HVAC Capacitor Sizing Chart
Typical dual-run capacitor values by tonnage, plus the field rules for testing a weak cap and picking the right replacement without a callback.
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In This Guide
What a Run Capacitor Actually Does
A failed capacitor is one of the most common reasons a condenser hums but won't start, and it is usually a fifteen-minute fix once you have the right part on the truck. The run capacitor stores and releases charge on every AC cycle to create a phase shift in the motor's start winding. That phase shift is what produces starting torque and keeps the single-phase compressor and condenser fan turning efficiently. When the cap weakens, the motor draws more current, runs hotter, and eventually stalls under load.
Most residential condensers use a single dual-run capacitor — one can with three terminals that services both the compressor and the fan motor. You will see the terminals marked:
- HERM — hermetic compressor start winding (the larger MFD value)
- FAN — condenser fan motor (the smaller value, usually 5 MFD)
- C — common, tied to the line side (L2) through the contactor
Capacitance is measured in MFD (µF)
MFD stands for microfarad. A dual cap printed 45/5 is 45 MFD on the HERM side and 5 MFD on the FAN side. The two ratings are independent — one section can fail while the other tests fine.
Dual-Run Capacitor Sizing Chart by Tonnage
The table below shows the microfarad values you will most often find on residential straight-cool and heat pump condensers. Treat it as a ballpark for stocking your truck, not a substitute for the nameplate. Two 3-ton units from different manufacturers can call for different caps depending on the compressor and fan motor.
| Unit Size | Typical Dual-Run Cap (HERM/FAN) | Notes |
|---|---|---|
| 1.5 Ton | 30/5 MFD | Smaller scroll or recip compressors |
| 2 Ton | 35/5 MFD | Very common residential size |
| 2.5 Ton | 40/5 MFD | Fan side occasionally 7.5 |
| 3 Ton | 40/7.5 or 45/5 MFD | Check the fan motor rating |
| 3.5 Ton | 45/5 or 50/5 MFD | Varies by compressor brand |
| 4 Ton | 50/7.5 or 55/5 MFD | Larger fan motors trend to 7.5 |
| 5 Ton | 60/5 or 70/5 MFD | Biggest common residential cap |
Caution: Tonnage alone never picks the cap. Variable-speed and ECM fan motors often do not use a run capacitor at all, and two-stage compressors can carry ratings that break this pattern. Always confirm against the printed MFD on the old cap and the unit data plate before you install.
Reading the Label: MFD and Voltage
Every capacitor carries two numbers you care about: the capacitance in MFD and the voltage rating in VAC. On a dual cap you will see something like 45 + 5 MFD, 440 VAC. Residential caps are almost always 370V or 440V rated.
Must match the original exactly. This is set by the motor design — it is not adjustable.
May be equal or higher than the original, never lower. A 440V cap safely replaces a 370V.
A common shop practice is to stock everything in 440V so one voltage covers both 370V and 440V applications. The higher voltage rating does not change how the cap behaves in circuit — it just means more headroom against dielectric breakdown, which often translates to longer service life in hot rooftop and attic condensers.
The Three Replacement Rules
Memorize these and you will never grab the wrong cap off the shelf:
1. Match MFD exactly
A 45/5 gets replaced with a 45/5. Do not round to the nearest stocked value. Undersizing weakens torque; oversizing overheats the start winding.
2. Voltage equal or higher
440V replaces 370V. Never go the other way — a 370V cap in a 440V-rated circuit is a premature failure and a safety hazard.
3. Replace at more than 6% off rated
A cap that measures more than roughly 6% below its printed MFD is failing. Replace it even if the unit still starts today — it is on borrowed time.
Do not split a dual cap into two singles unless you have to
In a pinch you can replace a bad 45/5 with a 45 MFD single on HERM and a separate 5 MFD single on FAN, jumpering both commons together. It works, but it is a temporary fix — order the correct dual cap and come back.
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How to Test a Capacitor
Safety first — discharge before you touch it
Kill the disconnect and the indoor breaker. A charged run capacitor holds enough energy to hurt you. Bleed each terminal pair through an insulated resistor (or a screwdriver with an insulated handle across the terminals) before handling any lead. Never assume a cap is dead.
The right way to test is with a meter set to capacitance, reading each section with at least one lead removed from the circuit so the motor windings don't skew the reading:
- Read C to HERM — compare to the large MFD value on the label
- Read C to FAN — compare to the small MFD value (usually 5)
- A reading of OL / open or near zero means the section is dead
- A bulged or leaking top is a visual failure — replace it regardless of reading
The 6% rule in numbers
For a 45 MFD section, 6% is about 2.7 MFD. So anything reading below roughly 42.3 MFD is out of tolerance and should be replaced. Manufacturers typically allow a ±6% band, so use that as your pass/fail line.
Worked Example: A Weak 45/5
The call:
A 3-ton condenser is humming at the contactor but the compressor won't spin up. The fan turns. The old dual cap is printed 45/5 MFD, 440 VAC.
Step 1: Kill power and discharge both terminal pairs.
Step 2: Meter the HERM section.
C to HERM reads 39.1 MFD (rated 45)
Step 3: Meter the FAN section.
C to FAN reads 4.9 MFD (rated 5) — good
Step 4: Apply the 6% rule to HERM.
45 × 0.94 = 42.3 MFD minimum. 39.1 < 42.3 → FAIL
Step 5: Replace with a matching 45/5, 440V cap. Wire HERM, FAN, and C to match the old one.
Result: fixed
The compressor starts cleanly. Confirm the fix by checking compressor amp draw against the nameplate RLA — it should now be at or below RLA, not pinned high. Only 1.7 MFD low was enough to stall a hot compressor under load.
Field Notes and Common Mistakes
- Photograph the wiring before you pull leads. HERM, FAN, and C are easy to swap in a hurry, and a miswire can take out the new cap or the fan motor.
- A repeatedly failing cap is a symptom, not the disease. Chronic cap failures point to a hot condenser — dirty coil, high head pressure, or a fan motor drawing over its rated amps. Fix the root cause.
- Heat is the enemy. Rooftop and attic units cook their capacitors. A 440V cap in a hot location routinely outlives a 370V one in the same spot.
- A hard start kit is not a substitute for the right cap. A start-assist device helps a marginal compressor break away, but if the run cap is out of spec, replace it — don't mask it.
- Bulging tops mean replace on sight. A domed or vented top is a cap that has already failed internally, even if it still meters close.
Keep the common residential values on the truck — 35/5, 40/5, 45/5, and 60/5 in 440V will cover most of what you meet. When in doubt, the printed label on the old cap and the unit data plate are the final word, not the tonnage.
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