- How the NM-JP Written Exam Is Built
- Domain 1: Drainage Waste and Vents
- Domain 2: Water Supply Systems
- Domain 3: Backflow Prevention
- Domain 4: Storm Drainage Systems
- Domain 5: Plumbing Fixtures, Water Heaters, and Accessibility
- Domain 6: General Knowledge and General Regulations
- Domain 7: Interceptors and Traps
- Domain 8: Isometric Analysis
- Domain 9: Septic and Sewer
- Open-Book References You're Actually Allowed to Bring
- Sequencing the Nine Domains Into a Study Plan
- Where the Domains Reappear on the Practical Exam
- FAQ
- The written exam has 70 scored questions across nine domains, plus up to ten unscored experimental items.
- Passing requires 53 of 70 points (75%) in 185 minutes, computer-based via PSI.
- Drainage Waste and Vents is the largest domain, so it deserves the most study hours.
- Both the written and practical exams are open book, but only certain approved references may enter the room.
How the NM-JP Written Exam Is Built
The New Mexico Journeyman Plumber (JP) Certificate of Competence Exam - administered through PSI Services LLC on behalf of the Construction Industries Division (CID) - organizes its written content into nine domains. Candidates answer 70 base questions in 185 minutes on a computer-based platform, and PSI may add up to ten additional unscored experimental questions (with corresponding extra time) that don't count toward your score but are indistinguishable from live items on screen.
The published passing threshold is straightforward: 53 of 70 points, or 75%. That number matters more than any percentage-per-domain breakdown, because CID does not publish a fixed weight for each of the nine areas beyond confirming that Drainage Waste and Vents is the largest single domain. Treat every domain as fair game, but allocate proportionally more review time to DWV given its size.
Domain 1: Drainage Waste and Vents
Drainage Waste and Vents (DWV)
As the largest domain on the exam, DWV covers how wastewater and gases move out of a building safely. Expect questions on pipe sizing, slope requirements, venting configurations, and the logic behind trap-to-vent distances.
- Minimum and maximum slope for horizontal drainage piping
- Vent sizing relative to fixture units and developed length
- Common venting mistakes: back-venting, wet venting, and combination waste-and-vent setups
- Cleanout placement and access requirements
Because DWV carries the most questions, weak spots here have an outsized effect on your score. Candidates who struggle with venting geometry should slow down and sketch systems by hand before attempting practice questions - a habit that also pays off directly on the Isometric Analysis domain covered below.
Domain 2: Water Supply Systems
Water Supply Systems
This domain tests potable water distribution: pipe materials, sizing methods, pressure considerations, and protection of the supply from contamination.
- Sizing water lines using fixture-unit demand tables
- Approved materials for copper and PEX distribution, including transitions between materials
- Water hammer arresters and pressure-reducing valve placement
- Hot and cold water separation and insulation requirements
Copper and PEX piping methods also show up directly on the practical exam's physical tasks, so studying this domain does double duty for both exam components.
Domain 3: Backflow Prevention
Backflow Prevention
Backflow questions focus on protecting potable water from cross-connection hazards. Expect scenario-based items describing a fixture or hazard level and asking which device applies.
- Air gaps versus mechanical backflow preventers
- Matching device type to degree of hazard (health hazard vs. non-health hazard)
- Installation clearance and testing requirements for assemblies
Domain 4: Storm Drainage Systems
Storm Drainage Systems
This smaller domain covers how rainwater is collected and conveyed away from a structure, distinct from sanitary drainage.
- Roof drain sizing based on drainage area and rainfall rate
- Secondary (overflow) drainage requirements
- Sizing and routing of storm leaders and horizontal storm piping
Candidates sometimes confuse storm and sanitary sizing tables under exam pressure. Keep a mental checklist that separates the two systems before answering any sizing question that mentions "roof" or "rainfall."
Domain 5: Plumbing Fixtures, Water Heaters, and Accessibility
Plumbing Fixtures, Including Water Heaters, Including Accessibility
This domain spans fixture installation standards, water heater setup and safety devices, and accessibility clearances for fixtures used by people with disabilities.
- Water heater relief valve, drain pan, and seismic strapping requirements
- Fixture supply and drain connection standards
- Accessible clearance dimensions around toilets, lavatories, and tub/shower units
- Combustion air and venting requirements for gas-fired water heaters
Tub and washer-box assemblies appear again on the practical exam, making this one of the domains where written knowledge and hands-on skill overlap most directly.
Domain 6: General Knowledge and General Regulations
General Knowledge and General Regulations
This domain is the catch-all for administrative and regulatory knowledge: licensing rules, permit requirements, and general code administration concepts that don't fit neatly into a technical system category.
- CID's role and general permitting expectations
- Definitions and terminology used throughout the plumbing code
- Scope of a journeyman plumber's authorized work versus a contractor's license
Remember that the JP certificate is an individual working credential, not a contractor license - a distinction this domain may test directly. For a deeper look at what the credential legally allows, see What Is NM-JP Certification?
Domain 7: Interceptors and Traps
Interceptors and Traps
This domain covers devices that separate or block unwanted materials from entering the drainage system, from simple fixture traps to grease and oil interceptors.
- Trap seal depth and evaporation prevention
- Grease interceptor sizing and location rules
- Prohibited trap configurations, such as double trapping
Trap-related questions frequently pair with DWV concepts, since a trap's function depends on adequate venting. Study these two domains close together rather than in isolation.
Domain 8: Isometric Analysis
Isometric Analysis
Isometric questions ask you to read or interpret a piping diagram - identifying fixture unit loads, pipe sizes, or code violations shown in a drawing.
- Reading isometric drawings for vent and drain routing
- Spotting improper slope, missing cleanouts, or incorrect vent connections in a diagram
- Calculating fixture unit totals from a drawn system
Key Takeaway
Practice sketching your own isometric diagrams from written scenarios, then check them against DWV and water supply sizing rules - this single habit strengthens three domains at once.
Domain 9: Septic and Sewer
Septic and Sewer
The final domain addresses building sewer connections and on-site septic considerations relevant to a journeyman's scope of work.
- Building sewer sizing, slope, and material requirements
- Connection requirements between building drain and building sewer
- Basic septic system components a plumber must recognize, even where design work falls outside JP scope
Open-Book References You're Actually Allowed to Bring
Both the written and practical exams are open book, which changes how you should prepare. You're permitted to bring approved reference books with permanent tabs and highlighting, plus a silent, nonprinting, nonprogrammable calculator. Handwritten notes and loose papers inserted into references are not allowed.
Two commonly misunderstood items are source-only: the written mathematics textbook and the liquid-waste regulations are referenced as the origin of certain exam content but are not themselves permitted in the exam room. Build your tabbing system around the actual approved references PSI lists in your bulletin, not around every document mentioned as a source. For exact fee amounts and what's included in your application total, see NM-JP Certification Cost 2026.
Sequencing the Nine Domains Into a Study Plan
Because Drainage Waste and Vents is the largest domain and overlaps conceptually with Interceptors and Traps and Isometric Analysis, it makes sense to study those three together early, then branch into supply-side and administrative domains.
DWV, Traps, and Isometric Analysis
- Master venting logic and trap seal rules together
- Practice reading and sketching isometric diagrams daily
Water Supply, Backflow, and Storm Drainage
- Drill fixture-unit sizing tables for supply and storm systems
- Match backflow devices to hazard scenarios
Fixtures/Water Heaters, Septic and Sewer, General Knowledge
- Review accessibility clearances and water heater safety devices
- Cover building sewer sizing and administrative regulations
Full Review and Timed Practice
- Run full-length timed practice sets across all nine domains
- Re-tab references based on where you're losing time
This is a general framework, not a rigid formula - adjust the weeks based on your existing field experience. For a more detailed week-by-week methodology, read the NM-JP Study Guide 2026.
Where the Domains Reappear on the Practical Exam
The 100-point, 120-minute practical exam isn't a separate universe of knowledge - several written domains reappear as hands-on tasks. Copper and PEX piping connect to Water Supply Systems. Tub and washer-box assemblies connect to Plumbing Fixtures. Fitting and tool identification draws on general product knowledge tied to multiple domains, and island vent assembly with drainage minimum-rise setup is a direct extension of DWV concepts.
| Written Domain | Related Practical Task |
|---|---|
| Water Supply Systems | Copper/PEX piping assembly |
| Plumbing Fixtures, Water Heaters, Accessibility | Tub and washer-box assemblies |
| Drainage Waste and Vents | Island vent assembly, drainage minimum-rise setup |
| General Knowledge and General Regulations | Fitting and tool identification |
Studying the domains in tandem with practical task lists means you're preparing for both exams simultaneously rather than treating them as unrelated hurdles. If you're still weighing whether the full certification effort is worthwhile for your career, Is the NM-JP Certification Worth It? breaks down the return on the time and fees involved, and NM-JP Salary Guide 2026 covers earnings context. You can also start testing your domain knowledge right away with practice questions on the main practice test site.
Once you've worked through each domain individually, cycle back through full-length mixed sets on our practice platform to simulate the real mix of questions you'll see across all nine areas in a single 185-minute sitting.
FAQ
Drainage Waste and Vents is confirmed as the largest of the nine domains, so it should receive the most review time relative to the other eight areas.
CID confirms the total is 70 scored questions across nine domains with Drainage Waste and Vents largest, but a precise question count per individual domain is not published - prepare across all nine rather than assuming a fixed split.
They overlap significantly. Written domains like Water Supply Systems, Plumbing Fixtures, and Drainage Waste and Vents connect directly to practical tasks such as copper/PEX piping, tub and washer-box assemblies, and island vent assembly.
No. Both exams are open book with approved references, permanent tabs, highlighting, and an approved calculator allowed, but handwritten notes and added papers are prohibited.
You need 53 of 70 points overall (75%) on the written exam; there is no indication that each domain requires a separate passing score. See NM-JP Passing Score 2026 for the full breakdown.