CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A technician measures a run capacitor rated 35 µF (±6% tolerance) and gets a reading of 28 µF on a capacitance meter. What should the technician conclude?
- AThe low reading means the meter, not the capacitor, is defective
- BThe reading indicates the capacitor is oversized and should be downsized
- CThe capacitor is weak and out of tolerance, and should be replaced
- DThe capacitor is within tolerance and can remain in service
Show answer & explanationAnswer & explanation
Correct answer: C. The capacitor is weak and out of tolerance, and should be replaced
A 35 µF capacitor with ±6% tolerance has an acceptable range of about 32.9 to 37.1 µF. A reading of 28 µF is well below this range, indicating the capacitor has degraded internally and lost capacitance, which reduces motor starting/running torque and should be replaced.
Why the other options are wrong
- A. There's no indication the meter is at fault; the reading pattern matches typical capacitor aging.
- B. Capacitance readings don't indicate oversizing; a low reading indicates degradation.
- D. 28 µF falls outside the ±6% acceptable range of 32.9–37.1 µF.
Capacitor Tolerance Testing
Run capacitors are rated with a manufacturer tolerance (commonly ±6%); a measured capacitance outside this range indicates a failing capacitor.
- Calculate range: rated µF × (1 ± tolerance)
- Weak capacitors cause hard starting, overheating, tripped overloads
- Always discharge a capacitor safely before testing
Memory trick: Out of range, time to change.