freight doesn't experience.
- Speed adjustment on curves and grades with liquid loads
- Rollover risk factors specific to tank vehicles
Domain 10: Proper Use of Emergency Systems
Covers correct operation of the vehicle's built-in emergency features.
- Emergency shut-off and venting system operation
- When and how to use tank-specific safety controls
Because these domains test comprehension of physical behavior rather than lookup-table facts, memorization alone rarely gets candidates across the 80 percent line. If you're trying to gauge how tough this mix really is compared to other CDL endorsements, How Hard Is the X Exam? Complete Difficulty Guide 2026 breaks down where most candidates struggle.
TSA Security Threat Assessment
The hazmat half of X carries a requirement the tank vehicle half does not: a TSA Security Threat Assessment, which includes fingerprinting. As of January 1, 2025, the fee is $85.25 for both new and renewing applicants. Drivers who already hold a valid TWIC card can qualify for a reduced rate of $41.00, provided their state accepts TWIC comparability for this purpose.
Entry-Level Driver Training (ELDT) Rules
Since February 7, 2022, anyone applying for a Hazardous Materials endorsement for the first time must complete hazmat-specific entry-level driver training from a provider listed on the FMCSA Training Provider Registry before taking the knowledge test. This applies to first-time H applicants specifically - it is not required to add or renew the Tank Vehicle endorsement, which has no ELDT component at all.
This asymmetry matters for planning: if you already hold H from before 2022 and are only adding N to reach the X combination, you likely won't need to repeat ELDT. If you're pursuing H for the first time, factor the training requirement into your timeline before you schedule a knowledge test. Structured training resources are covered in X Training.
What Legally Counts as a "Tank Vehicle"
The N endorsement - and by extension the tank vehicle half of X - applies to a specific legal definition, not just any vehicle hauling liquid. A tank vehicle is one used to transport liquid or gaseous material within a tank that has an individual rated capacity of more than 119 gallons and an aggregate capacity of 1,000 gallons or more. Vehicles below that threshold don't trigger the tank vehicle endorsement requirement, which is a distinction knowledge test questions frequently test candidates on.
| Element | Hazardous Materials (H) | Tank Vehicle (N) |
|---|---|---|
| Governing regulation | 49 CFR 383.121 | 49 CFR 383.119 |
| Typical question count | 30 | 20 |
| Passing score | 80% (383.135) | 80% (383.135) |
| TSA threat assessment required | Yes ($85.25 / $41.00 with TWIC) | No |
| ELDT required | Yes, for first-time applicants since Feb 7, 2022 | No |
Renewal Cycle and Ongoing Eligibility
The endorsement isn't a one-time achievement. It renews on a five-year cycle, and renewal requires completing a new TSA Security Threat Assessment - meaning the fingerprinting and background check process repeats each renewal period rather than being waived after the first pass. Drivers who let the hazmat side lapse will see their license revert to showing only N until H is reinstated, which effectively removes the X combination code until both endorsements are current again.
Who Hires Drivers With the X Combination
The X combination is specifically valuable to employers moving liquid or gaseous product that also happens to be a regulated hazardous material - fuel haulers, chemical transport carriers, propane and other gas distributors, and bulk liquid logistics companies. Because the combination signals both regulatory compliance knowledge and physical handling competence for shifting liquid loads, it tends to open doors that a single endorsement alone doesn't. For a closer look at where these roles show up and how demand is structured, see X Jobs, and for a broader view of whether the investment pays off, Is the X Certification Worth It? Complete ROI Analysis 2026 and X Salary Guide 2026: Complete Earnings Analysis are useful next reads.
Sequencing Your Preparation
Because you're preparing for two structurally different tests - one regulatory and memorization-heavy, one physics and judgment-heavy - sequencing matters more than it would for a single exam. A practical approach is to front-load the denser hazmat regulatory content first, since it involves more raw material (the Hazardous Materials Table, placarding rules, shipping papers), then shift toward tank vehicle handling once the regulatory foundation is solid.
Hazardous Materials Regulations
- Work through the Hazardous Materials Table and classification logic
- Drill marking, labeling, and placarding scenarios
Hazmat Handling & Emergency Response
- Review ORM-D rules, packaging, and accident reporting requirements
- Study emergency equipment operation and response sequencing
Tank Vehicle Handling Fundamentals
- Focus on cargo surge, baffled vs. non-baffled behavior, and braking by load state
- Compare tank construction types and their handling implications
Full-Length Review & Weak-Spot Drilling
- Take timed practice sets covering all ten domains
- Re-test grade/curvature scenarios and emergency system operation until consistent
If you want a structured walkthrough of this process rather than building it yourself, X Study Guide 2026: How to Pass on Your First Attempt lays out a more detailed plan, and running through practice questions on our practice test platform is one of the fastest ways to find which of the ten domains still needs work before test day.
Frequently Asked Questions
No. X is the code that appears on a CDL once a driver holds both the Hazardous Materials (H) and Tank Vehicle (N) endorsements at the same time. There is no single "X exam" - you pass the hazmat knowledge test and the tank vehicle knowledge test separately.
Both the hazmat and tank vehicle knowledge tests require 80 percent correct, as mandated by 49 CFR 383.135. Each test is scored independently, so you must clear the threshold on both.
No. FMCSA-registered entry-level driver training is required only for first-time Hazardous Materials (H) endorsement applicants as of February 7, 2022. The Tank Vehicle (N) endorsement has no ELDT requirement.
Effective January 1, 2025, the TSA Security Threat Assessment fee is $85.25 for new and renewing applicants, or $41.00 for applicants with a valid TWIC in a state that accepts comparability.
The hazmat endorsement, and therefore the X combination, must be renewed every five years, which includes completing a new TSA Security Threat Assessment.