Run Capacitor Load Test Calculator

What it does. Enter amps and volts on a running capacitor section and get real µF, a pass/fail against the label tolerance, and a visit-to-visit trend.

Detailed Overview

The Run Capacitor Load Test Calculator takes two meter readings from a running system and turns them into the capacitance a run capacitor section is actually delivering at operating voltage and temperature. It compares that figure with the tolerance printed on the can, shows where the reading lands inside the pass window, factors in physical-condition evidence, and estimates how many maintenance visits are left before the capacitor drifts out of tolerance.

Purpose

Run capacitors wear out. The metallized polypropylene film inside repairs itself after small internal faults, and every repair clears away a bit more electrode area, so capacitance slides downward over the years. Heat and voltage accelerate that aging. On every maintenance visit the question is the same: has this capacitor drifted far enough to justify replacement, and can the tech show the customer why?

Every summer, homeowner forums fill up with invoices showing a dozen capacitors swapped in a single visit. Proactive replacement done honestly is condition-based: a measured reading against the label, a trend across visits, and physical evidence. This tool builds that record in about a minute.

When and Where to Use It

• Cooling maintenance visits. Run each section under load and log µF so the next visit has a trend.
• No-cool and hard-start calls. Verify a weak capacitor before you condemn a compressor or fan motor.
• Replacement decisions in front of the customer. Put the reading, the pass window, and the trend on the table.
• Parts substitution at the truck. Check that the replacement carries a voltage rating equal to or higher than the original.
• Service managers. Drop the copied summary into invoices as a standard note so replacements are documented across the crew.

Inputs

• Rated capacitance (µF). Default 35 µF, taken from the companion post’s worked example.
• Label tolerance. ±5%, ±6% (default), or ±10%. Genteq and PROLINE spec ±6%; Cornell Dubilier’s standard is ±10%; other brands print ±5%. Use whatever the can says.
• Amps through the section (A). Taken on the wire between the capacitor section and the motor’s start winding. Default 5.2 A.
• Volts across the section (V). Taken across the same section’s terminals while it runs. Default 380 V.
• Line frequency. 60 Hz (default) or 50 Hz.
• Physical condition. Domed top, leaking oil, corroded or burnt terminals, open section.
• Previous readings (µF). Optional numbers from the last one or two visits.
• Original and replacement voltage ratings. 370 VAC, 440 VAC, or dual rated.

Outputs

• Calculated capacitance (µF) shown with the formula.
• Deviation from rating (%) and the pass window in µF.
• Pass window gauge. An inline SVG bar with the tolerance band, the rated value, and the reading plotted on it.
• Change per visit (least-squares slope over the readings) and visits to lower limit at the current drift rate.
• Verdict: replace on physical evidence, replace below tolerance, recheck a high reading, in tolerance but close to the limit, or in tolerance.
• Voltage substitution check (OK or do not install).
• Copy for the invoice button that copies a plain-text summary of ratings, readings, trend, condition, and result.

Context: Where This Tool Lives in HKIA’s Content

The tool was built to go with this HKIA content:

• “Capacitors Die of Heat: What Kills a Run Capacitor and When Early Replacement Is Honest” (Tech Edition, October 2026). The post covers self-healing film, heat and voltage stress, label tolerance, the under-load formula with a passing and a failing example, and condition-based replacement built on a three-visit trend. The presets in the tool mirror those examples exactly.
• “HVAC TIP: How To Test a Run Capacitor Under Load” (hvacknowitall.com). The original short tip that introduced the under-load method.
• HVAC Know It All Tech Edition newsletter, October 1, 2026. The newsletter’s capacitor section links here.

It pairs naturally with the Furnace Input Clock Calculator from the same issue, which produces the same kind of measured, documented record on the heating side.

Math & Logic

• Capacitance under load: µF = amps × 1,000,000 ÷ (2π × frequency × volts). The constant works out to 2,652 (2,652.6) at 60 Hz and 3,183 at 50 Hz. It comes from capacitive reactance, Xc = 1 ÷ (2πfC).
• Pass window: rated µF × (1 ± tolerance).
• Near-limit flag: a tool heuristic, not a manufacturer standard. If the reading lands in the lowest third of the band under the rating (below about 33.6 µF on a 35 µF ±6% capacitor), the verdict is “in tolerance, close to the limit.” The same verdict shows up when the trend would hit the limit within one visit.
• Trend: least-squares slope of µF per visit, calculated across the earlier readings plus the current one. Visits to the lower limit = (current µF − lower limit) ÷ drift per visit, and it only appears when the trend points down.
• Physical evidence overrides the reading. A domed top, leaking oil, corroded terminals, or an open section brings back a replace verdict no matter what the µF says.
• Voltage check: the replacement’s maximum rating has to be equal to or higher than the original. A dual 370/440 VAC part counts as 440 VAC.
• Presets: “Passing Herm section” (35 µF ±6%, 5.2 A, 380 V = 36.3 µF) and “Weak Herm section” (4.4 A = 30.7 µF, about 12% low) reproduce the companion post’s numbers. “Fan section” (5 µF ±6%, 0.7 A, 380 V = 4.9 µF) is an illustrative fan-section reading.

Limitations

• The label is the authority. The tool cannot know the tolerance on an unlabeled or worn capacitor, so enter what the manufacturer prints.
• Measurement placement matters. Amps have to come off the section’s own conductor, and volts across that same section. A reading well above the window usually means a measurement error, and the tool says so.
• No lifespan prediction in years. No published data backs a universal service life or a fixed-age replacement schedule. The trend projection uses the entered readings only.
• Temperature and life-class data are not modeled. Rated life (for example IEC 60252-1 Class B, 10,000 hours) is a survival probability under test conditions, not an expiration date.
• Contactors are not tested here. Check contacts and voltage drop on their own.

Sources Used

• Motor Run Capacitors Catalog, Cornell Dubilier Electronics. ±10% standard tolerance with tighter options; 60,000-hour rating at 70°C; -40 to 70°C standard range.
• Genteq Capacitors, Genteq (Regal Rexnord). ±6% typical tolerance; replacement voltage rating must equal or exceed the original.
• PROLINE Motor Run Capacitor Datasheet (PLC 001-02/10), PROLINE, 2010. ±6% tolerance; Class B, 10,000 hours with 94% survival.
• AC Motor Capacitors, Part 1: General (IEC 60252-1), International Electrotechnical Commission, 2010 with 2013 amendment. Operating classes A through D.
• The True Cost of Callbacks and How to Stop the Bleeding, Air Conditioning Contractors of America (ACCA), 2025. Callback cost context for condition-based replacement.