What it is

A run capacitor is a small can wired to the compressor and fan motor that gives those motors the phase shift they need to run efficiently. A dual run capacitor packs two capacitors into one can — one section for the compressor (HERM) and one for the condenser fan (FAN) — sharing a common terminal (C). When a cap weakens, motors start hard, run hot, draw high amps, or won't start at all. Testing it is a two-minute job that prevents a lot of misdiagnosed "bad motor" and "bad compressor" calls.

How it works

A single-phase motor needs a second, phase-shifted winding to get going and to run smoothly. The run capacitor creates that shift by storing and releasing charge slightly out of step with the line voltage, so the start (auxiliary) winding's current leads the main winding's current. That phase difference is what produces a rotating field and torque. As a cap ages, its internal capacitance drifts down (or it shorts/opens), the phase shift degrades, and the motor loses torque and efficiency. Microfarads (µF, sometimes written MFD) is the unit that measures that storage capacity, and it's what you compare against the printed rating.

A dual cap is just two of these in one housing. The common terminal (C) is one shared plate connection; HERM ties to the compressor's start winding and FAN to the fan motor's start winding. The C terminal usually carries the line-side feed.

In the field

Discharge it first — every time. A run cap can hold a dangerous charge even with the power off. Lock out the disconnect, then bleed each section through a resistor (a 20,000-ohm, 2-watt resistor across the terminals is the safe method). Don't dead-short it with a screwdriver as a habit — that arcs and can damage the cap and your nerves.

Identify the terminals. On a dual cap you'll see C, HERM (or H), and FAN (or F). The rating is printed on the can, usually as two numbers like "45/5 µF" — 45 for the HERM/compressor side, 5 for the FAN side, both at a voltage rating (370V or 440V).

Read it. Pull the wires off, set your meter to capacitance (µF). Measure HERM to C for the compressor section and FAN to C for the fan section. Compare each reading to its printed number.

Inspect it. A bulged top, leaking oil, or rust is an automatic condemn even if the numbers look okay — it's on its way out.

Normal values & targets

  • A good capacitor reads within ±6% of its printed rating. A 45 µF section is good from about 42.3 to 47.7 µF; a 5 µF section is good from about 4.7 to 5.3 µF.
  • Common residential dual caps: 35/5, 40/5, 45/5, 50/5, 60/5 µF. Single run caps for fan motors often run 5–10 µF; compressor run caps commonly 25–60 µF.
  • Voltage rating is a maximum, not a target — a 440V cap is a fine replacement for a 370V cap (higher is okay), but never go lower.
  • A cap reading below the minus-6% line is weak even if the motor still runs; replace it before it strands you on a hot day.

Common faults & what they mean

  • Reads 0 µF / OL on capacitance → open capacitor; the motor won't start or will only buzz.
  • Reads way over the rating or shows a short → failed cap, replace it.
  • Reads, say, 38 µF on a 45 µF section → weak; out of tolerance, replace.
  • Top is domed/bulged or oily → internal failure brewing; condemn regardless of reading.
  • Fan runs but compressor won't start (or vice versa) on a dual cap → only one section failed; the cap is still bad, replace the whole can.

Tech tips & gotchas

If you don't have a capacitance meter handy, you can confirm a totally dead cap by amp-clamping the start winding lead, but the right tool is a meter that reads µF — guess less, measure more.

Match microfarads exactly and match or exceed the voltage rating. Slapping a 40 µF in where a 45 belongs leaves the motor down on torque and running hot. The 5 µF fan side seems trivial, but a dead 5 µF section is why a perfectly good condenser fan won't spin.

A "spinning it up with a stick" condenser fan that then keeps running is the classic dead-start-cap symptom — the motor can run but can't start itself. Don't leave it like that; replace the cap.

Replace a swollen cap even if it still meters in range. The bulge means the internal pressure relief has worked; failure is imminent.

When a cap fails repeatedly, suspect the load that's stressing it — a dragging fan bearing or a hard-starting compressor pulls high current that bakes the cap. Fix the root cause or you'll be back.

Safety / code notes

Treat every capacitor as charged until you've bled it through a resistor — this is the most common shock source in routine AC service. Replacement caps must meet or exceed the original voltage rating and match the microfarad value; the run capacitor is part of the motor's listed circuit. Dispose of old caps per local rules — older units may contain oils you don't want in the trash stream.