Dual run capacitor

For homeowners

The dual run capacitor is a sealed aluminum can on the outdoor unit that gives the two single-phase motors (compressor and fan) the electrical kick they need to spin. Single-phase AC motors can’t start on their own — they need a phase-shifted current pulse, and that’s what the capacitor provides.

It’s called “dual” because it contains two capacitors in one housing. One section feeds the compressor (typically 30 to 50 µF), the other feeds the condenser fan motor (typically 5 to 7.5 µF). Both sections share a common terminal labeled C. The other terminals are labeled HERM (for the hermetic compressor) and FAN.

Capacitors are one of the most common failure points in residential AC. They cost about $15 and take five minutes to replace, but a failing capacitor can quietly destroy a $1500 compressor if not caught. Annual maintenance should include a microfarad test on the capacitor.

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For technicians

The capacitor exists because single-phase AC power can’t start an induction motor on its own. AC induction motors generate torque from a rotating magnetic field, and a single sine wave doesn’t rotate — it just oscillates. To create rotation, you need a second magnetic field offset in phase from the first by 90°, with the start winding positioned 90° physically from the run winding. The current through the start winding has to lead the run winding’s current by a quarter cycle. A capacitor in series with the start winding does exactly that — capacitive current leads voltage by 90°, so the start winding’s current ends up phase-shifted from the run winding’s current.

“Run capacitor” means the cap stays in the circuit continuously, both during startup and during steady-state running. Some larger compressors also have a separate start capacitor wired through a potential relay — only in the circuit for the first fraction of a second during startup, then dropped out. Most residential split systems have only a run capacitor, no start capacitor.

The dual run cap exists because the outdoor unit has two motors that both need phase-shifting. Manufacturers could put two separate single capacitors in the unit, but combining them into one can saves space, labor, and parts cost. The two motors share a common point (the line side opposite their respective C terminals), which is why the dual cap has a shared C terminal — both internal sections connect to the same shared foil.

Capacitance values are sized to the motors. A condenser fan motor at 1/4 to 1/3 horsepower needs 5 to 7.5 µF. A 2-5 ton residential compressor (2-5 horsepower) needs 30 to 50 µF. Replace with the exact same values; a 35/5 in place of a 45/5 means the compressor is running with insufficient phase shift and will draw locked-rotor current trying to start.

Voltage rating matters and people get this wrong. Run capacitors are rated 370V or 440V in residential. A 440V-rated cap can substitute for a 370V-rated cap of the same capacitance because higher voltage rating is always safer. The reverse — putting a 370V cap where a 440V is specified — fails earlier because the cap operates closer to its dielectric breakdown point. The price difference is a couple of dollars; use 440V if you’re keeping a small replacement inventory.

How to test a capacitor. Pull the disconnect outside, wait two minutes for residual charge to bleed, then short the terminals to yourself one last time with an insulated screwdriver — across HERM-C and FAN-C — to be sure. Pull the wires off. Set your multimeter to capacitance mode (µF). Measure HERM-to-C — should read within 6% of the printed value. Measure FAN-to-C — should read within 6% of the printed value. Out of tolerance on either reading, replace the cap. Don’t try to test by clicking it across the meter on ohms mode and watching the needle swing — that’s a 1960s technique that tells you almost nothing useful about microfarad drift.

Failure modes, in order of how often you see them:

Capacitance drift. The most common failure. The dielectric film slowly degrades, the effective capacitance drops below tolerance, the motor it serves starts pulling more current and running hotter. The compressor or fan is still running but its bearings are taking abuse. You can have a cap that’s “still working” by the user’s standard — system runs, makes cold air — but is 25% below rated and quietly killing the motor. Annual maintenance includes a microfarad check for this reason.

Bulging. The cap is sealed and oil-filled, and internal gas pressure builds when the dielectric fails repeatedly. The top cap bulges outward — sometimes barely, sometimes dramatically. A visibly bulged cap is dead; it might still pass a microfarad test for a few days but it’s going to fail completely soon, often catastrophically. The score line on top is the engineered failure point — better that the cap vents from a controlled opening than ruptures from an unpredictable seam.

Oil leak. Less common in modern caps because the dielectric oil is no longer PCB (banned 1979 as a known carcinogen). Modern run caps use non-toxic oil. A leaking cap means the can is compromised — replace it.

Dead short. Rare but happens. The internal dielectric punctures all the way through and the two foils touch. The cap reads zero microfarads and zero ohms resistance. The motor circuit downstream sees a hard short on its start winding. Usually you’ll find this with a tripped breaker and a unit that won’t run.

Open circuit. Wire breaks loose from a terminal internally. Cap reads zero microfarads on that section. Equivalent to no capacitor at all in the circuit.

A capacitor failing kills the motor it serves if not caught. The motor sits there drawing locked-rotor amps trying to start, the internal thermal overload kicks it offline, it cools and tries again. Repeat cycles. Eventually the internal overload itself fails, or the bearings cook, or the start winding burns out. A $15 capacitor and a five-minute swap, missed for a month, becomes a $1200 compressor replacement.