How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (2023)

April 17, 2023 413

Capacitors are devices (usually found on the exterior of the motor) that work by storing an electric voltage, which is then discharged to meet the demand of the air compressor system. Depending on the type, they can also run in a continuous duty cycle. Capacitors come in an array of variations, but how should you size one for your compressor?

To size a capacitor for a compressor, you must find the Amps and Voltage requirements of your air compressor. This will give you the required Capacitance in Micro Farad (uF). First, check the manual of your compressor to match the necessary size; otherwise, use the (2650 x Amps) / (Volts) = Capacitor size required (in uF) formula.

Capacitors do not run as often as compressors, so they typically last longer than the air compressor itself. They can also fail unexpectedly if you try to start the unit with an improper pairing- hence the importance of selecting the proper size.

In this article, we’ll discuss what an air compressor capacitor is, where it’s located, and how to select the appropriate size for your machine!

What Is an Air Compressor Capacitor?

A capacitor consists of two metal conductors, separated by some sort of insulating material (also known as dielectric). It is responsible for storing energy and releasing it into the system when necessary; it accomplishes this by swapping electrons between two conductive plates. Air compressors use capacitors to startup, build pressure, and pump air.

More simply put: They are the magic electrical jump-starters that get your air compressor going and keep it running over time.

The main purposes of a capacitor in an air compressor system is to:

How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (1)
  • Circumvent unwanted frequencies in the system.
  • Inhibit the flow of DC, while allowing the flow of AC.
  • Answer the energy signal from any section.
  • Shift phases and delay system actions.
  • Start the motor.
  • Filter ripples in a rectifier circuit.

The larger the capacitor is, the more power it is able to store and discharge over time.

Types of Air Compressor Capacitors

Your air compressor system usually contains 2 types of capacitors- a start capacitor and a run capacitor.

Capacitors also range in sizes from as little as 1 uF to as large as 50+ uF. They should never be wired directly across a line and should instead be used for startup/run purposes only.

While the names “start” and “run” capacitor are fairly self-explanatory, the details below will better explain the role each one plays.

Start Capacitor

A start capacitor is responsible for kickstarting your air compressor.

It works by receiving a signal from the capacitor start relay, which then turns on an electromagnet. When this happens, power flows through this electromagnet and creates a magnetic field, building up until it’s strong enough to pull in the armature.

How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (2)

The armature is what starts the motor, as it removes pressure from the system by pushing out oil or air from between its two split parts and generating a voltage “boost.”

This process continues until the motor reaches a speed that’s high enough to sustain itself. At this point, the capacitor will continue to maintain a steady RPM level with its continuous flow of power.

Run Capacitor

The run capacitor, which controls the current and voltage to the compressor’s run windings, is a continuous duty component.

It’s more of an energy-saving part, maintaining the motor circuit throughout the entire time your air compressor is in use.

How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (3)

To help you differentiate between the two- a start capacitor builds energy to jolt your machine on, while a run capacitor provides continuous energy spurts to the motor in order to keep it running. Both are crucial for your air compressor to function properly.

Where Is the Capacitor on Your Air Compressor Located?

The capacitor for your air compressor will most commonly be located near the motor- either on the exterior of your motor housing or beneath its surface.

It’s also possible for them to be at the front of an electric air compressor’s base, depending on your model. Capacitors are usually marked by a yellow or red stripe that runs down one side.

They look a little something like this:

In terms of where you can purchase a new capacitor, they can be found at local hardware stores, in various online retailers, and even on Amazon. Capacitors do not come in a one-size-fits-all, so make sure you calculate the correct size before ordering!

How Is a Capacitor’s Capability Measured?

The ability each capacitor has to store energy is known as “Capacitance.” In other words, it is the ratio of the amount of electric charge stored on a conductor to a difference in electric potential.

Capacitance is typically measured in what is called “Farad.” However, it’s easier to consider the measurements of smaller units like Kilo Micro Farad (KMFD), Micro Farad (MFD), Kilo Pico Farad (KPF), or Nano Farad (NF).

For reference purposes, one Micro Farad is equal to 1 millionth of a Farad. Remember these units when we talk about calculating the correct capacitor for your air compressor.

Determining the Correct Capacitor Size for a Compressor

As we discussed ad nauseum, capacitors are responsible for providing energy to power your compressor. So it makes sense that the capacitor size you need will be determined by how much current and voltage your machine requires.

If you run into an issue regarding a failed run or start capacitor within your air compressor system, follow the formula below to help find the best replacement!

How are Capacitor Sizes Expressed?

Capacitors sizes are determined by two factors:

  • Capacitance (measured in Micro Farad, MFD, or uF)
  • Voltage (measured in Volts or VAC)
How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (4)

Keep in mind that the voltage rating isn’t what your capacitor will produce but rather what it can handle. The voltage rating displayed on the capacitor includes the “not to exceed” rating, which means you can replace a 250 voltage with a 340 voltage, but you cannot replace a 340 voltage with a 250 voltage.

Here is what your capacitor size readings will look like, displayed on the outer surface:

Calculating Capacitor Size

The best practice is to first consult your air compressor’s user manual to see what capacitor size is recommended. If you can’t find that, you can also look at the script on the side of both your run and start capacitors and re-order that specific type.

If, for some reason, that’s illegible, don’t panic. You can use simple calculations to determine the size you need.

Most capacitors are sized in terms of Micro Farad (uF). In order to properly size a capacitor for an air compressor, use the following equation:

Note: 2650 = a full load of Amps

(2650 x Amps) / (Volts) = Capacitor size required (in uF)

To get the Amps and Volts:

  1. Measure the total Volts between the HERM and COMMON terminals on the run capacitor.
  2. Measure the Amps on the wire leading from HERM to START on the compressor.

Let’s say, for example, you get 5.0 Amps and 331 Volts. Your calculation would look like this:

(2650 x 5.0 Amps) / (331 Volts) = A 40 uF Capacitor required

The run capacitor should have the exact Micro Farad (uF) that the motor requires, while start capacitors should be +/- 10% of the closest Micro Farad (uF) to the motor requirement.

Air Compressor Capacitor Buyer’s Guide

Based on what we learned above, there are a few recommendations for top-performing capacitors- ready to help your air compressor do its best work.

These are the most highly rated air compressor capacitors on the current market:

The best part is- they’re all under $20 and available on Amazon! The average cost of utilizing a qualified technician to swap out your capacitor is around $150 (including parts and labor).

Genet is among the most popular line of capacitors. This company designs cutting-edge, high-quality comfort and electrical parts; their track record for high-quality, highly-dependable goods makes them the go-to capacitor supplier.

Genet Motor Run Capacitors are the most durable and dependable electric capacitors available. They replace or supersede obsolete GE Industrial capacitors, which were previously regarded as the most reliable on the market.

Dual capacitors (like the Powerwell branded ones) come in a variety of sizes, depending on their capacitance (uF or MFD), such as 40 plus 5 uF, and voltage.

Round cylinder-shaped dual run capacitors are often used for air conditioning to help start up the compressor and the condenser fan motor. A round capacitor can also be used to substitute oval-shaped ones with the same uF and voltage specifications.

How to Replace Air Compressor Capacitors

When a capacitor blows, it’s usually fairly obvious as the air compressor smokes and stops working. In order to change out either your run or start capacitor, you must remove the old one and insert a replacement.

Warning: Handle with care; most capacitors carry a lethal charge.

Follow these steps to replace your air compressor capacitor:

  1. Put safety glasses and gloves on.
  2. Make sure the machine is unplugged and cooled off.
  3. Remove the pressure relief valve, dial, switch, and wiring of each if they are in the way.
  4. Next, remove the motor housing (if the capacitor you’re changing is beneath its surface).
  5. Unwire the old capacitor.
  6. Dispose of the old capacitor in a safety bin, as it could contain harmful chemicals.
  7. Rewire the new capacitor and push it into position (inside the motor housing).
  8. Turn the metal bolt into place using your hands.

Turn your machine on to make sure the capacitor is installed correctly. Check out this video for a more detailed how-to on replacing a motor start capacitor!

What Happens if You Put a Bigger Capacitor on an Air Compressor?

As a good rule of thumb- the size of your new capacitor (meaning the voltage and capacitance measurement) should be the same as the old one you removed.

If it’s slightly bigger- no harm will come of it, but if it’s MUCH bigger, the entire air compressor system could be thrown off. This can lead to the damage of other electrical components or complete combustion.

An air compressor can not operate properly if the compressor does not provide enough energy or provides too much power for the motor.

Every compressor is made slightly differently, so you should consult the owner’s manual before deciding on a different capacitor than the original model.

Final Thoughts

Capacitors are used in a variety of different types of electrical networks where they store energy temporarily. These little (but mighty) parts are necessary in the event that a high voltage power source is needed for a short period of time.

Air compressor capacitors can also be used to boost a current or as a temporary storage device for alternating currents. They come in many different varieties and sizes, depending on how much capacitance you need for your system.

We recommend taking a look at Genteq Capacitors- one of the most popular on the market! The best part is- they’re all under $20 and available on Amazon.

If you need to calculate your machine’s capacitor needs ASAP and can’t find the manual or read the sizing on the old capacitor, use this formula:

(2650 x Amps) / (Volts) = Capacitor size required (in uF)

For any further questions, reach out to a qualified air compressor technician in your area!

How to Size a Capacitor for a Compressor (Tips and Buyer's Guide) (5)


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How do I choose a capacitor for my compressor? ›

Multiply the full load amps by 2,650. Divide this number by the supply voltage. The full load amps and the supply voltage can be found in the owner's manual. The resulting number is the MicroFarad of the capacitor you need.

How do I calculate what size capacitor I need? ›

Capacitor Size Formula

To calculate a capacitor size, divide the start-up energy by one half of the voltage squared.

What happens if you put too big of a capacitor on a compressor? ›

This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings.

Can I replace a compressor capacitor with a higher UF? ›

Can I Replace a Capacitor With a Higher UF in a Power Supply. Yes, it is possible to replace a capacitor in a power supply with one with a greater UF. Because power supply electrolytic capacitors have a tolerance of up to 100% greater, you can replace them with another capacitor of equal or higher uFs and voltage.

Can you oversize a run capacitor? ›

Yes, you can use a larger run capacitor, but only if the mfd or uf rating is equal to or greater than the original capacitor by up to 20%. Using a larger capacitor will not damage the motor or the run capacitor.

What is the 2 3 rule of capacitor placement? ›

From this, they developed the 2/3 rule for selecting and placing capacitors. For a uniformly distributed load, the optimal size capacitor is 2/3 of the var requirements of the circuit. The optimal placement of this capacitor is 2/3 of the distance from the substation to the end of the line.

Can I replace a 35 3 capacitor with a 35 5? ›

A: Hello. No do not trade the 35/3 from 35/5 it may not have enough power to run your motor.

How do I choose a replacement capacitor? ›

Choosing New Capacitors. Each capacitor has two values: a voltage rating and capacitance value. Both are important. The general rule for replacing capacitors is to use values that are equal to or higher than the originally-specified values.

Can I use a 7.5 capacitor in place of a 5? ›

Yes, you can use a 7.5 capacitor in place of a 5. But in most cases, it is determined by other capacitor specifications, such as voltage, as well as the application. When capacitance increases in a capacitive circuit, the capacitive reactance XC decreases, causing the circuit current to increase and vice versa.

Will a bad capacitor keep a compressor from running? ›

Air Conditioner Unit Doesn't Turn On

If your AC capacitor is bad, you may notice that your air conditioner takes a long time to turn on or doesn't turn on at all. In some cases, the damage to the air conditioner's capacitor may be severe enough to prevent the compressor from starting at all.

Can I use a 440v capacitor instead of a 370v? ›

Reason: The voltage rating on the capacitor displays should not be exceeded. This means 440 V capacitor can be replaced with a 370 V but it can not happen the other way round.

Does it matter which way a capacitor faces? ›

This positive terminal is super important and designates how this polarized capacitor needs to be wired. The positive side always gets connected to power, and the arc side connects to ground.

What are the 3 numbers on a capacitor? ›

The first number represents the tens, the second number the ones and the third number is the multiply factor. See below for examples. For example: A capacitor with the number 102 printed on it means a 10 with two zeros. The value in this case is 1.000 pF or 0,001 µF also named 1 nf.

What do the letters and numbers on a capacitor mean? ›

If the letter is p, n, or u, this tells you the units (pico-, nano-, or microfarad). Replace this letter with a decimal point. For example, n61 means 0.61 nF, and 5u2 means 5.2 uF. A code like "1A253" is actually two codes. 1A tells you the voltage, and 253 tells you the capacitance as described above.

Can a bad capacitor ruin a compressor? ›

Using the wrong capacitor rating or a poor quality capacitor can adversely affect the operation of the motor, the compressor or an entire HVAC system.

What does 30 UF mean on a capacitor? ›

A microfarad is a term to describe a capacitor's level of capacity. That means the higher the microfarad rating, the more electrical current it can store. A typical capacitor can range from 5MFD to 80MFD. If you are looking at your capacitor and you can't find the proper rating, it might also look like µF also.

What will bad capacitor do to compressor? ›

The most common problem that bad capacitors can cause is “hard starting.” This is when the compressor of an AC has difficulty starting up, stutters trying to turn on, and then shuts off a short while later.

What happens when you increase the size of a capacitor? ›

Larger plates provide greater capacity to store electric charge. Therefore, as the area of the plates increase, capacitance increases. Capacitance is directly proportional to the electrostatic force field between the plates.

What is the difference between oil filled and dry capacitors? ›

Oil impregnated capacitors have much higher partial discharge voltage as compared to DRY TYPE. Jaivic impregnated Oil filled Capacitors are hermetically sealed, hence do not have any atmospheric influence compared to DRY TYPE. Capacitor wound elements are encased in metal containers of adequate thickness.

What is the rule of thumb for capacitors? ›

Working Voltage and Ripple Current

Usually, capacitors are derated by the following rule of thumb: a capacitor is selected such that its voltage rating is two to three times greater than the expected operating voltage.

What happens when a capacitor is backwards? ›

Driving Event. Most tantalum capacitors are sensitive to the polarity of the applied voltage. Incorrect installation and/or improper circuit application that subjects the capacitors to reverse bias may lead to performance degradation or catastrophic failure (short circuit) of the capacitor.

What is the Kirchhoff's rule for capacitors? ›

When the capacitors are completely charged, the voltage produced by the capacitors is equal to the voltage produced by the battery. Using Kirchoff's current rule, we find that the charge on each capacitor is equal. We want to equate this to the case of one capacitor of equivalent capacitance.

Can a motor run without a run capacitor? ›

A run capacitor is an energy-saving device that is in the motor circuit at all times. If a run capacitor fails, the motor can display a variety of problems including not starting, overheating, and vibrating. A bad run capacitor deprives the motor of the full voltage it needs to operate correctly.

What is the difference between a run capacitor and a start capacitor? ›

The Two Types of Capacitors

Run capacitors are more common. They store the energy that is needed to keep the fan motor going so that the air conditioner continues to cool down your house. In contrast, the start capacitor specifically provides the energy needed to start up your air conditioner.

Does the physical size of an AC capacitor matter? ›

Using improperly sized capacitors can have a variety of detrimental effects on the motor. If the capacitor's µF rating is less than the motor was designed for, the motor winding current will be too high. If the capacitor's µF rating is higher than the motor was designed for, the motor winding current will be too low.

Is it OK to replace a capacitor with higher capacitance? ›

Yes, the smoothing capacitors of a power supply can be replaced by a higher µF capacitor. The smoothing capacitors smooth out the output voltage waveform of a power supply. The power supply's output may be stabilized even more by replacing the capacitor with a higher µF one.

How do you tell if a capacitor needs to be replaced? ›

Some capacitors do not require any test to determine failure. If a visual inspection of the capacitors reveal any sign of bulging tops, those need to be replaced. This is the most common failure in power supplies.

Can you use a smaller capacitor for AC compressor? ›

Keep in mind that the voltage rating of the replacement capacitor can be higher, but should never be lower The smaller capacitance is for the fan motor and the larger for the compressor.

What size capacitor do I need for a 1.5 hp motor? ›

H.P.0.5 H.P.1.5 H.P.
Running Capacitor25 Mfd36+25 Mfd
Starting Capacitor100/120 Mfd120/150 Mfd

How do you tell if capacitor is bad or it is the compressor? ›

Here are some common symptoms of a bad AC capacitor.
  1. AC Not Blowing Cold Air. An air conditioner that doesn't blow cold air is one of the first signs of a problem many homeowners notice. ...
  2. High and Rising Energy Bills. ...
  3. Humming Noise. ...
  4. Old HVAC System. ...
  5. AC Turns Off On Its Own. ...
  6. AC Doesn't Turn On Immediately. ...
  7. AC Won't Turn On.
May 16, 2018

Does a compressor use a start or run capacitor? ›

The run capacitor is used to improve the running efficiency of a compressor's motor. The run capacitor is placed in series with the start winding of the compressor and will remain in the circuit as the motor operates.

How long do compressor capacitors last? ›

In general, you can expect most AC capacitors to last about 20 years. This is a fairly long lifespan, which is why most heating and AC repair companies will take a look at the capacitor last. They expect the capacitor to last for quite some time.

What not to do with a capacitor? ›

Precautions for Your Safety
  1. Do not charge by higher current or higher voltage than specified. ...
  2. Do not reverse placement of (+) and (-). ...
  3. Do not solder directly to the capacitor. ( ...
  4. Keep capacitors out of children's reach. ...
  5. Do not heat, disassemble, nor dispose of in fire. ...
  6. Do not discharge by force.

What happens if you apply too much voltage to a capacitor? ›

If the voltage between the two electrodes exceeds the withstand voltage value of the capacitor, the dielectric between the two electrodes will be broken down, the two electrodes will be turned on, and the capacitor will not be able to store charge.

Do I need to discharge AC capacitor before replacing? ›

They can become damaged by a variety of issues from lightning strikes to high temperatures, but can still maintain a charge. Even if you can see the capacitor and it appears to be damaged, it is better to be safe and discharge electricity from the capacitor before removing it.

Does it matter if a capacitor is round or oval? ›

Round capacitors are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference. Fit is the only question here. If space in the mounting box is not limited, the case style does not matter.

What is a capacitor for dummies? ›

Unlike the battery, a capacitor is a circuit component that temporarily stores electrical energy through distributing charged particles on (generally two) plates to create a potential difference. A capacitor can take a shorter time than a battery to charge up and it can release all the energy very quickly.

Should you touch a capacitor? ›

A charged capacitor can be very dangerous, so it's important that you avoid coming into contact with the terminals at all times. Never touch the capacitor anywhere but on the sides of its body. If you touch the two posts, or accidentally connect them with a tool, you could get badly shocked or burned.

What does J mean on capacitor? ›

Type J – Dipped Tantalum Capacitors. Type K – Mica Capacitors. Type L – Polyester/Polystyrene Capacitors.

What does K and J mean in capacitor? ›

K means that the capacitance tolerance is 10% (F=1%, J=5%, K=10%. These are most common resistance and capacitance tolerances). MF does not mean Micro Farad. Probably it stands for Metallized Film, which indicates the capacitor type/material.

How do you calculate the size of a capacitor? ›

This calculates the capacitance of a capacitor based on its charge, Q, and its voltage, V, according to the formula, C=Q/V.

What capacitor should I use? ›

Class 1 ceramic capacitors offer the highest stability and lowest losses. They have high tolerance and accuracy and are more stable with changes in voltage and temperature. Class 1 capacitors are suitable for use as oscillators, filters, and demanding audio applications.

What numbers are important on a capacitor? ›

The nominal value of the Capacitance, C of a capacitor is the most important of all capacitor characteristics. This value measured in pico-Farads (pF), nano-Farads (nF) or micro-Farads (μF) and is marked onto the body of the capacitor as numbers, letters or coloured bands.

How do you decode a capacitor? ›

The value is expressed in terms of pico-Farads. To decode the value, take the first two digits, then follow them with the number of zeros indicated by the third digit. 104 becomes "10" followed by "0000," or 100000 pF, more succinctly written as 100 nF.

What size capacitor do I need for a 1200 watt amp? ›

Q: What size capacitor should I get? A: The rule of thumb is to put in 1 Farad of capacitance for every 1,000 watts RMS of total system power.

Does the size of capacitor matter for AC? ›

Using improperly sized capacitors can have a variety of detrimental effects on the motor. If the capacitor's µF rating is less than the motor was designed for, the motor winding current will be too high.

What happens if you put a smaller capacitor on a compressor? ›

Generally a smaller cap will reduce the starting torque, and may prevent it from starting properly under moderate load. As tecman said, it will reduce torque, both starting & running. This in turn will cause the motor to pull more current trying to either get the load started or keep the load going.

Can a bad capacitor ruin a motor? ›

When a capacitor short-circuits, the winding in the motor may burn out. When a capacitor deteriorates or opens, the motor has poor starting torque. Poor starting torque may prevent the motor from starting, which will usually trip the overloads.

What size capacitor do I need for a 300 watt amp? ›

300 Watt Amp With 500K Micro Capacitor | Oznium LED Lights.

What size capacitor do I need for 1000 watt amp? ›

Capacitor Selection Chart
Single Amp Systems
Power RMSCapacitor
Up to 500 watts0.5 Farad
Up to 1000 watts1.0 Farad
Up to 2000 watts2.0 Farad
1 more row
Jan 26, 2023

What size capacitor do I need for a 0.75 KW motor? ›

The Quality 0.75kw motor requires a 20uF Capacitor for the motor to start up.

What happens if you put the wrong capacitor in an AC unit? ›

A bad capacitor prevents the exterior unit from properly functioning, which hinders the cooling process as a whole. Second, improper voltage delivery to exterior unit components forces the system to work harder as it attempts to perform its job. Additional components often sustain damage due to a faulty capacitor.

Can I use any capacitor for my AC unit? ›

Voltage: Capacitors are usually rated for both 370 volts and 440 volts. You can use a new capacitor with a higher voltage than the original, but never choose one that has a lower voltage rating than the system voltage. Frequency: AC capacitors are typically rated for 50 and 60 hertz.

Is there a right or wrong way to fit a capacitor? ›

Does it matter which way you connect a capacitor? In theory, no. In practice, however, certain capacitors are polarized and a reverse polarity voltage will probably damage it. Examples are electrolytic and super capacitors.


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