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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Recently-asked questions & answers about how to select the proper start or run motor capacitor posted originally at HOW to CHOOSE a START / RUN CAPACITOR
Above/left: table of low, medium, and high voltage capacitor sizes or values mapped against electric motor size in KW.
On 2017-06-30 by (mod): how to choose a run capacitor if you know the start capacitor µF
There is no direct translation between an arbitrarily-specified 300 µF motor start capacitor and the required run capacitor for the same motor - of unspecified voltage in your question.
Electric motor "run" capacitors are typically in the 2 µF 70 or 80 µF range of capacitance and their voltage rating will be 370 Vac or 440 Vac. But that's a general remark not an exact specification.
If the run capacitor is too big or too small in µF rating, the result can be motor vibration, exceeding the motor's rated run temperature, motor vibration and noise, motor wear and thus reduced motor life, and more expensive motor operation.
Typical run capacitor sizes that will be in the “ballpark are
1/8hp motor: 4 to 5 µF
1/2 hp motor: 10 µF
1-2 hp motor: 10-15µF
3 hp motor: 20µF
5 hp motor: 40µF or more likley 30 µF
(Similarly, you will find tables matching motor size in kW and capactior value in µF for starting capacitors. But read on before slapping a capacitor you found in a toolbox or in a table of data.
When choosing the run capacitor for your specific motor, the µF you choose should match the run Capacitor rating on the motor's data tag. It's almost certainly going to be below 70 µF . But to repeat a key point: you need the manufacturer’s run capacitor rating to get the right capacitor.
There are rule-of-thumb equations for guessing at the run cap µF if you don't have the manufacturer's data tag, but those rely on measuring the motor when running under load - and can be in error depending on the existing motor and capacitor when the measurements are made.
So I cannot recommend that approach for choosing a new or replacement capacitor, even if the motor is running. An accurate measurement to choose the run cap size would require use of an oscilloscope and examination of the motor power phase to compare start vs run windings - not something a field technician is likely to do.
Another approach suggested by Lemau is to install a run capacitor of 30-50 uF/kW, measure the motor’s performance, and choose a subsequent capactor rating accordingly. Lemau offers a formulat ot calculate the capactor size that is a function of current, Hz frequency, applied voltage, a power factor, and a disspated power in watts.
Best bet: read the data tag, or if it's illegible, let's find the motor's manufacturer and model and then look up the specifications. When you have the motor's run-capacitor specification, typically the run capacitor you install can be +/- 6% (or by some writes +/- 5-10%) of that rating (according to what I've found by researching this question).
That's for capacitance. The voltage rating needs to be equal to or greater than the motor's voltage rating. Unlike the µF rating, a higher voltage rating won't cause the same troubles. A lower capacitor voltage means a shorter capacitor life. Industry standards for motor capacitor life and testing, including the Highly-Acclerated Life-Testing methods include the
We can dig out our file copies of these articles but they’re readily found online by a simple search as well.
On 2017-06-30 by israel
If the starting 300uf what would be the size of the run capacitor
On 2017-06-19 by Ravi Peiris
I am supposed to replaced a burned Stating Capacitor. Motor 230V/550K
If I use 105-140 Uf, will it work?
On 2017-06-12 by (mod) re: match the voltage rating of motors to capactitors during selection
The voltage rating of electric motor starting capacitors should be rated at about 1.5 x the line voltage supplied to the motor.
To me that suggests that your 330V cap is the right voltage.
Electric motor starting capacitors are typically 125, 165, 250 or 330 VAC and are in the 25 µF to 1,400 µF range in microfarads.
But watch out: I've read (not experienced personally) that a too-large capacitor can cause excessive start-up torque that can actually damage equipment.
The opposite problem: if a starting capacitor is too small then it won't be able to get the motor spinning: the motor may hum but not start. (Risking overheating and damage).
Electric motor "run" capacitors are typically in the 2 µF 80 µF range of capacitance and their voltage is usually 370 Vac or 440 Vac.
Keep in mind that many modern motors, especially smaller ones, use a combined Start/Run capacitor in one physical device with multiple connecting wires.
The voltage and MFD range of the run capacitor should be as close to factory spec as you can find. Typical capacitor tolerance is +/ 6% of the factory spec.
Now review the mfd specifications. The mfg of the capacitor should be close to the original - it is ok if it is off just a bit, say 5%.
What happens if we install a mis-matched capacitor:
1. them motor may not start and may be damaged
2. the motor may start with excessive torque and be damaged
3. the motor may run too slowly
4. the motor may overheat and be damaged
5. the motor may be noisy or may damage the equipment it operates
6. If the capacitor voltage rating is too small it will have a short life
Smart would be to check the data tag right on the motor as that should give the proper capacitor specifications.
The voltage rating of the capacitor should ALWAYS be equal to or greater than the motor's voltage.
Or find the manufacturer's tech support number and check with them.
Here's a nice capacitor explanation article that's geared to the non-engineer:
On 2017-06-12 by dayalan
I need to replace a start cap on a pressure cleaner
It currently has a 165 volt 324/389mfd cap
Or supply is 230 volt
Can I replace it with a 330 volt cap
Another cap is 330 volt 124/149mfd . Can I replace that with a 280 volt cap
On 2017-04-21 by Ted
I have some monster capacitors I wanted to know if they can be used to build a phase converter, Ronken 3.5 Mf 3000 volt AC internal resistor. and Ronken 2 Mf 3500 VAC. Both of theses caps are 3.375" x 2"x 9.875" high.
Thanks for any help.
On 2017-04-15 by (mod) re: typical capacitor uf rating for an A/C compressor
From a quick peek at some Hitachi catalogs finding models in the range of your Hitachi FL1152-SE compressor motor I see that it's probably a 1/6 hp. motor - in the 100 - 120 Watts range. Take that to the table 2 above and I see a 30/40 uf capacitor - something to check also with your electrical supplier.
I no success finding your Matsulovea Electric - compressor motor - Panasonic keep showing up as search results, in the 250W range -
On 2017-04-15 by Azmil
I just found 2 refrigerator compressor without any mark on it..without wires or capacitor..i already check all 3 leads which is run, start and common and test it and both still working great.
But now am having problem to choose which capacitor to use for each of it.
Here is the details i found on both compressor.
1: Hitachi FL1152-SE
Made by Hitachi
Its for Hitachi Refrigerator
2: Model DA57C11RAY5 (on top it shows DA57C11RAY51202b7)
I plan to use and turn both unit as Air Compressor.
Made by Matsulovea Electric Ind. Co. Ltd
Its for Panasonic Refrigerator
On 2017-04-13 by (mod) re: +-5% or +-10% of capacitor rating is an acceptable range
I'm not an EE but it seems to me you are perfectly within range. Some standards specify +/- 6 to -7%
On 2017-04-13 by Anonymous
I am looking at replacing a starting capacitor on my AC outdoor unit fan. The one in the unit is 330V 176-216MFD. Does the new one have to be exact match or will one +-5% or +-10% work just as well?
On 2017-04-13 00:33:33.655839 by Joe
Starting capacitor for 1 hp 4432 flo tech
On 2017-04-07 by Alejandro Theriot
I am in the process of building a flywheel energy generator system. Planning to use a 2HP electric motor to drive a 10 Kw generator. Am looking to use a PV system to start the electric motor and then by pass it once the generator is at opt operational conditions. Want to know what the technical specs of a supercapaictor should be? The PV system will be used to recharge the supercapacitor. My personal email is: firstname.lastname@example.org
I am in the process of building a flywheel energy generator system. Planning to use a 2HP electric motor to drive a 10 Kw generator. Am looking to use a PV system to start the electric motor and then by pass it once the generator is at opt operational conditions.
Want to know what the technical specs of a supercapaictor should be? The PV system will be used to recharge the supercapacitor. My personal email is: email@example.com
On 2017-03-28 by (mod) re:installing a higher -rated motor start capacitorBill,
On 2017-03-28 by Bill P
I have a Century Gould 1hp motor rpm 3450 -8150240-2 -13-6/6.8 -Thermally protected - 115/230 --Mallory Capacitor 139852-45 - 400-480-mfd-110vac -m235-7629k-04 - psu 110-125 vac -start capacitor 72-88 ,this is all numbers I can find.
The motor hums, use a rubber stick like to turn the compressor just a little or small nudge, very lightly. it starts up, shut it down, and it will start, next time I have to give it a nudge again, .
I went to WW Grainger, they gave me a new Capacitor (their part #2MDT9), it did the same, I ask if I could get a more electrical power, he said no, it seems it just needs a little more start power not much, what size can I get to work for me. the motor runs great when I get it started, no heat or any problems. Could you email me back with a Capacitor information ( numbers) so I can get one that will work.
On 2017-03-20 by (mod) re: capacitor for a 1.5 Hp submersible pumpAvinash
On 2017-03-20 06:16:15.947604 by Avinash PhatakI have a 1.5 Hp submersible pump in my bore well. What kind of capacitor is ideal for this motor. I have medium voltage available (around 230 - 250 V)
On 2017-03-10 14:42:06.886418 by (mod)Charles
On 2017-03-10 04:18:28.467824 by Charles ApplegateI need someone to look at my model 820030 1.5 hp motor and install a starting capacitor.
On 2017-02-03 17:50:10.879829 by Ed
I have a 1/3 hp motor on my furnace fan. The motor seized up so I got a new one. They suggested getting a new capacitor at the same time. The old capacitor was rated as 5 mfd. and would start/run the fan at three speeds, high medium and low.
Upon checking the low was never hooked up. When I got the new 1/3 hp motor and capacitor I see it can be hooked up to run four speeds and the capacitor they sent was rated at 7.5 mfd. I hooked the new motor up the same way as the old one using the two higher speeds and the old capacitor. Is this going to work? The fan seems to run OK.
On 2017-01-30 00:36:39.509389 by DannyThe run capacitor for my air compressor motor is gone so I have no reference Pinot to start with. How do I find out what size to buy?
On 2016-10-28 20:21:59.722493 by (mod) re: what happens when you install a bigger capacitorBigger capacitor gives more starting oomph for the motor. However too big, combined with a motor that's failing may simply result in an electrical burn-up.
On 2016-10-28 02:55:47.741689 by SteveIf I want to increase starting torque of motor do I need to increase or decrease the size of the start capacitor or is it fixed by the manufacturer
On 2016-10-20 14:49:47.581865 by AnonymousI have an old Craftsman wood lathe, 1/3hp motor needing a new start capacitor. The old is 70-78 MFD 110VAC and is a can 3 1/2x3 1/2x3/4" and mounted in the motor base. Can anything like this be had today.
On 2016-08-15 23:26:57.740472 by AnonymousVirendra, there will be hundreds of starting and start/run capacitor combinations; regardless of language all will be specified in microfarads, as you suggest.
On 2016-08-15 10:13:23.245802 by VirendraHai
On 2016-08-06 14:53:24.264776 by (mod) re: capacitor for a Bowling Green electric lawn mowerIt might be smartest to take the motor to a local electric motor company or to contact one rather than guessing; E.g. in Bowling Green, contact Covington Electric Motor Repair, 1-800-775-1071 as it's a fair guess that your motor's name reflects a Bowling Green company.
On 2016-08-05 01:39:06.768827 by wallacei have an old bowling green electric lawn mower which has been lying in the garage for a while. it is driven by a single phase/1.5hp/4.4A/240vac motor. it has a run capacitor of 20uf but the start capacitor is missing. how do i go about selecting the correct start capacitor for this lawn mower
On 2016-07-17 11:41:37.172112 by AnonymousI have a universal motor 420w 1300rpm .what run capacitor should i choose ???
On 2016-06-11 00:14:22.556290 by (mod)I'd call the manufacturer, or look on the old cap to see its manufacturer - that makes finding a replacement easier.
On 2016-06-09 21:58:51.928304 by Rick
I have a pool/spa pump and motor assembly that came with a CSC capacitor (10,000AFC, 25+10UF +-6%, 370 VAC/B, 50/60 Hz), It is round and is located on top of the horizontally mounted pump/motor assembly.
It was installed in Sept. 2013. I cannot find a replacement capacitor anywhere in the "pool equipment supply world" and I can't find CSC, I can find a few CSC capacitors available around the Internet (used or surplus) but not the capacitor this assembly was provided with. Would you have any advice? Thanks!
On 2016-04-30 19:16:56.794661 by (mod)John, the start/run cap may be and often is outside of the motor. BUt if your motor does not have a visible capacitor it may be a design that does not use one.
On 2016-04-30 15:35:00.952718 by johnhow do you figure the size of a cap--if there is not a cap in the motor---230v--1.5hp--1.5amps--1.23sf
On 2015-07-10 19:18:09.219060 by (mod)Desi,
On 2015-07-09 23:58:03.386660 by DesiHow can you tell if is a start or run capacitor?
On 2015-06-12 11:09:33.883250 by (mod)Mark
On 2015-06-12 01:00:52.258250 by marki have a 20" disk sander very hard start load i know for a fact it spikes to a 100a draw at start. its a 220v single phase 1750rpm, 1.5HP 10 amp, kw1.1. i been blowing start capacitors. i have no idea what start capacitor came with it the # where not readable. what it does randomly is: start then makes a lot of noise sounds and vibrates. i turn it off when it does this. normally it will make a click noise after its up to speed. im guessing this is when the start cap takes over. i have tried so far a 270-324 iuf and 415-500 iuf both worked for a bit then blew, both would randomly not disengage after it was up to speed. i changed the run cap too.
(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.
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.
(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.
(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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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.