Testing and Replacing Run Capacitors
The run capacitor is the single most common electrical failure on a condensing unit. Here is how to discharge it safely, read the MFD, catch a weak cap before it kills a compressor, and choose the right replacement.
Not sure what you're looking at?
Walk a no-cool call step by step with Ventora's AI troubleshooting flow — from contactor to capacitor to compressor.
In This Guide
What a Run Capacitor Actually Does
A run capacitor stays in the circuit the entire time a motor runs. It creates a phase shift between the run and start windings of a single-phase PSC (permanent split capacitor) motor, giving the motor the rotating magnetic field it needs to turn efficiently. Without that phase shift, the motor either won't start at all or draws locked-rotor amps trying to.
On a typical residential condensing unit you'll find a single dual run capacitor — one can with three terminals that serves both loads:
- HERM — feeds the hermetic compressor (the larger MFD value, e.g. 45)
- FAN — feeds the condenser fan motor (the smaller value, e.g. 5)
- C (Common) — the shared line side that both sections reference
A cap printed 45/5 MFD means 45 microfarads on the HERM side and 5 on the FAN side. Because it's a single component doing two jobs, one failed section can take out the compressor, the fan, or both.
Symptoms of a Weak or Dead Cap
A failing capacitor rarely announces itself cleanly. Watch for:
- Compressor or fan hums but won't spin — the classic dead-cap signature
- Fan blade that will only turn after a nudge with a stick — the fan section has lost capacitance
- Hard starting — compressor grunts, dims the lights, then trips on its internal overload
- Elevated running amps on the compressor or fan compared to nameplate RLA/FLA
- Short cycling on the overload as an underpowered motor overheats
- Physically bulging, domed, or leaking can — the top is designed to swell before it ruptures
Field tip: A bulged top is a definite failure, but a capacitor can read badly out of tolerance while still looking perfectly flat. Never condemn or clear a cap by looks alone — put a meter on it.
Safety: Discharge Before You Touch It
A run capacitor stores a charge even after the power is off. A fully charged 440V cap can deliver a painful — and occasionally dangerous — jolt, and it can weld a screwdriver tip or arc across your meter leads. Treat every capacitor as charged until you've proven otherwise.
Lockout and discharge, every time
- Open the disconnect at the condenser and the breaker at the panel.
- Verify 0 volts across the contactor line terminals with your meter before reaching in.
- Discharge each terminal to Common through a 20,000-ohm, 2-watt resistor (the proper way) or by bridging with an insulated-handle screwdriver in a pinch.
- Discharge HERM-to-C and FAN-to-C separately — both sections hold charge.
A resistor bleeds the charge in a controlled second or two without the bang and pitting you get from a dead short. Keep one in your bag.
How to Test: Step by Step
The most reliable test uses a meter with a capacitance (µF / MFD) function. Measure with at least one lead disconnected so the motor windings don't skew the reading.
Step 1 — De-energize: Kill the disconnect and breaker, then confirm 0V at the contactor.
Step 2 — Discharge: Bleed HERM-to-C and FAN-to-C through a 20k resistor.
Step 3 — Document: Photograph the wiring, then lift the leads off the terminals (or at least the wire off the terminal you're reading).
Step 4 — Measure the fan section: Set the meter to capacitance and read FAN to C. Compare to the smaller printed value.
Step 5 — Measure the compressor section: Read HERM to C. Compare to the larger printed value.
Step 6 — Judge each section: Apply the 6% tolerance below. Either section out of spec condemns the whole cap.
No capacitance meter? Use the amp-clamp method
You can calculate capacitance under load with a clamp meter and a voltmeter using MFD = (2652 × I) / V, where I is the amps flowing through the capacitor lead and V is the voltage measured across the capacitor. It works, but you're probing a live 240V unit — a dedicated capacitance meter is safer and faster.
Ventora — Your AI HVAC Companion
Snap a photo of the cap's label, get the MFD spec and a matched replacement in seconds. Free on iOS.
Reading MFD and the 6% Rule
Every run capacitor is stamped with a microfarad value and a tolerance — usually ±6%. That tolerance is your pass/fail line: if a measured section drifts more than 6% away from its rated MFD, the phase shift is off enough to stress the motor, and the cap should be replaced.
The tolerance window
Anything outside this band fails. In the field, caps almost always fail low — they lose capacitance as the dielectric degrades.
| Rated MFD | Low Limit (−6%) | High Limit (+6%) |
|---|---|---|
| 5 MFD | 4.7 | 5.3 |
| 35 MFD | 32.9 | 37.1 |
| 45 MFD | 42.3 | 47.7 |
| 70 MFD | 65.8 | 74.2 |
A meter that reads OL on a discharged cap means it's open (dead); a reading near zero with a beep on continuity means it's shorted. Both are automatic replacements.
Worked Example: A Weak Dual Cap
The call:
A 3-ton condenser is running but not cooling. The compressor cycles on and off, the fan spins fine. The cap is stamped 45/5 MFD, 370V. You discharge it and take readings.
Step 1: Fan section (FAN–C): reads 4.9 µF against 5 rated.
Window is 4.7–5.3 → in tolerance. Fan side is good.
Step 2: Compressor section (HERM–C): reads 39.8 µF against 45 rated.
Low limit is 45 × 0.94 = 42.3. Measured 39.8 is below it.
Step 3: Percent off = (45 − 39.8) / 45 × 100 = 11.6% low.
Result: Replace the cap
The HERM section is 11.6% low — nearly double the 6% limit. That weak phase shift explains the hard-starting compressor tripping its overload. Even though the fan section still passes, you replace the whole dual cap.
After replacement
With a fresh 45/5 installed, the compressor starts crisply, run amps drop back to nameplate RLA, and the system holds a normal temperature split. If it still hard-starts on a healthy cap, that points toward a tired compressor and a hard start kit conversation — not another capacitor.
Choosing the Right Replacement
Getting the replacement right comes down to two numbers — MFD and voltage — plus a couple of practical rules:
- Match MFD exactly. A 45/5 gets a 45/5. Never round to what's on the truck — the wrong microfarad value throws off the phase angle and shortens motor life. When in doubt, match the nameplate spec, not just the old can.
- Voltage can be equal or higher, never lower. A 440V cap safely replaces a 370V cap; a 370V cap must not replace a 440V. Common ratings are 370V and 440V, and the popular 370/440V dual-rated caps cover both.
- No exact dual on hand? Two matched single run capacitors (one sized for HERM, one for FAN) wired to a common line will do the job of a dual cap.
- Respect polarity markings. Many caps mark the terminal wired to line/Common with a dash or arrow. Keep C on Common and land HERM and FAN correctly.
Don't stop at the cap
Capacitors don't usually die of old age alone. A pitted, chattering contactor, high head pressure from a dirty condenser, or a bearing-worn fan motor all drive a cap to fail early. Fix the root cause or you'll be back for the same part next season.
Capacitor Sizing Quick Reference
Typical dual run capacitor values by unit size. Always verify against the equipment nameplate — these are common factory pairings, not a substitute for the printed spec:
| Unit Size | Typical Dual Cap (Comp/Fan) |
|---|---|
| 1.5 Ton | 30/5 MFD |
| 2 Ton | 35/5 MFD |
| 2.5 Ton | 40/5 MFD |
| 3 Ton | 40/7.5 or 45/5 MFD |
| 3.5 Ton | 45/5 or 50/5 MFD |
| 4 Ton | 50/7.5 or 55/5 MFD |
| 5 Ton | 60/5 or 70/5 MFD |
Voltage on these is almost always 370V or 440V. A 370/440V dual-rated replacement is a safe, common stock item that fits either.
Ventora — The AI HVAC Assistant in Your Pocket
Identify a capacitor from a photo, pull sizing specs, and troubleshoot no-cool calls step by step. Built for techs who'd rather diagnose than dig through manuals.