Starting capacitors in place on an air conditioning compressor (C) InspectAPedia.comElectric 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.

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How to Find, Choose, & Buy a Replacement Electric Motor Starting Capacitor

Typical start capacitor values by motor size - AFCAP

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 Ratings

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. [2]

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)

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


"FAN", and

"HERM", which stand for the Common, Fan, and HERMetically sealed compressor.[5]

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.[2]

The capacitance must stay the same within 5% of its original value.[2]

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.[2]

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:

Start & Run & Dual Capacitor Specification References

Examples & Sources of Start / Run Motor Capacitors

Dayton dual motor run capacitor, round (C) InspectApedia


Continue reading at HOW TO WIRE up a START CAPACITOR or select a topic from the More Reading links or topic ARTICLE INDEX shown below.

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