🔋 Capacitor Type

Check your old capacitor label for the type

⚡ Motor Details
µF

From the label on your old capacitor

📊 Tolerance & Voltage

How close the replacement µF must match

VAC

Always meet or exceed this. 370V or 440V most common.

CAP
— µF
Estimated capacitor rating (live)
Select your motor details above

Calculating capacitor specifications…

🔋 Your Capacitor Results
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Recommended Capacitor Rating
— µF
at 370V minimum
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VAC
Min Voltage
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µF
Acceptable Range
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Cap Type
⚙️
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HP
Motor Size
Capacitor Confidence Level
Low High —

🔋 Standard Replacement Sizes

Capacitor Rating
Voltage
% Difference
Fit

📋 Full Specification Summary

Specification
Value
Note
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⚠️

Capacitors Store Dangerous Electrical Charge

A capacitor can hold a lethal charge even after the power is off. Always discharge the capacitor before handling using an insulated screwdriver and a 20,000-ohm resistor, or have a licensed HVAC technician do the replacement. Never touch the terminals with bare hands.

🛠️ Replacement Tips

HVAC Capacitors — What They Do and Why They Fail

Capacitors are small but critical. When one fails, your AC stops working. Here’s what you need to know — explained in plain language.

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What Does a Capacitor Do?

A capacitor stores electrical energy and releases it in a controlled burst. In HVAC systems, run capacitors keep the compressor and fan motor running smoothly. Start capacitors give the motor an extra energy kick to get it spinning from a dead stop.

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Run vs. Start Capacitor

A run capacitor stays in the circuit while the motor runs. It improves efficiency and keeps voltage stable. A start capacitor is only used for the brief moment when the motor starts. It disconnects automatically once the motor reaches speed. Many units only have a run capacitor.

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Dual Run Capacitor

A dual run capacitor serves both the compressor and the fan motor from one single cylinder. It has three terminals: HERM (for the compressor), FAN, and C (common). Replacing a dual cap with two single caps is allowed if the µF ratings match exactly.

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What Does µF Mean?

µF stands for microfarads. It’s the unit of capacitance — how much energy the capacitor can store. A higher µF means more stored energy. Use the exact rated µF for your motor. Going too high or too low hurts motor performance and lifespan.

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Signs of a Bad Capacitor

The most obvious sign is a bulging or leaking cylinder. Other signs: the AC hums but doesn’t start, the fan spins slowly, the compressor makes a clicking noise then shuts off, or the unit runs but doesn’t cool well. A failed capacitor is one of the most common AC repair calls.

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Why Capacitors Fail

Heat is the number one killer. Capacitors that live in hot outdoor units degrade faster. Voltage spikes also weaken them over time. Most capacitors last 5–20 years. If yours is over 10 years old and showing symptoms, replacement is usually worth it before a more expensive failure happens.

📐 How We Estimate Capacitor Size

Capacitor µF ≈ (HP × 2652) ÷ Voltage Acceptable Range = Rated µF × (1 ± tolerance%) Health % = (Measured µF ÷ Rated µF) × 100

The HP-to-µF formula is an approximation used in field sizing. Real-world values vary by manufacturer and motor design. Your existing capacitor’s label is the most accurate source — always match or stay within tolerance of that number. For dual run capacitors, the compressor (HERM) and fan ratings are sized independently using the above formula, then combined.

Capacitor Size Reference Chart

Typical capacitor values for common residential HVAC equipment at 230V.

EquipmentTypical µF (Run)Voltage RatingType
1/3 HP Condenser Fan5 µF370VRun
1/2 HP Condenser Fan7.5 µF370VRun
1.5–2 Ton Compressor25–35 µF370VRun
2.5–3 Ton Compressor40–50 µF370VRun
4–5 Ton Compressor55–70 µF370–440VRun
Dual Run (2–3 ton)35/5 µF370VDual Run
Dual Run (3–4 ton)45/5 or 50/5 µF370VDual Run
Start Capacitor (most)88–108 µF250VStart

Frequently Asked Questions

Yes — within tolerance. The AHRI standard allows ±6% of the rated value. Most HVAC technicians use ±10% as an acceptable field replacement rule. Going higher than rated causes the motor to overheat. Going lower causes the motor to run inefficiently and eventually burn out. Always stay as close to the original rating as possible.
Yes, absolutely. You can always replace a capacitor with one rated for a higher voltage — 440V instead of 370V is perfectly fine and often a smart upgrade in hot climates. What you cannot do is use a lower-voltage capacitor. If the capacitor is rated below your system voltage, it will fail quickly and possibly dangerously.
Yes. A dual 35/5 µF 370V capacitor can be replaced with one 35 µF run cap for the compressor and one 5 µF run cap for the fan motor. The total µF for each motor circuit must match the original. This is a common field workaround when the exact dual cap isn’t available. Wire HERM and FAN as they were, and join the two C (common) terminals together.
First, discharge the capacitor safely using a 20,000-ohm resistor across the terminals. Set your multimeter to capacitance mode (µF). Touch the probes to the capacitor terminals. The reading should be within ±6–10% of the rated value. If it reads significantly lower, or if you get zero or OL, the capacitor has failed and needs replacement.
There’s no fixed replacement schedule. Capacitors typically last 5–20 years. In hot climates (like the American South or Southwest), they often fail within 5–10 years. Have your HVAC technician test the capacitor during each annual tune-up. If it reads below 90% of its rated value, proactive replacement is cheaper than an emergency service call on the hottest day of summer.
This calculator gives accurate sizing information. Capacitor replacement is a job many DIY-capable homeowners do themselves. But the safety risk is real. Capacitors hold dangerous charge even with the power off. If you’re not comfortable with electrical work, have a licensed HVAC technician do the job. The part itself usually costs $15–$60, while a service call typically runs $75–$200.
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