Motors, Controls, and HVAC Equipment
What you must understand
Motor questions test full-load current tables, overload protection sizing, branch-circuit short-circuit and ground-fault protection, disconnecting means, and controller requirements. Know that motor branch-circuit protection is sized from the tables, not the nameplate, and that overload protection is based on the nameplate current. Understand HVAC equipment: the difference between the compressor, condenser fan, and evaporator fan loads, and how the nameplate minimum circuit ampacity and maximum overcurrent protection are used. Know control-circuit fundamentals, contactors, starters, and the disconnect requirements for motors and HVAC units.
Traps
A common trap is using the nameplate current to size branch-circuit conductors and short-circuit protection when the table value is required, or vice versa for overloads. Questions often test that the disconnecting means must be within sight and within a specified distance of the motor. HVAC questions frequently hinge on using the nameplate maximum overcurrent device rather than calculating it. Watch for the 125 percent multiplier applied to the largest motor and for the requirement that the disconnecting means open all ungrounded conductors.
Rule of thumb and worked example
Rule of thumb: motor branch-circuit conductors are sized at 125 percent of the table full-load current. Example: a single-phase motor with a table FLC of 16 amperes requires conductors rated at least 16 × 1.25 = 20 amperes, so a 12 AWG copper conductor at 20 amperes is the minimum. The overload heater is then selected from the nameplate current, and the short-circuit protection from the table, not the nameplate.