Electrical, Controls, and Components
Fundamentals
The electrical section rewards fluency with Ohm's law (E = I × R, P = E × I), series versus parallel circuits, and reading a ladder diagram. In a series circuit the current is the same through every load and voltages add; in parallel the voltage is common and currents add. Most appliance control circuits are 24-volt parallel circuits fed from a step-down transformer, with switches (thermostat, pressure switch, float switch, limits) wired in series with the coil they control.
Components
Know PSC versus ECM blower motors, start components (capacitors, potential relays, start thermistors), contactors, sequencers, defrost boards, and lockout boards. A run capacitor's measured microfarad value should be within about 10% of the nameplate rating - always discharge it before testing and use a meter with a capacitance function. A bulged top or leaking body means replace regardless of reading.
Safety devices and code basics
Overcurrent protection, conductor ampacity, disconnects within sight of the equipment, grounding and bonding, GFCI protection near water, and minimum circuit ampacity on the nameplate are all fair game. The nameplate MCA and MOP dictate wire and breaker sizing, not the compressor RLA alone.
Worked example
A capacitor rated 35 µF measures 29 µF. That is 6 µF low - about 17% below rating, outside the ±10% tolerance, so replace it. Separately, a 24-volt circuit with a 12-ohm coil draws I = 24 ÷ 12 = 2 amps; knowing that tells you whether a blown 3-amp fuse is a symptom of a shorted coil.