What it is

A capacitor stores electrical charge and dumps it back into a motor winding to create the phase shift a single-phase motor needs. Three flavors show up on the truck, and mixing them up gets people in trouble:

  • Run capacitor — stays in the circuit the whole time the motor runs. It trims the motor's efficiency and torque continuously. Lower microfarad value, oil-filled metal can, built to handle continuous current and heat.
  • Start capacitor — only in the circuit for the first second of startup, then switched out by a relay. Big microfarad value, usually a black plastic case, NOT rated for continuous duty. Leave one in the circuit and it cooks.
  • Dual-run capacitor — two run capacitors in one can, sharing a common terminal. One section feeds the compressor, the other feeds the condenser fan. Saves space in a condenser.

This article is about telling them apart and sizing the replacement. For the step-by-step bench test, that's its own topic.

How it works

A single-phase motor can't tell which way to spin from one winding alone. It needs a second winding fed with current that's shifted in time. The capacitor provides that shift by charging and discharging out of step with the line voltage, so the current in the auxiliary (start) winding leads the current in the main (run) winding. That phase difference creates a rotating magnetic field and torque.

A run cap gives a modest, continuous shift — enough to keep the motor efficient and quiet while it runs. A start cap gives a big kick of phase shift to break the rotor loose against a load, then has to leave because that big shift would overheat the windings at running speed. The switching-out is done by a potential relay (voltage-sensing) or, on older gear, a current relay or PTC device.

In the field

Identify which type you're holding. Run and dual-run caps are metal cans, oval or round, with microfarad values typically from about 2 to 80. Start caps are black plastic, often cylindrical, with much bigger numbers — commonly 88 µF and up, printed as a range like "88–108 µF." If it's a range, it's a start cap.

Read the rating off the can. A dual-run is printed as two numbers, like "40/5 µF" — the bigger number (40) is the compressor/HERM section, the small one (5) is the FAN section. A single run cap shows one value. Both show a voltage: 370V or 440V.

Match microfarads exactly. This is non-negotiable on run caps. A 45 µF where a 40 belongs (or vice versa) throws off the phase shift, the motor loses torque, runs hot, and pulls high amps. Start caps tolerate the listed range — anything inside the printed band is fine.

Match or exceed the voltage rating. Voltage is a maximum, not a target. A 440V cap is a perfect replacement for a 370V cap. Never go lower — a 370V cap in a 440V spot will fail early.

Split a dead dual cap with two singles if that's what's on the truck. A 40/5 dual equals a 40 µF single (HERM to C) plus a 5 µF single (FAN to C), each at the same-or-higher voltage. Wire the two C legs together. It's a legit repair when you don't have the exact dual.

Normal values & targets

  • Good run cap reads within about ±6% of the printed value. A 45 µF cap is good from roughly 42.3 to 47.7 µF.
  • Common residential dual-run values: 35/5, 40/5, 45/5, 50/5, 55/5, 60/5 µF. Furnace/air-handler PSC blower run caps often 7.5–15 µF.
  • Condenser fan single run caps: commonly 5–10 µF. Compressor run caps: roughly 25–60 µF on residential.
  • Start caps: big — typical residential start caps run 88–108, 108–130, 130–162, 145–175, 189–227, 216–259, 270–324 µF bands. Always paired with a potential relay and usually a bleed resistor across the terminals.
  • Voltage ratings you'll see: 370V and 440V on run caps; start caps often 110V, 165V, 250V, or 330V depending on the application.

Common faults & what they mean

  • Run cap reads 0 / OL → open; motor buzzes or won't start.
  • Run cap reads low (e.g., 33 on a 45) → weak, out of tolerance, replace even if the motor still limps.
  • Domed, bulged, or oily top → internal pressure relief has vented; condemn regardless of the meter reading.
  • Start cap with no bleed resistor across it → it'll arc the relay contacts and shorten relay life; a 15,000–20,000 ohm, 2-watt resistor across the start cap terminals is standard.
  • Start cap blown apart / case ruptured → almost always a stuck-closed potential relay left it in the circuit; replace BOTH the cap and the relay, or you'll just blow the new one.
  • Repeated run-cap failures → a dragging bearing or hard-starting compressor is pulling current that bakes the cap. Fix the load.

Tech tips & gotchas

The single most common mistake is treating a start cap like a run cap (or the reverse). A start cap will not survive continuous duty, and a run cap doesn't have the microfarads to break a stiff compressor loose. They are not interchangeable.

When you add a hard-start kit, you're adding a start cap and a potential relay to a system that only had a run cap. The run cap stays — the start components work alongside it.

The 5 µF fan section of a dual cap seems trivial until it's the reason a perfectly good condenser fan won't spin. Always read both sections.

A start cap on the bench will often read fine because the fault is intermittent or relay-related. If a start cap blew, don't just replace it and leave — the relay that should have switched it out is the prime suspect.

Safety / code notes

Treat every capacitor as charged until you've bled it through a resistor — even a run cap holds a shock-capable charge with the power off. Don't make a habit of dead-shorting caps with a screwdriver; it pits the terminals and can hurt you. Replacement caps must meet or exceed the original voltage rating, and run-cap microfarads must match. The capacitor is part of the motor's listed circuit, so a like-for-like replacement keeps the listing intact. Dispose of old caps per local rules.