How to Accurately Test Run Capacitors in the Field

Testing run capacitors is a key part of preventative maintenance—but how you test matters. While checking capacitance with a meter on the bench is quick and simple, verifying performance under live conditions gives you a clearer picture of whether the component is actually doing its job when the system is running.

Testing Under Load: A Field-Proven Method

Here’s a practical way to test a capacitor while it’s still wired and the system is operating. This method combines standard electrical checks with a little math to calculate actual microfarads—no shutdown or removal required.

Step-by-Step Process:

1. Read Voltage and Amperage:
    Begin by taking your standard voltage and current readings on the system.
2. Measure Start Wire Amps:
    Clamp your meter on the wire running between the run capacitor and the compressor (often labeled HERM). Record the amperage.
3. Check Capacitor Voltage:
    Measure voltage across the capacitor terminals. For compressors, that’s between HERM and C. For fans, use FAN and C. Record the reading.
4. Calculate Microfarads (μF):
    Use the formula:
    Amps × 2652 ÷ Voltage = Microfarads
5. Compare to Spec:
    Your calculated value should be within ±6% of the rating printed on the capacitor. If it’s outside that range, replacement is likely needed.
6. Repeat for Other Capacitors:
    Perform the same check on any additional run capacitors in the system to confirm all are operating within spec.
7. Verify Correct Sizing:
    Don’t assume the last tech installed the right part—always cross-check with the motor’s data plate to ensure the capacitor rating is correct.

Quick Tip: Check the Terminals

Before making any calls about bad capacitors, inspect the terminals. Corrosion or loose connections can cause voltage drops and skew your readings which can lead to unnecessary repairs and callbacks. A quick visual check can save you time and your customer money.

At WeSupplyTrades, we stock a full line of run capacitors, dual capacitors, and testing tools designed for field use. Whether you're maintaining rooftops, package units, or split systems, we’ve got the components and gear pros rely on.

Questions? Email us at [email protected].