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Home > Free Tools > Capacitor Sizing Guide
Your AC capacitor has failed and you need the right replacement. This guide shows you exactly how to read your old capacitor’s specs, find the correct MFD and voltage rating, and choose between single and dual run capacitors — so you order the right part the first time.
The easiest way to get the right replacement is to read the label on your existing capacitor. Every capacitor has these specs printed on it:
Turn off power at the disconnect. Remove the side access panel on your outdoor AC unit (usually 1-2 screws). The capacitor is the cylinder inside — silver or black, about the size of a soda can.
This is the most important number. It looks like “45+5 MFD” or “45 μF”. MFD and μF mean the same thing. A dual capacitor will show two numbers (e.g., 45+5); a single capacitor shows one number (e.g., 45).
Usually 370V or 440V. You can always go UP in voltage (use a 440V to replace a 370V) but never down. A 440V capacitor works in any application that calls for 370V or 440V.
Dual run capacitor (3 terminals: C, FAN, HERM): Powers both the compressor and fan motor. Most common in residential AC units. Has two MFD ratings (e.g., 45+5).
Single run capacitor (2 terminals): Powers one motor only. Used in some older systems or as a replacement when splitting a dual into two singles.
Start capacitor (2 terminals, usually black): Provides extra startup torque. Much higher MFD (88-108, 145-175, etc.) but only rated for intermittent use. Usually paired with a start relay.
The MFD rating MUST match exactly (within the ±5% OEM tolerance). Using the wrong MFD stresses the motor windings:
| If your capacitor says | You need | Notes |
|---|---|---|
| 45+5 MFD 440V | 45+5 MFD 370V or 440V dual run | Most common residential AC capacitor (3-3.5 ton units) |
| 40+5 MFD 440V | 40+5 MFD 370V or 440V dual run | Common in 2.5-3 ton units |
| 35+5 MFD 440V | 35+5 MFD 370V or 440V dual run | Common in 2-2.5 ton units |
| 55+5 MFD 440V | 55+5 MFD 440V dual run | Larger 4-5 ton units |
| 60+5 MFD 440V | 60+5 MFD 440V dual run | 5 ton units |
| 25+5 MFD 440V | 25+5 MFD 370V or 440V dual run | Smaller 1.5-2 ton units |
| 30+5 MFD 440V | 30+5 MFD 370V or 440V dual run | 2 ton units |
| 50+5 MFD 440V | 50+5 MFD 440V dual run | 3.5-4 ton units |
| 45 MFD 440V (single) | 45 MFD 370V or 440V single run | Compressor only (no fan section) |
| 5 MFD 370V (single) | 5 MFD 370V or 440V single run | Fan motor only capacitor |
| 7.5 MFD 370V (single) | 7.5 MFD 370V or 440V single run | Fan motor (some models) |
| 10 MFD 370V (single) | 10 MFD 370V or 440V single run | Fan motor or blower |
| AC Size (tons) | Typical Dual Run Capacitor | Compressor MFD | Fan MFD |
|---|---|---|---|
| 1.5 ton | 25+5 MFD 440V | 25 | 5 |
| 2 ton | 30+5 or 35+5 MFD 440V | 30-35 | 5 |
| 2.5 ton | 35+5 or 40+5 MFD 440V | 35-40 | 5 |
| 3 ton | 40+5 or 45+5 MFD 440V | 40-45 | 5 |
| 3.5 ton | 45+5 or 50+5 MFD 440V | 45-50 | 5 |
| 4 ton | 50+5 or 55+5 MFD 440V | 50-55 | 5 |
| 5 ton | 55+5 or 60+5 MFD 440V | 55-60 | 5 |
These are typical ranges. Always verify against the label on your existing capacitor or the unit’s data plate. The model number is the most reliable way to find the exact spec.
If you’re not sure whether the capacitor has failed, here’s the quick test:
Not all multimeters have this mode. If yours doesn’t, you can use the ohms test below instead.
Use an insulated resistor (20,000 ohm, 5-watt) across the terminals for 5 seconds. Then verify 0V with your meter.
Touch the meter probes to the C and HERM terminals (compressor side) and read the MFD. Then test C and FAN terminals (fan side).
OEM tolerance is ±5%. If the reading is more than 5% below the rated MFD, the capacitor is weak and should be replaced. Example: a 45 MFD capacitor reading 42.75 or below is out of spec.
| Rated MFD | Minimum OK (−5%) | Replace if below |
|---|---|---|
| 5 | 4.75 | 4.75 |
| 25 | 23.75 | 23.75 |
| 30 | 28.50 | 28.50 |
| 35 | 33.25 | 33.25 |
| 40 | 38.00 | 38.00 |
| 45 | 42.75 | 42.75 |
| 50 | 47.50 | 47.50 |
| 55 | 52.25 | 52.25 |
| 60 | 57.00 | 57.00 |
For capacitors: yes, OEM matters. Here’s why:
OEM capacitors are manufactured to ±5% MFD tolerance. Generic aftermarket capacitors typically run ±10-15% out of spec. On a 45 MFD compressor, that means a generic could deliver anywhere from 38-52 MFD — an 8-15% deviation that places constant stress on the compressor windings.
The compressor is the most expensive component in your AC ($1,500-$2,500 to replace). Protecting it with a $10-15 more expensive OEM capacitor instead of a $20 generic is obvious insurance. We’ve seen systems where a cheap generic capacitor killed a compressor within 2 years.
| Symptom | What’s happening | Severity |
|---|---|---|
| AC hums but won’t start | Capacitor can’t provide startup energy to the compressor. Motor stalls. | Replace now |
| Fan spins slowly or won’t spin | Fan section of dual capacitor has failed. Compressor may still work. | Replace now |
| AC shuts off after a few minutes | Weak capacitor causes motor to overheat, triggering thermal overload. | Replace soon |
| Higher electric bills | Weak capacitor forces motor to draw more amps to compensate. | Replace soon |
| Capacitor is bulging or leaking oil | Physical failure. The dielectric inside has broken down. | Replace immediately |
| Clicking sound from contactor, no compressor start | Contactor engages but capacitor can’t deliver startup boost. | Replace now |