Air compressor capacitor: How to size a capacitor for a compressor

When either one or both air compressor motor capacitors fail, you will need to replace them. But what capacitor does my compressor need? And how do you size the starter capacitor? These are two questions many compressor owners ask on this site. This is because OEM parts can be expensive and finding a source for the capacitor of a low-cost air compressor can be a challenge.

Here’s how you can figure out what air compressor start capacitor is required for your air compressor, and how to replace the compressor capacitor.

Safety Note

Be careful not to touch the capacitor terminals as a capacitor is a high-discharge device. You may get a severe electric shock from the stored energy. If unsure, please seek professional guidance while fixing the air compressor. It is also advisable to test the capacitor with a voltmeter to check the state of charge.

What are air compressor motor capacitors?

Without getting into too many details as this information is covered on other capacitor information pages on this site, the compressor capacitor typically sits on the side of the compressor motor on the outside of an air compressor, near the motor shaft. It may be under a motor capacitor cover.

Some air compressors more than one start capacitor. Each compressor will have a start capacitor while some have a start and run. Others have a combination of start/run capacitors. Start capacitors are only used once switching on the air compressor, while run capacitors help with the ongoing running of the compressor motor.

Where to find the capacitor for an air compressor motor

In the image below the red dot identifies the capacitor housing on a typical compressor motor. The capacitor sits inside the housing. Your compressor motor might just have the capacitor itself attached or adjacent to the motor rather than inside a housing as the one in the picture is.

What Capacitor Does My Compressor Need?

To help you better visualize one, here’s a capacitor from a Campbell Hausfeld air compressor motor. In the quest to find an air compressor capacitor replacement, know that compressor motor capacitors have two ratings.

Voltage

On the side of the capacitor, there should be a label. This label shows the ratings of the capacitors, one of which is the voltage. Typically the voltage range will be 120-240 Volts. In this image the capacitor voltage is 250v AC, meaning 250 volts and an alternating current type capacitor. As long as you get a capacitor that has the same voltage range as the one you currently have, then the voltage will be satisfactory for your air compressor motor.

Capacity

Another rating noted on the label will be the microfarad capacity. It may show as an MFD rating, MFD being the acronym for micro-farad.

These are different ways that identify the microfarad rating of that capacitor.

Wondering what starter capacitor my compressor motor needs? You need to get one that has the same microfarad rating as the existing capacitor, or, one that shows a microfarad range into which the existing capacitor range fits. If your replacement capacitor is too low the air compressor will not start, if it’s too high then you are likely to cause electrical faults.

Replacing the start capacitor for air compressors

The last thing you need to figure out is whether or not the new capacitor will fit on your compressor motor. Typically they all have terminals to which you connect the wires to the motor, yet what I’m referring to here is whether the new capacitor will fit inside the existing capacitor housing or not?

You see, if the new capacitor has the same voltage range, the same microfarad range, and you can attach it to the existing mounting of the motor capacitor on your compressor, you can use it. Even if it’s a different shape, a different color, longer or shorter, or a bigger circumference, as long as the voltage and microfarad ratings are the same, it is a suitable capacitor for your compressor motor.

A simple Google search string such as “buy capacitor for _ air compressor” will typically return a number of sources. Pick one that suits you according to the details noted above, and you are in business.

How to test capacitors

If you’re unsure if your capacitors are being problematic then check out our how-to test air compressor capacitors page.

What if the Capacitor keeps blowing?

Check out our page on compressor motor start capacitor blowing.