Starting capacitors in place on an air conditioning compressor (C) InspectAPedia.comElectric motor starting & run capacitor troubleshooting, selection, installation & repair
Installation Guide to Air Conditioning Compressor Motor & Other Electric Motor Start-Boost or Run Capacitors

  • CAPACITORS for HARD STARTING MOTORS - CONTENTS: How to install & use air conditioner compressor motor starting & run capacitors including hard-starting motors. Use of starting capacitors on electric motors for air conditioning, pumps, etc. Start or Run Capacitor Diagnostic Checks: How to Use a VOM or Multimeter to Test a Motor Starting Capacitor. How to fix or replace a hard-starting air conditioner compressor or other hard starting electric motor. How to inspect, test, or install an air conditioner motor starting capacitor. Warnings about remaining life of hard-starting air conditioner or heat pump compressor motors
  • 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 instructions:

Tthis 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.

Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. If you observe this we suggest that to avoid damage you turn off the system while waiting for repairs.

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Air Conditioning Compressor or Other Electric Motor Starting Capacitors

Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.Capacitors are electric devices that get an electric motor running at start-up by providing a "jolt" of stored electrical energy, or that help keep a motor spinning once it has started.

The starting capacitor helps a motor start spinning by creating a high-torque, rotating, electrical field in the motor. In many electric motors there are actually two capacitors, one boosting the start winding (the start capacitor) and a second that remains in the circuit while the motor continues running (the run capacitor).

If the start capacitor has failed the symptom is that the motor won't start. If either or both start and run capacitors are defective the motor may try to start but will hum and won't keep running. You may hear a compressor or fan motor humming or observe that it's getting hot.

Watch out: If you observe a humming electric motor that is not starting we suggest that to avoid damage you turn off the system while waiting for repairs.

Normal start & run capacitors on compressors, fan motors, other electric motors

Air conditioner compressor motors (and lots of other electrical motors) that run on two-phase (220V) or single phase (120V) electrical power usually include a capacitor in the start circuit to help get the motor spinning; a capacitor can be put into the "run" circuit of the motor as well to increase motor efficiency.

Starting capacitor: the starter capacitor gives extra torque or boost to get a motor spinning in the right direction by providing about double that nominal system voltage.

Run capacitor: once the motor has started, a run capacitor may be used to help the motor retain full power, providing 1.5 x the nominal system voltage and varying as needed depending on the load on the motor.

In some old-school class views adding a run capacitor is similar to making two-phase out of one-phase electricity and is a common practice on air conditioners. What these folks really are saying is that "With capacitive reactive power of about 75% of the nominal power of the motor, the comparison of power is slightly lower than that of a three-phase motor of equal size." [2]

Technical notes: Single phase electric motors such as those used in air conditioner compressors and fan motors contain two different stator windings: an auxiliary starter winding fed by an electric capacitor and a main run winding fed by the principal electrical circuit. The auxiliary starter winding is used to provide an extra boost (and proper direction) to get the motor spinning at start-up. (Three phase electric motors do not use start/run capacitors.)

Electric motors that use start/run capacitors may be PSC (permanent split capacitor) and CSR / CSCR (capacitor start, capacitor run) designs. Unlike a PSC motor, a CSR/CSCR motor must also have a starting relay that will cut the start capacitor out of the electrical circuit once the motor has gotten up to run speed. The run capacitor then remains in the circuit to keep the motor spinning properly.

What are Hard-start start/run capacitors or "booster" capacitors?

Photograph of  This new compressor was placed directly against
a brick wall. One third of its condenser coil cooling ability was blocked.If your air conditioner has stopped running, a possible problem is the failure of the starting capacitor found on the outside compressor/condenser unit. If that unit has electrical power but the compressor and/or its cooling fan are not running, one of the components to check (and that is easy to replace) is the starter.

When an electrical motor is having trouble starting, such as an air conditioning compressor motor, blower motor, a refrigerator motor or a freezer motor, or even a fan motor, the repair technician may install a simple and inexpensive hard-start capacitor.
See HARD STARTING COMPRESSOR MOTORS for complete details about choosing & wiring a hard-start capacitor.

The starting capacitor is a simple electrical device which can give an extra voltage jolt or "boost" to get the hard-starting motor spinning.

The starting capacitor in our page top photo is oval in cross section, but most replacement and many newer air conditioner motor starting capacitors are simply cylindrical in shape (photo at left).

Field report: describes diagnosing & replacing a bad starter capacitor on a heat pump that had stopped working

Start Capacitor Global (C) InspectApediaOur Heat Pump was new in 2002. It is a 3 ton unit ProSeries brand by Service Masters. We just had a big birthday party at our house for my 1 year old daughter. 95 degrees outside. House full of Aunts, Uncles, friends, Great Grandparents.

We got through that before the failure. Next morning it is just real gooey in our house. I can feel the blower fan is on. No cool air at all. I had to go to work so I left the house. Come back home in the mid afternoon to find house is hotter than before and my wife reports that the air hasn’t been cooling at all. It’s around 86-88 degrees in the house.

I check the little screw in fuse on the side of the blower unit. Hoping to find a $2 fuse blown. Nope. Fuse looks fine. Getting a little sense of panic because I remember how expensive our last service call cost for this heat pump. Plus it takes two days to get a technician to come out.

The key that I found on your web pages was the fact that the fan on the top of the condenser unit wasn’t running and the Compressor wasn’t running either. This pointed [possibly] to the start capacitor. In my system it is actually a pair of them, a dual capacitor unit. Aha! Finding the start capacitor took about twenty minutes. I work really slowly when I’m trying to not drop a handful of sheet metal screws in the grass [or worse into an electrical component where left in place a dangerous short circuit could occur].

I could see a little bit of bulging on the capacitor and some rust and grunge on the top.

I found a local HVAC Supply house in the phone book and read them the nomenclature from the side of the old Start Capacitor. The order clerk I spoke to was amazingly patient with me. About an hour and a half later I was plugging the new Start Capacitor in and flipping the circuit breaker back on. I put my hand on the coolant line and felt the copper getting cool fast. - James Oiler

Watch out: remember to turn off electrical power to any equipment before opening or working on it. Otherwise you could be electrocuted. Also, remember that an electrical capacitor can retain a large electrical charge even after electrical power has been turned off. You could still be shocked!


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.

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

The following links to chapters in this motor capacitor article series have been retained for compatibility with cached article data.

Article Series Contents

Question: the capacitor is leaking oil

(June 15, 2015) Larry said:

I noticed my air conditioner's run capacitor is leaking oil - what does this mean?


The capacitor is shot and should be replaced.

Reader Question: replaced both start & run capacitors on AC compressor but compressor won't turn on

(June 15, 2015) Larry said:

Trane Xl1600 heat pump cools down to set thermostat setting and then will not turn compressor back on and blow hot air in the home. This seams to happen when Phoenix temperatures reach 100 degrees. Replaced both start and run capacitors a couple months ago as issue started last summer was working fine until 100 degrees. Any assistance with this would be greatly appreciated.

This question was posted originally at DIAGNOSTIC GUIDES A/C / HEAT PUMP


Larry if replacing the start/run caps didn't fix the trouble I suspect either a failing control board / relay or a compressor motor that is failing beyond the ability of the starting cap to kick it off. Perhaps your tech will check the current draw of the various motors involved to see if that's going on.


Continue reading at MOTOR CAPACITOR TYPES or select a topic from the More Reading links shown below.

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CAPACITORS for HARD STARTING MOTORS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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Frequently Asked Questions (FAQs)

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(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.

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.

Question: clarifying dual start-run capacitor nomenclature

(June 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

(June 18, 2014) Mark said:
I always was taught that 35/5 was a 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 start & run capacitor.

35/5 : The first two numbers are the capacitance 35 uF (start) and 5 uF (run).
440 refers to the ability to run voltages up to 440VAC

(Aug 30, 2014) Bob said:
I was told my A/C unit has two capacitors. The unit stopped running, no fan at all, I located the dual capacitor and it checks out fine, so where is the second capacitor?

Does Lennox use a dual and a single capacitor

(Aug 30, 2014) Anonymous said:
I have located the dual capacitor, what would the second capacitor look like


Thanks Mark, I've included your helpful explanation in the article above.

Anon, thank you for griping about the difference between start and run capacitors. I'll review the text above for clarity and edit as needed.

Run caps are sold as single capacitor (two terminals) for a single purpose (keep the motor running) and rating (rated in mfd or uf) for the motor (such as 45uf for a compressor or 5 uf for a fan motor)

Dual run caps combine two capacitors in a single unit, one for compressor and one for fan, such as a 45/5 MFD unit, and will have 3 terminals as detailed in the text above (compressor, fan and common)

Start caps have the single job of getting a compressor motor spinning and of course are rated at a much higher MFD number. Mark summed it up nicely below and again in the article text above.

Bob if your unit has a start-run capacitor, a dual capacitor, that unit may be the only one present.

Capacitors may be found for the compressor/condenser motor and separately for the fan motor (on some units) - follow the power terminals of the equipment back to their power source.

The capacitors used on your particular Lennox unit will be described in the installation, maintenance and wiring manual or obtained by giving Lennox a call with your product model number.

Question: Marine AC compressor shuts down

(Sept 16, 2014) Ira said:
This is a question regarding a marine a/c unit, but is virtually the same as any other unit.
The problem is that when I initially start the unit in the morning it will run for about 30 seconds and then the compressor shuts down, but the fan remains on. I shut the unitI wait a minute and turn it back on and it then runs perfectly all day, wether I leave it on to recycle or turn it on and off manually.



has anyone checked for a failing start relay?

Question: clicking noise in Goodman Heat Pump

(Oct 6, 2014) Diane said:

I hope someone can help me fix my goodman 3 ton heat and air unit heat pump.
The trouble I'm having is I heard this clicking noise like the fan trying to come on, but never did until
I took a stick and pushed the blades and they started moving than I went inside the house and the air coming out was not cool could anyone tell me where to start checking, I,m going to try to fix this myself .

(Nov 20, 2014) Daniel said:
I woke up this morning, the ac unit was on the fan was working but there was no air coming from the vents inside the house, I went outside to see what I could do everything seem to be working but I think the condenser motor it was humming. and real hot to the touch. can someone give advice on what it could be?



If the indoor air handler unit is running but no air is coming out of the air supply registers I suspect that the blower assembly has failed or duct connections have come loose.

Check first for a blocked air filter or crimped air ducts. Watch out: an air filter that got sucked into the blower assembly can cause a fire.

Question: suspect stuck rotor in Panasonic AC Compressor

(May 6, 2015) Mohiddin said:
Sir, this is about Panasonic 1.5 ton AC compressor. The compressor ceases to start, tried with replacing the 45Mfd/440V capacitor with new one. Still the comp ceases to run and getting heated up. Checked the winding continuity and found to be OK. Suspecting the rotor has stuck. can I try to run the comp in reverse to release the stuck rotor or can i increase the starting torque boosting by adding a parallel capacitor to the existing 45Mfd capacitor?


You might get such a motor running but I'm doubtful it has a useful remaining life.


It sounds to me as if you need a new compressor. Even if you got it running by banging it around, its continued service is doubtful.

Question: old AC unit startup trouble

(May 6, 2015) Bruno , (Architect 5512 (RET> ) said:
Unit has been great, for many years
This year ,when I tried to use it, (It didn't seem to,"get to the 2nd step" (See Above ? )
Using ..(my intuition only , I am a "Mustang Architect,and 10 year carpenter tradesman prior)
So Do you think I am on the "right track ??

Thank you for your absolutely great Web-Site Bruno


Sorry Bruno I don't understand the question


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