CSLB C-20 HVAC ContractorControls, Electrical and SafetyHard
A technician replacing a failed dual run capacitor rated 45 µF for the compressor has only two single 25 µF capacitors in stock. If wired in parallel to substitute for the original capacitor, what is the resulting capacitance, and is it acceptable?
- A25 µF, since capacitors in parallel average their values
- B50 µF, which exceeds the ±6% tolerance for a 45 µF capacitor and is not recommended
- C12.5 µF, which is far too low and unacceptable
- D50 µF, which is within acceptable tolerance for a 45 µF capacitor
Show answer & explanationAnswer & explanation
Correct answer: B. 50 µF, which exceeds the ±6% tolerance for a 45 µF capacitor and is not recommended
Capacitors wired in parallel add their capacitance values: 25 µF + 25 µF = 50 µF. For a 45 µF capacitor with a typical ±6% tolerance, the acceptable range is about 42.3 to 47.7 µF. A substituted 50 µF value exceeds this range and could cause the compressor to run with excessive current draw or reduced efficiency, so it is not an acceptable substitute despite being the same general size.
Why the other options are wrong
- A. Parallel capacitors add together; they do not average.
- C. 12.5 µF would result from a series connection, not parallel, and doesn't match parallel wiring.
- D. 50 µF is outside the acceptable ±6% range, so this option incorrectly calls it acceptable.
Capacitors in Parallel
Capacitors wired in parallel have their capacitance values added together, unlike resistors, which is the opposite of how they combine in series.
- Parallel: C_total = C1 + C2
- Series: C_total = (C1×C2)/(C1+C2)
- Substituted capacitance must stay within manufacturer tolerance (commonly ±6%)
Memory trick: Parallel capacitors just add up, like teammates.