Motors, Controls, and Power Distribution
Motor circuits
Motors are governed by their own article because starting current and overload protection differ from general loads. Know that branch circuit short-circuit and ground-fault protection is sized from the motor's table values, while overload protection is sized from the nameplate and protects the motor from overheating. Understand that the disconnecting means must be within sight in general, and that the controller is a separate requirement from the disconnect.
Controls and distribution
Know basic control circuits, start-stop stations, magnetic starters, and the difference between two-wire and three-wire control. Understand transformer connections, three-phase configurations, and how a motor starter's holding circuit maintains operation after the start button is released. Recognize common protective devices: fuses, circuit breakers, overload relays, and ground-fault protection.
Traps in questions
Common errors include sizing overload protection from the branch circuit device, assuming the disconnect must be the controller, and confusing wye and delta voltage relationships. Another trap: forgetting that a motor's nameplate current, not the table value, governs overload sizing.
Worked example
A question gives a motor nameplate current and asks about overload protection. Rule of thumb: overload equals nameplate current times the permitted percentage; short-circuit protection uses table values. Keep the two calculations separate and label them.
Practice approach
Draw ladder diagrams for start-stop control from memory, then compare to a reference and correct your own errors.