Electric Motor Starting Capacitor Selection
Installation Guide to Air Conditioning Compressor Motor & Other Electric Motor Start-Boost or Run Capacitors
HOW to CHOOSE a START / RUN CAPACITOR - CONTENTS: how to choose & buy an electric motor start capacitor, hard start capacitor, or run capacitor that is properly rated for and matches the requirements of the electric motor such as an AC compressor motor or fan motor where the capacitor is to be installed.
POST a QUESTION or READ FAQs about installing a hard-start capacitor to get an air conditioner motor, fan motor, or other electric motor running.
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Electric motor start-run capacitor selection guide: this article explains how to choose & buy an electric motor start capacitor, hard start capacitor, or run capacitor that is properly rated for and matches the requirements of the electric motor such as an AC compressor motor or fan motor where the capacitor is to be installed.
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.
These electric motors use a capacitor to start and run the motor efficiently. We explain the choice & wiring procedures for a hard start capacitor designed to get a hard-starting air conditioner compressor motor, fan motor, refrigerator, or freezer compressor or other electric motor (such as a well pump) going.
How to Find, Choose, & Buy a Replacement Electric Motor Starting Capacitor
The best option if you are replacing a starting capacitor or a start/run capacitor is to match the existing device on your system.
Capacitor energy ratings are expressed in farads - the amount of electrical power stored in the capacitor, where uf = microfarad (10-6 farads) and is the same as mfd (microfarads) written in other references.
Capacitors are also rated for the voltage range in which the capacitor can safely operate, such as 220V or 440V.
You can substitute capacitors on a cooling system but the substitute capacitor must be able to handle the voltage. For example you can't sub a 110V-rated capacitor into a 220V system.
Depending on the application, the micro-farad range of starting capacitors varies according to the motor size. Run capacitors typically range in micro-farads from 1.5 to 50uf. Start capacitors typically range from 20-30 uf up to 250-300 uf. The example capacitor charts (left) are adapted from AFCAP. 
You can also check a capacitor to compare its performance with its microfarad rating by using an ohmmeter. In a working capacitor ohms will build-up and then fall off (when the capacitor discharges).
If you reverse the + and - leads of the DC ohmmeter. leads it will repeat. If you do not see any resistance in the capacitor then it has an internal short and it's shot - you need a new one.
HVAC suppliers sell general-use starting capacitors that are intended for use across a range of electrical motors and motor sizes.
But at least some industry sources (the Sealed Unit Parts Company or Supco) make a more careful argument explaining that it's best not to install a significantly oversized starting capacitor on an electric motor. According to Supco, [quoting]
If the start capacitor is too large for the application, the capacitor can actually mask the developed voltage in the start windings and keep the start capacitor in the circuit continuously. .... The ... run-start voltage is suppressed below the trigger voltage of the start device. As a result, the start capacitor remains in the circuit as the motor runs.
A secondary, fail-safe method is necessary to ensure that the start device is ultimately removed from the circuit.
... A start device that fails to remove the start capacitor from the circuit has the potential to cause premature failure of the start windings in the compressor. ... If the capacitor is never removed from the start windings, premature winding failure could occur. As such, care should be taken when selecting capacitor sizes for an application.
Care should also be taken regarding products that tout a "bigger capacitor is better" approach to compressor starting. SUPCO E-Class devices provide a secondary timing safety device to ensure that the start capacitor is dropped from the circuit in a fail-safe mode. [Italics are ours-Ed.].
Reader Question: distributor says the order of numbers on the capacitor doesn't matter
6/16/2014 Danno said:
I'm replacing the capacitor in my AC condenser. The original/stock unit says 35/5 440 AC. This is the cap I ordered from a distributor, but upon receiving it, the box says "5/35 440" (the 5 and 35 are transposed. Distributor says its the same thing. Is this correct? Thanks for any 411
Danno I think the scrambled order is not an issue as long as the key numbers on the capacitor match its application or match the old one AND as long as you connect the proper wires.
Example start/run capacitor markings
(NOT specifying your particular capacitor)
Voltage, e.g. 110-125V
Capacitance, e.g. 189-227 uF or micro Farads or MFD (Start capacitors usually are over 60 uF)
Capacitance, e.g. for a dual run capacitor: 35/5 refers to 35 MFD (for a compressor) and 5 MFD (for a fan motor)
Temperature range, e.g. -40 - + 65C
Cycles, e.g. 50-60 Hz
What may be important also in capacitor selection (see our article above for details) is the distinction between
Start capacitor - just used to get the motor spinning from a stop
Run capacitor - keeps the motor spinning
Dual capacitor - combines two capacitors in a single physical device, one to run a higher-amperage motor such as the compressor in an air conditioner compressor/condenser unit, and a second smaller capacitor to run a smaller motor such as the cooling fan motor in the compresor/condenser unit.
Decode a 35/5/440 marking on a motor starting capacitor
Reader Mark (18 June 2014) has given us a helpful and detailed translation of the markings found on start capacitors, repeated here:
Run capacitors: I always was taught that 35/5 was a [dual] run capacitor. 35 is the microfarad rating for the compressor & 5 microfarad was for the fan. (three terminals) not:
Decode a 35/5/440 marking on a motor starting capacitor:
The capacitor you are describing marked 35/5 440 is probably a dual run capacitor.
35/5 : The first two numbers are the capacitance 35 uF (for the compressor) and 5 uF (for the fan motor).
A dual "run capacitor" supports two electric motors, such as in large air conditioner or heat pump units, with both a fan motor and a compressor motor.
It saves space by combining two physical capacitors into one case. The dual capacitor has 3 terminals labeled
"HERM", which stand for the Common, Fan, and HERMetically sealed compressor.
440 refers to the ability to run voltages up to 440VAC
Dual run capacitors come in a variety of sizes, depending on the capacitance (µF), such as 40 plus 5 µF, and also the voltage. (Be sure to connect properly to compressor motor, fan motor and common)
A 440 volt capacitor can be used in place of a 370 volt, but not a 370 in place of a 440 volt.
The capacitance must stay the same within 5% of its original value.
Round cylinder-shaped dual run capacitors are commonly used for air conditioning, to help in the starting of the compressor and the condenser fan motor.
An oval dual run capacitor could be used instead of a round capacitor, but the mounting strap should be changed to better fit the oval shape
Watch out: Take a look at the wiring sheet that should be with your capacitor or wiring marked on the device itself to be sure that you are properly connecting the Start and Run wires and you'll be OK.
As we noted in the article above, the dual capacitor terminals may be labeled so that the three leads can be wired correctly:
S = start wire connector
R = Run wire connector
C = common connector
Start & Run & Dual Capacitor Specification References
Essex, Brown: "Motor Repair Supplies" (Catalog), Essex Group, Inc., 1601 Wall St., Fort Wayne, Indiana 46801, Tel: 219-461-4633, Website: www.superioressex.com, retrieved 6/20/14, original source: http://www.essexbrownell.com/uploadedFiles/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: www.supco.com, Email: info@supcocom, Supco Catalog, retrieved 6/20/14, original source: http://www.economicelectricmotors.com/cdrom/catalogs/Supco_catalog.pdf - see pp. 2-6.
van Roon, Tony, "Capacitors", [online article], retrieved 6/20/14, original source: http://www.sentex.ca/~mec1995/gadgets/caps/caps.html, 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 Amazon.com] 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 Amazon.com sales
Supco™ Universal (broad application) Capacitor, 10 MFD at 370V
Yes ... maybe; it depends on the condition of the pump casing, parts, an in my opinion, an accurate diagnosis of the trouble; at some point rebuilding is not cost effective. Indeed there are specialists (H Shreck in Poughkeepsie used to be one of them) who rebuild electric motors of all kinds.
There are also many water pumps whose design makes it quite reasonable to replace pump impellers.
So to answer your question, I dunno - it depends on what's broken.
Question: fan will start but won't keep running.
(June 1, 2014) Anonymous said:
The fan on my Conquest 80 will not stay on (interior fan). It starts and then stops after a few seconds. It was installed in 2005. Help.
The start capacitor is for getting a motor started, not keeping it running.
Often motors have two windings, a start winding and a run winding. Your motor's run winding may be damaged.
Or your fan motor may require a dual capacitor (start and run) or a separate run capacitor to keep it spinning.
Or your system may have a faulty control.
Question: will a hard start capacitor reduce current drawn and stop tripping a breaker
(June 7, 2014) Joel said:
I have a commercial hood & exhaust fan (120v) running in a food truck that is used for catering. We occasionally need to run off a generator (3000w) and we've found that the fan cause the generator overload to trip. The fan has been tested and observed to draw 8 amps running at full speed. Given this, would it be possible to add a hard start kit, such as SUPCO SPP4E? Will need to confirm the motor size, just wondering if this would help.
The total draw of all items is less than 20 amps and the generator is capable of supplying 25 amps constant.
Thanks for the advice.
Joel I think the problem lies elsewhere and needs some further diagnostics. I suspect that your total current draw is exceeding the ability of the generator - you may be running more than the fan, such as lighting, a cooler, toasters, other electrical appliances. If it were just the fan, drawing 8A, it has no business tripping the breaker.
Put another way if the problem is the fan and there are NO other appliances running, then there is a failing fan motor drawing high current, or an electrical short circuit or other unsafe condition to find and fix.
A start capacitor or a run capacitor won't change the current drawn by the motor.
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Supco, Sealed Unit Parts Company, PO Box 21, 2230 Landmark Place, Allenwood, New Jersey, 08720, Tel: 732-223-6644, 201-449-3300, email: firstname.lastname@example.org, provided the compressor starting capacitor and packaging information (purchased by the author from an air conditioning parts supplier in New York) - our example uses a Sealed Unit Parts Company Solid State part No. RSC 10 115V starting capacitor which was designed for installation on refrigerators and freezers. See www.supco.com/
 "The E Class Advantage", Supco (op cit), describes the company's advanced start/run capacitor products. Web search 08/04/2011, original source: http://www.supco.com/eclassadvantage.htm Quoting from that article:
The SUPCO E-Class Series comprise the most advanced developments in start device technology:
1. Voltage sensing technology that monitors for motor start (current sensing devices require internal fuse protection).
2. A 2-wire connection that simplifies installation
3. A secondary timing circuit that ensures that the capacitor is not permanently left in the start winding circuit
4. A fully electronic device - minimizing the limitations of mechanical devices and secondary fusing associated with triac devices
5. A start device matched with an appropriately sized capacitor to cover the range of compressors for the intended application (one size does not fit all)
The use of compressor start devices results from a need to ensure that a compressor (usually air conditioning) will start under voltage conditions that are less than ideal. As discussed, several options exist in the market to address compressor start concerns. Start devices exist in many forms for specific applications. SUPCO provides a full range of products in all relevant technologies to effectively match the proper start device to the application. Care should be taken to utilize a device that meets the requirements of the job. Extra caution should be observed when employing the "one-size-fits-all" and "a bigger capacitor is better" approach to applying a start device. Consult SUPCO, a manufacturer with a complete product range, to ensure the greatest success in the start device application.
 "Motor Start and Run Capacitors", AFCAP (African Capacitors Limited), web search 08/05/2011, original source: http://www.afcap.co.za/manual/Part2.pdf
George Fazio, reader, contributed comments on failed starter capacitor diagnosis by noting the bulged capacitor ends. 09/25/2009
Troubleshooting Compressor Problems," Henry Puzio, Fuel Oil & Oil Heat with Air Conditioning Magazine, June 1993, p. 39
Tom Morris, Engineer, capacitor discussion and correction to the original data. Email to D Friedman 5/29/2006 - Thanks Tom for critical editing. The text
above explaining about capacitors was suggested by Mr. Morris. The original text of the 1993 compressor diagnosis article had the resistance explanation backwards.
Thanks to reader Diane McGivney for asking about air conditioner compressor motor starting capacitor costs and typical air conditioner service call fees - (May 2010)
Thanks to reader James Oiler for reporting on the replacement of a heat pump starter capacitor, August 2010.
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