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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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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.
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.
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:
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
Continue reading at HOW TO WIRE up a START CAPACITOR or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
(Feb 21, 2014) Anonymous said:
is it possible to rebuild a submersible well pump
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.
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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