Switches, Receptacles, and Other Devices
Device questions test wiring logic, ratings, and protection features rather than deep theory.
Switching circuits
A single-pole switch breaks one conductor-always the ungrounded one. A three-way circuit uses two three-way switches joined by two travellers, with the common terminal of each connected to the supply and the load respectively; the travellers are interchangeable, which is why miswiring the common is the classic error. A four-way switch is inserted between the two three-way switches in a four-way run. Dimmers must be rated for the load type (LED, fluorescent, motor) and may require a neutral at the box.
Receptacles and protection
Standard receptacles are rated 15 A or 20 A; a 20 A receptacle may be supplied by a 20 A circuit, while 15 A receptacles are permitted on 20 A circuits in many configurations but not for a single dedicated outlet. Polarity matters: the grounded conductor to the silver terminal, the ungrounded to the brass. GFCI protection is required for personnel protection in wet and damp areas and is achieved either at the receptacle or upstream; downstream receptacles are protected only if wired to the load terminals. AFCI protection addresses arc faults in branch wiring. Tamper-resistant, weather-resistant, and special-purpose devices carry their own listing requirements.
Traps
Confusing line and load terminals on a GFCI, assuming a switch may be placed in the neutral, installing a dimmer on a motor load, and ignoring receptacle spacing or dedicated-circuit requirements.
Rule of thumb
For three-way switching, always identify the common terminal first; if both switches are wired common-to-traveller, the light will work only in some positions.