NM Journeyman Plumber common mistakes
NM Journeyman Plumber · Understand the traps and how to avoid them.
Applying UPC fixture unit values and trap sizes from memory instead of the New Mexico Plumbing Code's adopted edition, especially where NM amends or adopts a specific UPC year with state-specific amendments.
Most candidates study a national UPC/IPC study guide or a neighboring state's code and never verify which edition New Mexico has adopted and which sections are amended, so they answer from the wrong table.
What to do instead
Download the current adopted edition and the NM amendments from the Construction Industries Division (CID) website, tab the fixture-unit and trap-size tables, and write the edition year and amendment numbers on your reference sheet. Practice every sizing question against those tabbed tables, not a generic chart.
Sizing a building water supply or gas pipe by adding fixture units or BTU load and then selecting pipe from the wrong row or column of the table, ignoring the length-of-run or developed-length column.
Candidates learn the fixture-unit method conceptually but freeze on the two-entry table lookup (load vs. developed length), and under exam pressure they grab the pipe size from the first column that matches the load.
What to do instead
Drill the two-step procedure explicitly: total the load, compute developed length including fittings and vertical rises, then intersect the correct row and column. Do ten timed lookups per study session using the actual code table until the row/column habit is automatic.
Confusing a fixture's trap arm length and fall limits with the trap-to-vent distance, and misapplying the rule that the trap arm must fall no more than one pipe diameter before the vent.
The 'one pipe diameter of fall' rule and the trap arm length table look similar, and candidates memorize a single 'trap arm length' number without attaching it to the pipe size or the fall condition.
What to do instead
Build a one-page card pairing trap arm size, maximum developed length, and maximum fall for each size, and rehearse the logic: the vent must be taken off before the arm falls more than one diameter. Work problems where the arm is level versus sloped so the distinction becomes concrete.
Failing to distinguish a wet vent from a common vent or a circuit vent, and applying wet-vent sizing rules to a branch that is actually a combination waste and vent.
These vent types share vocabulary and the code tables overlap, so candidates pattern-match on the word 'vent' instead of reading the conditions (which fixtures drain through the vent, sizing increase required, distance to the stack).
What to do instead
Make a comparison matrix with columns for definition, permitted fixtures, sizing, and restrictions for wet, common, circuit, and combination waste and vent systems. Answer practice questions by first naming the vent type from the conditions given, then applying that column.
Treating every drain connection as needing a trap and failing to recognize fixtures that must not be trapped or that require a special trap, such as a trap for a clothes washer standpipe versus a grease interceptor for a commercial sink.
Candidates memorize 'every fixture gets a trap' and then cannot handle exceptions like a floor drain that receives only clear water, a fixture with an integral trap, or the prohibition on double-trapping and on trapping in series.
What to do instead
Study the trap requirements section alongside the prohibited-trap list, and list exceptions with the reason: double trapping, crown venting, and S-traps are prohibited because they siphon or obstruct. For each fixture in the code's fixture table, note whether the trap is required, integral, or special.
Calculating water heater relief valve, expansion tank, and thermal expansion requirements as if the system were open, ignoring the check valve or backflow preventer that makes it a closed system.
Candidates learn T&P valve sizing and temperature settings but skip the closed-system concept, so they miss questions about thermal expansion control and the required relief valve discharge piping termination.
What to do instead
Learn the closed-system chain: backflow preventer or check valve creates a closed system, which requires a thermal expansion tank or equivalent, plus a T&P valve with full-size discharge terminating safely. Practice identifying where the check valve sits in a given piping description and what that triggers.
Mixing up combustion air methods: using the indoor air infiltration allowance when the space is tightly constructed, or sizing a combustion air opening from the wrong table (two permanent openings vs. one opening, vertical vs. horizontal ducts).
The combustion air rules depend on whether the space is confined, whether it is tight, and whether the openings connect to outdoors or another space, and candidates collapse these branches into one rule.
What to do instead
Draw a decision tree: confined or unconfined, tight or conventional, then the specific free area per BTU and the opening location rules. Rehearse with numbers for a 50,000 BTU appliance in each branch so the arithmetic follows the tree rather than a memorized figure.
Sizing a gas line from the appliance input rating without derating for altitude, and without converting appliance input to the gas's heating value and specific gravity used in the table.
Candidates use the nameplate BTU input directly and ignore that New Mexico elevations change the derating and that code tables are keyed to a specific gas heating value and specific gravity.
What to do instead
For every gas sizing problem, write the altitude, the derated input, the gas heating value and specific gravity, and the table's basis before touching the table. Practice converting nameplate input to derated input at typical New Mexico elevations so the step becomes automatic.
Answering permit and inspection questions from general knowledge rather than New Mexico's specific process: who may pull the permit, when the permit expires, required inspection stages, and who enforces the code at the state and local level.
This is the most state-specific domain, and national study material barely covers it, so candidates guess at jurisdictional authority, permit expiration, and the sequence of rough-in, cover, and final inspections.
What to do instead
Study the CID permitting and inspection rules directly: permit application and expiration, the inspection sequence (underground, rough-in, gas, final), and the authority having jurisdiction. Write a one-page timeline of a project from permit to final inspection and quiz yourself on who signs at each step.
Computing storm drainage and subsoil drainage using the wrong rainfall intensity or applying sanitary drainage sizing rules to storm leaders and area drains.
Storm drainage depends on the design rainfall rate for the location and on roof area, and candidates reuse the fixture-unit method from sanitary drainage because it looks familiar.
What to do instead
Separate the two systems on your study sheet: sanitary uses fixture units and developed length; storm uses roof area times rainfall intensity divided by a conversion factor, with leader and horizontal storm drain tables sized by area. Practice with an assumed intensity and a stated roof area, and note that New Mexico's design intensity differs from coastal defaults.