Electrical Systems, Controls, and Motors
Power, Protection, and Control Circuits
Start with fundamentals: Ohm's law, power relationships, single-phase versus three-phase distribution, conductor ampacity from the applicable table, termination temperature ratings, overcurrent protection, grounding and bonding, and voltage drop, where the common guideline is 3 percent on a branch circuit and 5 percent total. On equipment nameplates, read RLA or FLA, LRA, minimum circuit ampacity, and maximum overcurrent protective device. Minimum circuit ampacity is generally 125 percent of the largest motor load plus the sum of the other loads; the branch-circuit conductors must be at least the MCA, and the overcurrent device must not exceed the MOP. A disconnecting means must be within sight of the equipment.
Control circuits are typically 24 volts derived from a transformer, with thermostat terminals for R, C, W, Y, G, O, and B depending on configuration. Safety devices, including high-pressure and low-pressure switches, float switches, freeze stats, and overtemperature limits, are wired in series so any one opens the control circuit. Motors are commonly PSC, constant-torque ECM, or variable-speed; know how to test run and start capacitors with a capacitance meter, verify microfarad rating, check amp draw against nameplate, and confirm rotation on three-phase equipment. Before servicing, lock out and tag out, then verify the circuit is de-energized with a meter.
Worked example: compressor RLA of 14.5 amps with a condenser fan FLA of 1.2 amps gives MCA of 1.25 times 14.5 plus 1.2, or 19.3 amps. Conductors must be at least that size, and the nameplate MOP sets the largest permitted breaker.