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Master Mechanical exam answers

Plain-English breakdowns of the gas, plumbing, refrigeration, and venting questions you’ll see on the exam — with state-specific code citations.

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california

How does the California Master Mechanical exam work, and why is it the C-20 instead of ICC?
California uses the C-20 Warm-Air Heating, Ventilating, and Air-Conditioning Contractor license through CSLB, not the ICC Master Mechanical exam. Full 2026 guide: code book, exam structure, fees, study order, FAQ.

missouri

What is on the Missouri Master Mechanical exam, and how do KC and St. Louis differ?
Missouri does not have a statewide mechanical license. KC uses the ICC Master Mechanical (M-3) exam plus Chapter 18 amendments. St. Louis and Springfield differ. Full study path, fees, and the three places candidates fail.

plumbing

What is the minimum slope for horizontal drain piping?
IPC and UPC minimum slope for horizontal drain piping: 1/4 inch per foot for pipe under 3 inches, 1/8 per foot for 3-6 inch, 1/16 per foot for 8 inch+.

venting

What is the difference between Category I and Category III venting?
Category I venting is negative-pressure non-condensing (older furnaces). Category III is positive-pressure non-condensing — needs gas-tight vent like AL29-4C.

gas piping

How do I size a gas line for a 100,000 BTU furnace and 40,000 BTU water heater?
Sizing a residential gas line for 140k BTU total load (100k furnace + 40k water heater): typically 3/4 inch trunk, 1/2 inch branches, sized per NFPA 54 Table 6.2(a).
What are the bonding requirements for CSST gas piping?
CSST gas piping requires a #6 AWG copper bond to the electrical service ground per NFPA 54 7.13 — to prevent lightning-induced arcing through the thin tubing wall.

refrigerants

What does the A2L refrigerant safety classification mean for HVAC installs?
A2L refrigerants (R-32, R-454B, R-1234yf) are low-toxicity, mildly flammable replacements for R-410A. Install requires charge-per-room limits, leak detection, A2L-rated tools.