Q
Run capacitor
✂ — — — — — — — — — —
A
Stays in the circuit continuously to improve the running efficiency and torque of a single-phase motor by shifting the phase of current in the start winding.
Q
Start capacitor
✂ — — — — — — — — — —
A
Provides an extra torque boost only during startup, then is switched out of the circuit (often by a relay) once the motor reaches a set speed -- much higher capacitance than a run capacitor.
Q
Dual run capacitor
✂ — — — — — — — — — —
A
A single can housing two capacitors (commonly for the compressor and the condenser fan motor) sharing a common terminal -- marked with distinct microfarad (uF) ratings for each.
Q
Contactor
✂ — — — — — — — — — —
A
An electrically controlled switch that uses a low-voltage coil (commonly 24V from the thermostat circuit) to close high-voltage/high-current contacts feeding the compressor and condenser fan.
Q
Potential relay
✂ — — — — — — — — — —
A
Uses back-EMF (induced voltage) from the start winding to sense motor speed and disconnect the start capacitor once the motor is up to speed -- common on larger single-phase compressors (PSC/CSR systems).
Q
MFD / uF (microfarad)
✂ — — — — — — — — — —
A
The unit of capacitance printed on HVAC capacitors -- replacement capacitors should match the original MFD rating within the tolerance printed on the label (often +/-6%).
Q
Compressor overload protector
✂ — — — — — — — — — —
A
A thermal/current-sensing device (internal or external) that opens the compressor circuit if winding temperature or current gets too high, protecting the windings from a locked-rotor or overheating condition.
Q
MCA (Minimum Circuit Ampacity)
✂ — — — — — — — — — —
A
The nameplate value used to size the branch-circuit conductors feeding HVAC equipment -- always larger than the equipment's actual running current to account for multiple motors and safety margin.
NEC 440.4, 440.32
Q
MOCP (Maximum Overcurrent Protection)
✂ — — — — — — — — — —
A
The nameplate value giving the largest breaker or fuse size permitted to protect the HVAC circuit -- pick a standard size at or below this value.
NEC 440.22
Q
HVAC disconnect location requirement
✂ — — — — — — — — — —
A
A disconnecting means must be within sight of and readily accessible from the air-conditioning or refrigeration equipment.
NEC 440.14
Q
PSC motor (Permanent Split Capacitor)
✂ — — — — — — — — — —
A
A single-phase motor design using a run capacitor permanently wired in series with the start winding -- no start capacitor or start relay needed. Common in condenser fan motors.
Q
Locked-rotor amperage (LRA)
✂ — — — — — — — — — —
A
The current a motor draws the instant it starts, before it reaches running speed -- much higher than run-load amperage (RLA), and used to size starting components and check voltage sag.
Q
RLA (Rated Load Amperage)
✂ — — — — — — — — — —
A
The current a compressor draws under normal running/rated load conditions -- used for troubleshooting against nameplate values, distinct from the code-driven MCA figure used for wire sizing.
Q
Crankcase heater
✂ — — — — — — — — — —
A
A heating element that keeps refrigerant from migrating into and pooling in the compressor oil during off-cycles in cold weather, preventing oil dilution and a slugging/liquid-flooding startup.
Q
Low-voltage control circuit (24V)
✂ — — — — — — — — — —
A
The thermostat-to-equipment control wiring, stepped down by a transformer from line voltage -- carries the signals (Y, W, G, R, C) that energize contactors and relays rather than the load current itself.