Mississippi Mechanical Contractor
Practice at the real exam's pace
Every practice exam shows your time per question against the real exam's clock (2:24 here), and each topic lands in one of these:
What's on the exam
Questions per topic, out of 100Topics
Boilers and Hydronics
Key Concepts
Boiler types. Conventional boilers operate at water temperatures above 140°F (60°C); if boiler water drops below that, combustion gases condense and become highly corrosive, reducing boiler life and damaging the flue. A conventional (noncondensing) boiler system must include thermostatic controls keeping return water above 140°F (60°C) (MRAC 22nd ed., Sec. 38.1.1, p. 890). Condensing boilers are designed to condense combustion gases and drain the condensate, use secondary heat exchangers primarily of stainless steel, are over 90% efficient, and may be gas- or oil-fired (p. 891). Steel boilers pass combustion gases through tubes surrounded by water; copper-tube boilers use finned copper tubes and are classified low-mass, low-volume (MRAC 22nd ed., Chapter 38 — Hydronic Heating Fundamentals).
Gas Systems
Key Concepts
Scope and pressure limits - The fuel gas piping provisions cover natural gas at 125 psig or less and LP-gas at 20 psig or less, from the point of delivery to the outlet of the appliance shutoff valves (IFGC 2021 §101.2.2, Page 1-1). - Maximum operating pressure for piping inside buildings shall not exceed 5 psig, except where listed conditions are met (welded/brazed joints, industrial, warehousing, boiler/mechanical rooms, etc.) (§402.7, Page 4-2). - Plastic pipe: outdoors underground only; shall not operate above 100 psig for natural gas or 30 psig for LP-gas (§404.17.1, Page 4-43). - A replacement heating appliance serving an occupied portion of a structure may be operated before inspection where a request for inspection…
Cleanouts
Key Concepts
Where cleanouts are located on building sewers. Cleanouts on building sewers shall be located as indicated in Section 708 (IPC 2021, Section 703.5).
Interval spacing (horizontal drains and building drains). Horizontal drainage piping inside buildings and building drains shall have cleanouts at intervals of not more than 100 feet. Where manholes are used instead of cleanouts on building drains, the manhole interval may be up to 400 feet (IPC 708.1.1; IPC 2021, Page 7-7).
Manholes on large building sewers. Building sewers 8 inches and larger shall have a manhole located not more than 200 feet from the junction of the building drain and building sewer, and manholes thereafter at intervals of not more than 400 feet (IPC…
Plumbing Fixtures and Equipment
Key Concepts
Minimum fixtures by occupancy (IPC Table 403.1, Page 4-3) - One- and two-family dwellings and lodging houses with five or fewer guestrooms: 1 water closet, 1 lavatory, 1 bathtub/shower, 1 kitchen sink, and 1 automatic clothes washer connection — each per dwelling unit. - Apartment house: 1 water closet, 1 lavatory, 1 bathtub/shower and 1 kitchen sink per dwelling unit, but 1 automatic clothes washer connection per 20 dwelling units. - Fixtures shown are the minimum for the number of persons indicated or any fraction thereof; occupant load is determined by the International Building Code (Section 403.1 and Table 403.1 note a). - Urinals shall not be substituted for more than 67 percent of required water closets…
Water Heaters
Mississippi Mechanical Contractor — 2 questions (2% of exam)
Key Concepts
Attic installations (IPC 2021, Section 502.3) - Unobstructed passageway: not less than 30 inches (762 mm) in height and 22 inches (559 mm) in width, and not more than 20 feet (6096 mm) in length measured along the centerline from the opening to the water heater. - Continuous solid flooring not less than 24 inches (610 mm) in width. - Level service space not less than 30 inches by 30 inches at the front or service side. - Clear access opening not less than 20 inches by 30 inches (508 mm by 762 mm) where that is large enough to remove the heater.
Piping, valves, controls
Key Concepts
Materials selection - Water service pipe shall be not less than 3/4 inch in diameter (IPC 2021 § 603.1). - Steel pipe is not generally used to circulate refrigerant; the major exception is ammonia refrigeration systems, because ammonia may chemically react with copper tubing. Steel pipe is primarily used for gas lines or water lines (Modern Refrigeration & A/C, 22nd ed., Ch. 5, p. 74). Galvanized pipe (zinc anticorrosion) is used mainly in water systems; black pipe is used for gas lines and applications not carrying water. - Plastic pipe cannot be used to circulate refrigerant; three plastics used in refrigeration-related work are ABS, PVC and CPVC (Ch. 5). - Minimum weight of steel pipe for Group…
Sheet Metal and Ducts
Mississippi Mechanical Contractor | 6 questions (6% of exam)
Key Concepts
Rectangular gage and reinforcement selection - The greater duct dimension determines the duct gage for all four sides, for both reinforced and unreinforced duct (Rectangular Table Reading Guide, SMACNA HVAC-DCS 3rd ed., §2.1.1). - Flat slips and drives must not be less than two gages lower than the duct gage, and never below 24 gage (0.70 mm) — §2.1.2, p. 2.2, and the note to Table 2-47, p. 2.108. - Crossbreaking/beading is required on sides 19 in. and over, 20 ga (1.00 mm) or less, with more than 10 sq ft of unbraced panel area, unless lined or externally insulated (S1.15, pp. 2.2 and 2.4). It is not…
Water Supply
Key Concepts
Water service sizing and separation - Water service pipe shall be not less than 3/4 inch (19.1 mm) in diameter (IPC 603.1, Page 6-2). - Where water service piping shares a trench with a building sewer not constructed of materials listed in Table 702.2, the two shall be horizontally separated by not less than 5 feet of undisturbed or compacted earth. The separation does not apply where the bottom of the water service pipe, located within 5 feet of the sewer, is not less than 12 inches (305 mm) above the highest point of the top of the building sewer (IPC 603.2, Page 6-2).
Hangers and Supports
Key Concepts
Steel pipe support spacing (Pipefitter's Handbook, 3rd ed., Pages 2-67 to 2-68) - The spacing tables for standard weight pipe carrying water, and gas or steam, are based on average conditions. The footnote states the table "does not include allowance for flanges, fittings and valves, but does include allowance for insulation." Heavy fittings such as valves should always be located close to supports. - "Data under 'Horizontal Runs' are based on stress, those under 'Sloping Runs' are based on gradient." - The two sloping-run columns are for grades of 1 in. in 10 ft and 1 in. in 20 ft. Spacing for a sloping run shall not exceed the value given for the appropriate temperature in the…
Drain, Waste, and Vent
Key Concepts
Slope and drainage installation - Horizontal drainage piping shall be installed in uniform alignment at uniform slopes, not less than Table 704.1; piping upstream of a grease interceptor shall slope not less than 1/4 inch per foot (2-percent) regardless of size (IPC 704.1). Horizontal drainage piping 2½ inches and smaller requires 1/4 inch per foot (Table 704.1; IRC P3005.3). - Drainage pipe size shall not be reduced in the direction of flow (704.2). - Horizontal branches shall connect to the base of a stack not less than 10 times the diameter of the drainage stack downstream of the stack; the same rule applies downstream of a horizontal stack offset (704.3). - Double sanitary tee patterns shall not…
Backflow
Key Concepts
General cross-connection rule (IPC 608.1). A potable water supply system shall be designed, installed and maintained so as to prevent contamination from nonpotable liquids, solids or gases being introduced into the potable water supply through cross connections or any other piping connections. Backflow preventer applications shall conform to Table 608.1, except as specifically stated in Sections 608.2 through 608.17.10.
Hazard level and application (Table 608.1). Each device is listed with a degree of hazard (low hazard = pollution; high hazard = contamination, per Section 202) and an application (backpressure and/or backsiphonage, per Section 202 definitions). Key entries: - Reduced pressure principle backflow prevention assembly (and RP fire protection assembly): high or low hazard, backpressure or backsiphonage, sizes…
Traps and Interceptors
Key Concepts
Fixture traps and dimensions (IPC 2021 §1002.1, Page 10-1) - Each plumbing fixture shall be separately trapped by a liquid-seal trap; a fixture shall not be double trapped. - Vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches (610 mm) — a maximum. - Horizontal distance shall not exceed 30 inches, measured from the centerline of the fixture outlet to the centerline of the trap inlet. - Exceptions include fixtures with integral traps, and a combination fixture on one trap where one compartment is not more than 6 inches deeper than the other and the waste outlets are not more than 30 inches apart.
Developed length
Key Concepts
Pipe bending — developed length is measured on the centerline. "The length of arc in a pipe bend is measured along the centerline of the pipe, and the radius is also measured as extending to the centerline. In any bend, Length of Arc = No. of Degrees × .01745 × Radius" (Pipefitter's Handbook, 3rd ed., Page 1-1). Length of pipe in any bend depends on (1) length of bending radius and (2) number of degrees of bend.
Setback locates the start of a bend. "Setback is the distance from the beginning of the bend to the point where extended centerlines of straight pipe on each leg of the bend would meet… Setback = Radius × Tangent ½…
Isometric Analysis
Source: Pipefitter's Handbook (3rd ed.), Parts 1, 2 and 4
Key Concepts
- Setback locates the beginning of a bend. In a piping layout it is the distance from the beginning of the bend to the point where the extended centerlines of the straight legs would meet. Setback is measured along the centerline (Page 1-2).
- Length of arc in a bend is measured along the centerline of the pipe, and the radius is also measured to the centerline (Page 1-1).
- 180° bends: because the extended centerlines never meet, the bend is regarded as two 90° bends, in each of which the setback equals the radius (Page 1-50).
- Offset layout: after marking the end of the first…
Safety
Key Concepts
Fire protection (Subpart F, §1926.150) - A fire extinguisher rated not less than 2A must be provided for each 3,000 square feet of protected building area, or major fraction thereof; travel distance from any point of the protected area to the nearest extinguisher shall not exceed 100 feet — §1926.150(c)(1)(i). - One 55-gallon open drum of water with two fire pails may substitute for a 2A extinguisher — §1926.150(c)(1)(ii). A ½-inch garden-type hose line not to exceed 100 feet, discharging a minimum of 5 gpm with a minimum stream range of 30 feet horizontally, may also substitute — §1926.150(c)(1)(iii). - One or more extinguishers rated not less than 2A on each floor; in multistory buildings at least…
Joints and Connections
Key Concepts
Joint preparation (IMC 2021, Section 1203.2). Pipe shall be cut square, reamed and chamfered, and shall be free from burrs and obstructions. Pipe ends shall have full-bore openings and shall not be undercut. Joints and connections shall be of an approved type and shall be tight for the pressure of the hydronic system (Section 1203.1). Joints between different piping materials shall be made with approved adapter fittings (Section 1203.1.1).
Brazed joints (IMC Section 1203.3.1). Joint surfaces shall be cleaned, an approved flux applied where required, and the joint brazed with a filler metal conforming to AWS A5.8.
General Plumbing Regulations
Key Concepts
Scope and administration. The IPC governs erection, installation, alteration, repair, relocation, replacement, addition, use and maintenance of plumbing systems; appendices do not apply unless specifically adopted (Section 101.2). The exception to Section 101.2 permits detached one- and two-family dwellings and townhouses not more than three stories above grade plane with a separate means of egress — and accessory structures not more than three stories above grade plane — to comply with the IPC or the IRC (Page 1-1). Where a general and a specific requirement conflict, the specific governs; where different sections specify different requirements, the most restrictive governs (Section 102.1). Where the code and the manufacturer's instructions conflict, the more restrictive applies (Section 301.7). The code…
Fire Sprinklers
Key Concepts
Rack storage flue spaces (NFPA 13 2019, Chapter 20) - Nominal 6 in. (150 mm) transverse flue spaces must be maintained between loads and at rack uprights in single-row, double-row, and multiple-row racks storing Class I through Class IV commodities (20.5.3.3.2.1). Random variations in flue width and vertical alignment are permitted (20.5.3.3.2.2). - A longitudinal flue space is not required for Class I through IV and Group A plastics in double-row and multiple-row open racks for storage up to and including 25 ft (7.6 m) (20.5.3.3.1.1); nominal 6 in. longitudinal flues are required in double-row racks for storage over 25 ft (20.5.3.3.1.2). - Shelving less than 50 percent open, or loads blocking openings that otherwise serve as…
Refrigerants and Refrigeration
Key Concepts
System divisions. The compressor and the metering device are the two components that divide the low side from the high side. The compressor draws low-pressure vapor from the evaporator and pushes high-pressure vapor into the high side; the metering device separates the high side from the low side at the other end (Ch. 8 — Basic Compression Refrigeration and Air-Conditioning Systems, p. 128).
Vacuum and micron measurement. A micron is the equivalent of 0.001 mm Hg; one in. Hg equals 25.4 mm Hg or 25,400 microns. A perfect vacuum is 0 microns, and pressure increases as the micron number rises (Ch. 11, p. 192 area). A high-vacuum pump produces a vacuum higher than 28.92 in. Hg vacuum…
Heating and Cooling Principles
Key Concepts
Load calculation manuals. ACCA publishes Manual J—Residential Load Calculation and Manual N—Commercial Load Calculation. Manual N is the commercial procedure; Manual J applies to low-rise residential buildings (Pages 715 and 737). Per the Code Alert, for low-rise residential the method is typically based on ACCA Manual J; for commercial buildings it is typically based on ANSI/ASHRAE/ACCA Standard 183.
Outdoor design temperatures (Page 721). Three separate values, each for a different case: - 99% — well-constructed, well-insulated buildings with a standard window area - 99.6% — residences and small, poorly insulated office buildings - 97.5% — large buildings with considerable thermal capacity and small window area
Load Calculations
Key Concepts
Thermal properties - K-value (thermal conductivity): heat passing through one square foot of material one inch thick, in one hour, with a 1°F temperature difference between the two sides. Units: Btu·in/hr·ft²·°F. (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 32, p. 717) - C-value (thermal conductance): same idea but based on the material's actual measured thickness, not a standardized 1 inch. (Ch. 32, p. 717) - R-value: resistance to heat transfer; for a composite assembly, add the R-values of the individual materials. Higher R = slower heat transfer. - U-value (thermal transmittance): like C-value but includes an adjustment for the boundary air films on the inside and outside surfaces. U-values assume a **15 mph wind on the…
Testing, Adjusting and Balancing
Mississippi Mechanical Contractor (1 question, ~1% of exam)
Key Concepts
Airflow measuring instruments. An air handler flowmeter is placed in the air stream of an air handler to measure the total airflow of a central system; these devices are typically sized to fit over a large return grille or in the space normally occupied by the system's air filter (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 31 — Ventilation System Service, p. 701). For balancing, flow through supply outlets and returns can be measured by a flow hood, a velometer (with probe), a vane anemometer, or a thermal anemometer; duct flow is measured directly with an anemometer, velometer, or a manometer and pitot tube, all of which use…
HVAC Equipment
Key Concepts
Compression cycle and component roles (Ch. 8) - The evaporator absorbs heat; the compressor draws low-pressure vapor and raises its pressure and heat content; the condenser rejects heat and condenses vapor to liquid; the metering device restricts flow and lowers pressure into the evaporator (Sections 8.1–8.5). - The compressor and the metering device are the two places that divide the system's low side from the high side (Section 8.1.1). - Flash gas is the instantaneous evaporation of some liquid refrigerant. Some flash gas is acceptable in the evaporator, but "excessive flash gas reduces the efficiency of the evaporator. When a system has an expansion valve as its metering device, flash gas in the liquid line can cause…