Correctly Sizing a Capacitor I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.  Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size! Testing Volts / Amps With the condensing unit operating under a load, you will need to measure the total volts between the HERM and COMMON terminals on the run capacitor (i.e. 345 VAC). 

Next, measure the amperage on the wire leading from HERM to START on the Compressor (i.e. 4 amps). Use the equation below to verify the size of the capacitor. The resulting microfarad (uf) should match the size of the installed capacitor. An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor.  This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.  The run capacitor should have the exact microfarad (uf) that the motor is rated for.  Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation.  This is where the rule of +/- 10% of the rating came from, for Start Capacitors ONLY!  The voltage rating should be no less than the listed amount for the motor, for central heat pumps and air-conditioners this is usually a minimum of 370VAC. 

Most new condensing units are specified for 440VAC capacitors, and are more durable during fluctuations in power supply.  I have seen some universal type dual run capacitors rated for as much as 700VAC, as this voltage rating does not impact uf performance.  can you put a window ac unit on its sideThe changes in uf will impact amperage draw and reflect on kilowatt hour usage though.ac only package unit When completing your early season maintenance this year for your customers, do them a service by testing capacitors and verifying their size.  ac unit without windowYou could just increase your service ticket total and save some all important power.  You may even prevent a call-back during the next heat wave! 0 members like this

< Previous Post Next Post >User ReviewedHow to Determine Amperage of Circuit Breaker Three Methods:Finding the Amperage Rating of a Circuit BreakerFinding the Amperage Draw in a CircuitReading a Device's Amperage RatingCommunity Q&A Each circuit breaker has a specific rated amperage, or amount of current. When that amperage is exceeded, the circuit breaker shuts down the flow of current to prevent damage to the wiring. Learn how to calculate the actual amperage of the current and compare it to the rated amperage, so you can avoid unnecessary power interruptions. Examine the electric panel. Each circuit breaker should have its amperage marked on the handle. This is the maximum amperage that circuit can take before the circuit breaker trips. In the United States, standard household circuits are rated for 15 amps, or 20 amps for newer homes and high-load circuits. Multiply the amperage by 0.8. For everyday use, it's a good idea to expose the breaker to a maximum of 80% of the rated amperage.

It's fine to exceed this for short periods of time, but continuous current above this amount could trip the breaker. Your electrical panel might have a note about the breakers operating at 100% of the rated value. If it does, you may skip this step. Compare this to the current on the circuit. Find the wattage of a device. Pick any device attached to the circuit you're investigating. Find the wattage (W) listed on the data plate — usually on the back or underside of the device. This is the maximum power rating of the device, which can be used to calculate amperage. Some devices will list the amperage directly, often labeled FLA. If it does, skip down to the next section to interpret that rating. Check the voltage on the circuit. For household circuits, you can usually assume your house follows the voltage standards of your country. (For example, 120V in the US, or around 230V for most of the EU.[1]) If you think you are working with an exception, [the voltage] using a multimeter.

Divide the wattage by the voltage. Repeat for each device on the circuit. Add the amperages of devices that are always running.On top of the continuous amperage, add the amperage of other devices that might be on at the same time. If any combination gets above 100% of the circuit breaker's rating, it will trip the circuit. You can solve this by moving a device to a different circuit, or by remembering not to use high-power devices at the same time. Electrical circuits never operate perfectly. Some of the energy is lost to heat, and devices may draw more current to make up for this. Waste is low in most household circuits (below 10%), but it is still possible to trip the breaker if the amperage on paper is slightly below the breaker rating. Measure amperage directly with a clamp multimeter (optional). A clamp multimeter (or clamp ammeter) has a pair of "jaws" at the top that close shut to encircle a wire. When set to measure amps, the device will display the number of amps running through that wire.

[2] To test a circuit, expose the wire leading to the load side of a circuit breaker. With the clamp multimeter set up as described, have a friend turn on other devices in the house. If the device is on the same circuit, you will see the amperage display increase. Do not attempt this unless you have electrician's gloves and a basic understanding of electrical safety. These wires are live and can be very dangerous. Look for the data plate. All appliances should have a data plate with electrical information. Look on the back or underside of the device, or check the product manual. The information on this plate will help you determine how many amperes the device draws, and therefore the rating you will need for the circuit breaker. This section covers devices that list the amperage directly on the data plate, which should include all appliances with a motor. If your device only lists wattage (W), calculate the amperage from that value. This is not an appropriate method to determine safety features to protect the motor itself.

[3] The circuit breaker protects the wiring of the electrical supply. High-power devices such as air conditioning units and ovens are best installed by a trained electrician. Check the device's voltage rating. The amperage drawn depends on the voltage of your electrical circuit. The intended voltage (V) of the device should be listed so you can confirm that it matches your electrical system. If the device can run at two different voltages, it will usually list two values like this: 110V/240V. In this example, if you were running the device on a 110 volt supply, you would only refer to the first number listed on each line. Most electrical codes allow a ± 5% tolerance for the voltage (or slightly more).[5] Do not run a device on a voltage supply outside this range. Household outlets in the US and some other countries are on a 120V standard. Most of the world uses 220–240V. Look for the FLA. This is the number of amps the motor will draw at the rated horsepower. In the United States, if this device will be left on for more than three hours, the circuit breaker should be rated at 125% of this value.

(Multiply the FLA by 1.25.) This allows for additional load due to other factors, mainly heat. This value may also be listed as full load amperage, running amps, rated amp, or just amps. Some circuit breakers are rated to 100% of the listed amperage, meaning you can skip the 125% calculation. This information will be clearly listed on the circuit breaker electrical panel if you have this type of breaker.The LRA, or locked rotor amps, is the amount of current drawn when the motor is not turning. This is required to start the motor, and can be much higher than the FLA.[10] Modern circuit breakers are designed to allow this brief surge of current. If your circuit breaker is rated high enough for the FLA but still trips when the device is plugged in, it could be a faulty breaker, or just an old model. Move the device with high LRA to another circuit or have an electrician inspect your wiring. Do not confuse this with the RLA, a value listed on air conditioner units. Take other devices into account.

If multiple devices are running on the same circuit, add them together as follows: If your circuit breaker is rated at 100%, just add all of the amperages together. If your circuit breaker is rated at 80% or you do not know its rating, add the amperages of all devices running for more than three hours at a time and multiply by 1.25. Add to the result the amperage of all devices running for shorter periods of time. In either case, if the answer exceeds the circuit breaker amperage, move a device to another circuit. Use the MCA and MOP ratings for an air conditioning unit. These values are rarely listed except in North American air conditioning units. The Minimum Circuit Amperage tells you the minimum wire size necessary for safety. The Maximum Overdraft Protection is the highest circuit breaker amperage allowable. When in doubt, use the MOP value when choosing a circuit breaker to avoid unnecessary breaker tripping. These values are often surprising if you do not have HVAC experience, and are further complicated by new technologies that allow for lower amperage than MOP implies.