Mechanical, Electrical, and Plumbing Systems
You don't need to be an engineer, but you must understand how MEP systems coordinate with structure and code. In plumbing, know supply vs. drain-waste-vent (DWV), the purpose of traps (keep sewer gas out), vents (equalize pressure), and slope for drain lines (commonly 1/4 in per foot for 3-in and smaller). Understand water heater basics and backflow prevention.
In HVAC, understand heating and cooling load basics, duct sizing, and the importance of return air. Know the difference between forced-air and hydronic systems and the role of filters and commissioning. Understand that make-up air and combustion air are required for fuel-burning appliances.
In electrical, know the basics of circuits, ampacity, wire gauge (lower gauge = larger wire), grounding, and GFCI protection (required in wet locations) and AFCI (arc-fault). Understand panel capacity and the difference between line and load. Know that only licensed electricians perform electrical work in most jurisdictions, but the GC must coordinate rough-in sequencing.
Worked rule of thumb: Ohm's law: Power (watts) = Volts × Amps. A 1,500-watt load on a 120-volt circuit draws 12.5 amps. On a 15-amp circuit, that's 83% of capacity - near the 80% continuous-load limit, so it should be on a 20-amp circuit.
Traps: sizing wire by load only (ignoring voltage drop); skipping vents on plumbing; forgetting combustion air; ignoring GFCI/AFCI locations; sequencing MEP rough-in after drywall.