Start Capacitor Global (C) InspectApediaElectric Motor Hard Starting Capacitor Wiring & Installation
Installation Wiring Guide to Air Conditioning Compressor Motor & Other Electric Motor Start-Boost or Run Capacitors

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How to hook up an electric motor start or run capacitor:

This article gives electric motor start-run capacitor installation & wiring instructions for electric motor capacitors designed to start & run an electric motor such as an AC compressor, heat pump compressor or a fan motor, and how to wire up a hard-starting air conditioner compressor motor, fan motor, to get an air conditioner, heat pump, refrigerator, or freezer compressor or other electric motor (such as a well pump) going.

This electric motor capacitor article series explains the selection, installation, testing, & use of electric motor starter start and run capacitors used on various electric motors found in or at buildings such as air conditioner compressors, fan motors, some well pumps and some heating equipment.

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How to Install and Wire Up an Air Conditioner Compressor, Blower Motor, or Fan Motor Hard Starting Capacitor

Make These Simple A/C Compressor Checks Before Adding a Hard-Start Capacitor

A/C motor resistances (C) D FriedmanMost electrical problems in air conditioning systems are in the compressors and their relays or motor overload switches.

In a single phase (common residential A/C) compressor you can verify with an ohmmeter whether or not the A/C compressor is bad.

[click to enlarge the image at left]

Watch out: live high voltage may be present at a capacitor, capable of giving a tremendous electrical shock even after electrical power has been disconnected at the equipment. Never work on live electrical equipment. Also

A fractional horsepower electrical motor should show different electrical resistance between the three terminals (Start, Run, and Common) as we illustrate just below.

Find the two highest resistance terminals.

The third one will be the common terminal.

Our example is for a Frigidaire compressor motor.

In our capacitor testing and wiring sketch at left, you note we use the letters S, C, and R to identify the usual terminals to which a start/run capacitor is wired. On many systems these terminals may be labeled so that the three leads on a start/run capacitor can be wired correctly:

  • S = start wire connector
  • R = Run wire connector
  • C = common connector

See How to INSTALL & WIRE Up an Air Conditioner Compressor, Blower Motor, or Fan Motor Hard Starting Capacitor.

Electric Motor run speed side note: Incidentally while most electric motors are marked with a data tag indicating the motor run speed (in RPMs) it's worth noting that the number of run coils is what determines the run speed of the motor. Two-coils marks a motor that runs at 3450 rpm (3600 rpm "nominal"), while 4 coils marks a 1725 rpm motor. (120V, 60 cycle/sec x 60 sec/min = 3600 rpm).

Electric motor diagnostic procedures are given in detail

Simple Relay and Hard Start Capacitor Wiring Instructions - Example 1

Hard start capacitor wiring (C) D Friedman Sealed Parts Co.

Relay and hard start capacitors such as the Starter Pow-R-Pak sold by Sealed Unit Parts Co., can be installed with no wiring changes to the original system whatsoever. Quoting from Part No. SPP-5, a relay and hard start capacitor sold by that company:

Connect the two wires from the SPP-5 in parallel with the [existing, already installed] run capacitor (one wire each side) without removing any original wires.

Use special "piggy back" terminal of the SPP-5 if all the run capacitor terminals are being used. [Install only on PSC units equipped with run capacitor.]

Simple Relay and Hard Start Capacitor Wiring Instructions - Example 2

wiring diagram for a motor starting capacitor

Here are some sample capacitor installation instructions for adding a motor starter capacitor to an air conditioning compressor motor - taken from the product package for a relay and start capacitor intended for use on a refrigerator or freezer. Similar starter capacitors are available for air conditioning compressors.

[Click to enlarge any image]

  1. Remove the power supply cord from the electrical outlet - in other words, be absolutely certain that electrical power has been turned off to the equipment being serviced.
  2. Remove the old starting relay, leaving the old overload protection in place.
  3. Push the wire with the one single pin terminal onto the "start" terminal of the air conditioning compressor. (See the wiring diagram above).
  4. Push the other wire with the pin terminal onto the "run" terminal of the air conditioning compressor.
  5. Connect the line from the old starting relay to the spade terminal on the "run" wire (insulating sleeve).
  6. Restore electrical power

Start / Run Capacitor Mounting Positions

Starting capacitors in place on an air conditioning compressor (C) InspectAPedia.comStart/run electric motor capacitors can be mounted in any direction or position. However there are some other capacitor mounting considerations that can affect capacitor life: basically you want to minimize the capacitor's exposure to vibration and heat.

As Afacp points out, ..."the temperature on the surface of the capacitor cannot exceed, even under the worst conditions, the maximum permitted temperature. It is advisable to make an experimental measurement of the temperature reached by the capacitor under the working conditions in the final application and after the thermal equilibrium has been achieved."[2]

Watch out: Air Conditioner Motor Starting Capacitor Safety warnings:

When testing a compressor, one must discharge the capacitor first! It'll otherwise have enough power stored on it to be at least very painful. (Author and others have been zapped!) Some systems will automatically discharge the capacitor, but shorting its leads [to ground] with a screwdriver (after verifying that the power's off) is a safe way to ensure that you won't get shocked. Motor starting capacitors can hold a charge for days!

If oil has leaked out of a capacitor: Don't touch any oil that leaked out: old capacitors may contain PCB oils, an extremely carcinogenic (cancer causing) material which require special disposal.

Once the capacitor has been discharged (as described just above), then it can be tested with a multi meter. Either use the meter's built in capacitor test function, or use this trick: Charge the capacitor by using the sense current the meter puts out when set to ohms. You should observe a rapidly rising resistance before the meter indicates over range/infinity. Disconnect the test leads, and switch over to volts. Then, reconnect the test leads. A voltage reading should be observed, approaching zero.

If the capacitor doesn't hold a charge, or the resistance reading never approaches infinity, it probably needs replacement.

Also, the capacitor may be defective if the compressor hums but does not start. Visual inspection may reveal it to be bulged, or have a blown out safety plug.

Start & Run & Dual Capacitor Specification References

Motor run capacitor by Dayton - oval, 2 terminal (C) InspectApediaAt left is a simple two-terminal run capacitor.

  • Essex, Brown: "Motor Repair Supplies" (Catalog), Essex Group, Inc., 1601 Wall St., Fort Wayne, Indiana 46801, Tel: 219-461-4633, Website:, retrieved 6/20/14, original source: Content/Products/MR%20Supplies%20Catalog-s.pdf - see pp. 86-89.
  • Sealed Unit Parts Co., Inc., PO Box 21, 2230 Landmark Place, Allenwood NJ 08720, USA, Tel: 732-223-6644, Website:, Email: info@supcocom, Supco Catalog, retrieved 6/20/14, original source: - see pp. 2-6.
  • van Roon, Tony, "Capacitors", [online article], retrieved 6/20/14, original source:, gives a very detailed history of the invention and history of electrical capcitors beginning with van Musschenbroek's Leyden jar in 1745. This article includes
    "Capacitor Nomenclature" by Dean Huster.
  • Kaiser, Cletus J., The Capacitor Handbook: A Comprehensive Guide For Correct Component Selection In All Circuit Applications. Know What To Use When And Where, 2d Ed., [at] C.J. Publishing (2011), ISBN-10: 0962852538, ISBN-13: 978-0962852534 - product description
    This book provides practical guidance and application information when using capacitors in electronics and electrical circuit design. This easy-to-use book covers the following capacitor types: Ceramic, Plastic Film, Aluminum Electrolytic, Tantalum, Glass, Mica, and others. This book also has a very comprehensive Glossary and Index. The Selection Guidelines and the Symbols and Equations sections have the answers to all of your daily application questions. This book is one in a series of component handbooks.

Examples & Sources of Start / Run Motor Capacitors

  • Packard 370V 45+5 MFD Round Run Capacitor
  • Supco™ 30+5 MFD 440V 440 Volt 50/60 Hz Round Motor Capacitor (typical application for Amana, Carrier, Rheem, Trane equipment)
  • Supco™ 45+5 MFD 440V 440 Vol 50/60 Hz Round Motor Capacitor (typical application for Amana, Carrier, Rheem, Trane equipment)
  • Supco HS6 (SPP6) Hard Start Relay Capacitor (230V) gives a starting torque boost of up to 500%. Quoting from product sales literature:
    The SUPCO SPP6 Super Boost hard start capacitor increases starting torque up to 500%. Features: Installs in seconds across run capacitor terminals Contains specially designed relay and large start capacitor for severe hard start problems. Applications: Room A/C units, Residential and commercial PSC A/C units and heat pumps, For all PSC A/C units from 4,000 thru 120,000 BTU's (1/2 thru 10 H.P.) Can be used on 120 thru 288 VAC units, For severe low voltage and hard starting compressors. SPP6 Specifications: Voltage: 115V - 230V. Increased Torque: 390 ounce inches. - retrieved 6/16/14 sales
  • Supco™ Universal (broad application) Capacitor, 10 MFD at 370V


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