Capacitor Basics and Tech Tips

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Be sure to turn off all power supplies to the unit before making repairs.

Capacitors hold an electrical charge and present a shock hazard.  To discharge a capacitor, touch a tool with an electrically insulated handle across the terminals to dissipate the charge.  Capacitors should only be serviced by a trained and licensed professional in accordance with all national, state, and local codes.


A capacitor is a device that stores electrical energy and alters the flow of current to the windings of a single phase AC electric motor or compressor.  If a capacitor fails, the motor may not start, may hesitate, may not perform properly, and will overheat.
The capacitance of a capacitor is measured in microfarads, which is abbreviated as either uF or mfd.  There are two types of capacitors commonly used in HVAC equipment, run capacitors and start capacitors.


Due to changes in capacitor materials and manufacturing over the years there have been slight variations in case diameter. This is especially true if replacing an older plastic capacitor with a new metal case capacitor. If your capacitor mounts through a round hole bracket it may be necessary to enlarge the hole slightly with a file or grinder on some older units.  Please verify your capacitor's physical size before ordering a replacement.


Run capacitors are energized the entire time the motor or compressor is running and are designed for continuous duty.  Run capacitors normally have a metal or gray molded plastic case.  Typical microfarad ratings for run capacitors (uF or mfd) range from 3 to 100 microfarads.  Run capacitors also have a maximum voltage (VAC) rating.  The typical VAC ratings in HVAC applications are either 370 VAC or 440 VAC.  A capacitor rated at 440 VAC is more robust and can be used to replace a 370 VAC model provided that the microfarad rating is the same and the physical size and shape of the capacitor will fit in the mounting location.  A 370 VAC capacitor CAN NOT be used to replace a 440 VAC rated capacitor.

Run capacitors are available in single section types or dual section types.  A single section run capacitor has two terminal posts and one microfarad rating.  Single section run capacitors serve one motor or compressor.   A dual capacitor has two uF ratings assembled in one can, and has three terminals.  These are commonly designated as "40+5 uF" or "40/5 uF" on the rating label.  There is a common terminal (C), a fan motor terminal (F) and a hermetic compressor terminal (H or Herm).  Dual capacitors are used to serve both the compressor and fan motor in an outdoor unit.  If you are replacing a single rated compressor capacitor with two terminals, a dual capacitor can be used.  Simply hook up to the Common and Hermetic terminals and don't use the Fan terminal.

Run capacitors are either oval shaped or round shaped.  The shape has no effect on the performance of the capacitor.  If the uF rating is the same and the VAC rating is equal or higher than the existing model it will work.  The limiting factor is the physical size and mounting location.  If it is necessary to use a different physical size capacitor, a mounting strap can be made from perforated metal plumber's strap commonly available at hardware stores.


Start capacitors are energized momentarily on startup to provide additional starting torque, and then are disengaged after the motor or compressor are up to speed.  This is usually accomplished with a start relay.  Start capacitors are not designed to be energized continuously.  Start capacitors have a round black molded case and two terminal posts.  Start capacitors are rated for a given microfarad range, such as 189-227 uF, or 124-156 mfd.  The most common VAC ratings for start capacitors in HVAC equipment are normally either 250 VAC or 330 VAC.


Often a capacitor failure will be obvious, the case will be bulged or leaking fluid.  Some capacitors have a vent hole in the top to relieve any pressure upon failure.  If the capacitor appears to be physically intact, the capacitor should first be discharged then checked with a microfarad meter.  If a microfarad meter is not available, an ohm meter can be used to perform a basic check.  The ohm meter will “jump” when the probes are touched to the terminals and then drop back down to zero.  When the probes are reversed on the terminals the ohm meter will “jump” again.  There should be no continuity from any of the terminals to the capacitor case.  High amp draw, overheating, and tripping on internal overload may also indicate a defective or improperly sized capacitor.