Electrical Systems and Controls Flashcards
7 cards from real 313D practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Systems and Controls flashcards as text
A residential thermostat's R terminal is energized and the Y terminal is calling for cooling, but the contactor does not pull in. Voltage measured at the contactor coil terminals is 0V. The MOST likely cause is:
Answer: An open safety switch or broken wire in the Y control circuit
Zero volts at the coil means the 24V signal never reached the contactor, pointing to an open in the control circuit such as a tripped safety switch.
Which NEC article covers the installation of air conditioning and refrigerating equipment?
Answer: Article 440
NEC Article 440 specifically addresses air conditioning and refrigerating equipment, including motor-compressor circuit requirements.
What does the minimum circuit ampacity (MCA) rating on an AC unit nameplate tell the installer?
Answer: The minimum wire size required to safely supply the unit
MCA specifies the minimum ampacity the supply conductors must have, ensuring wiring can handle continuous operation without overheating.
A residential outdoor unit has a maximum overcurrent protection (MOCP) rating of 35A on its nameplate. The installer should use a breaker that is:
Answer: 35A or less
The MOCP is the maximum allowable breaker size; you may use any standard size up to and including 35A.
Which tool is used to verify that a 240V circuit is correctly wired with both hot legs and a ground, without a neutral?
Answer: Non-contact voltage tester on each leg and a multimeter leg-to-leg and leg-to-ground
You confirm 240V leg-to-leg and ~120V from each hot leg to ground using a multimeter; a non-contact tester confirms both conductors are energized.
When a compressor is tested and found to have equal resistance between all three terminals (C, S, R), this indicates:
Answer: The compressor windings are internally shorted
In a healthy single-phase compressor, R-to-S resistance should equal C-to-R plus C-to-S; equal readings on all three terminals indicates a winding short.
What is the purpose of the crankcase heater on a residential compressor?
Answer: To keep the oil warm and prevent refrigerant migration into the crankcase during off cycles
Refrigerant migrates into cold compressor oil during off cycles; the crankcase heater keeps oil warm to prevent refrigerant absorption that could cause oil dilution and compressor damage on startup.