How To Determine Capacitor Size For AC Unit

The capacitor on AC unit should be replaced with the same specs. A wrong size capacitor could cause the ac compressor to overheat and shorten its life. However, you need to make sure that you have the exact capacitor size for your ac unit.

An undersized AC capacitor (smaller than needed microfarad) can contribute to a motor winding overheating, leading to motor winding failure.

An oversized AC capacitor will not cause much of any problems (especially for a start capacitor). But, depending on the capacitor and motor, it could decrease the air conditioner efficiency very slightly.

Issues Caused By The Wrong Capacitor Size

The compressor will fail to start because the ohmic value of the capacitive reactance conveyed to the start winding will be very high. Hence the AC compressor will suffer very low current for its’ start winding, rendering very poor starting torque. The internal overload switch will frequently, cyclically, open/close.

The condenser fan will suffer high current through its’ start winding and thus burn out. This is because the ohmic value of the capacitive reactance will be too low; hence high current. The fan may seem to run well for a while, but it will eventually burn out its’ start winding.

The main breaker will eventually trip. The sum of currents will be greater than the design. Hence, hopefully, the main breaker will trip before the condenser fan motor’s start winding burns out.

How To Calculate Capacitor Size For Ac Motor

To calculate capacitor size, you can go plus or minus 10% on a run capacitor. But, there are no other standard capacitor sizes within 10% of 35 50 mfd , and thus no substitutions can be made. You’ll find that you cannot apply the 10% rule to any cap lower than 50 mfd.

A capacitor can vary up to 10% of its rating before it is condemned. In that case, logically, a capacitor of a different rating that falls within 10% of the rating of another capacitor should be allowed as a substitution for it.

You should measure the capacitance of the replacement capacitor and verify that the actual capacitance is within 10% of the capacitor rating it is intended to replace.

For capacitor sizes lower than 50, you will not be able to apply this rule.

2 Basic Capacitor Types Used In Ac Compressor

Run capacitors

Run capacitors are rated in a range of 3-70 microfarad (mfd). Run capacitors are also rated by voltage classification. The voltage classifications are 370V and 440V. Capacitors with ratings above 70 microfarads (mfd) are starting capacitors.

Run capacitors are designed for continuous duty and are energized the entire time the motor is running. Single-phase electric motors need a capacitor to energize a second phase winding.

This is why determining capacitor size for ac units is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven.

This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.

Start capacitors

Starting capacitors are housed in a black plastic case and have an mfd range instead of a specific mfd rating on run capacitors. Start capacitors (ratings of 70 microfarads or higher) have three voltage classifications: 125V, 250V, and 330V.

Examples would be a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V start capacitor. Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly.

Start capacitors are designed for momentary use. Start capacitors stay energized long enough to rapidly bring the motor to 3/4 of full speed and are then taken out of the circuit.

You should understand everything written on a capacitor to determine the capacitor size for the ac unit. There are many numbers, codes, and specs on a capacitor, and it would be wise to understand what they all mean. Similar specs capacitor – within even the same brand – might not mean the same things completely.

About Mas Broto

Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.