HVAC Capacitor Sizing Guide
How to select, size, test, and replace run capacitors, start capacitors, and dual run capacitors for residential and light-commercial AC systems.
Safety Warning: Capacitors store a lethal electrical charge even after power is disconnected. Always discharge capacitors before handling. See the Safety section below.
Types of HVAC Capacitors
Run Capacitor
- Function: Stays in the circuit continuously while the motor runs, improving torque and efficiency.
- Shape: Oval or round metal can, oil-filled.
- Terminals: 2 terminals.
- Voltage: 370V AC or 440V AC.
- Typical MFD: 5–80 µF.
- Used for: Compressor motor, condenser fan motor, blower motor.
Start Capacitor
- Function: Provides extra starting torque; disconnected from the circuit once the motor reaches ~75% speed.
- Shape: Round plastic or phenolic case.
- Terminals: 2 terminals.
- Voltage: 125V AC or 250V AC.
- Typical MFD: 88–1200 µF.
- Used for: Hard-start kits, high-torque start applications.
Dual Run Capacitor
- Function: Combines compressor run capacitor and fan motor run capacitor in a single can.
- Shape: Round metal can, larger than single run.
- Terminals: 3 terminals labeled HERM, FAN, COM.
- Voltage: 370V AC or 440V AC.
- Rating format: e.g., 45/5 MFD (45 for compressor, 5 for fan).
- Used for: Most modern residential condensing units.
How to Read Capacitor Labels
Every capacitor carries a label with at least three key pieces of information. Understanding them prevents incorrect replacements.
| Label Item | What It Means | Example | Replacement Rule |
|---|---|---|---|
| MFD / µF | Capacitance in microfarads — the primary sizing spec | 45 MFD | Must match within ±5–10%. Never go higher. |
| VAC | AC voltage rating — maximum operating voltage | 370V AC | Equal or higher is acceptable. Never lower. |
| Dual notation | Two MFD values separated by slash — dual run capacitor | 45/5 MFD | First value is HERM (compressor), second is FAN. |
| Tolerance | Acceptable manufacturing variance from rated MFD | ±6% | Typical range ±3–6%; affects acceptable measurement range. |
| Temperature | Maximum operating temperature rating | 70°C or 85°C | Match or exceed. 85°C-rated caps are more reliable in hot locations. |
Single Run Capacitor Label Example
Replace with: 35 MFD, 370V AC or 440V AC
Dual Run Capacitor Label Example
HERM = 45 MFD (compressor), FAN = 5 MFD (fan motor)
AC Capacitor Size Chart by Tonnage
| System Tonnage | Approx. BTU/h | Compressor Run (HERM) | Fan Motor Run (FAN) | Voltage Rating |
|---|---|---|---|---|
| 1.5 ton | 18,000 | 25–35 MFD | 5–7.5 MFD | 370V or 440V |
| 2 ton | 24,000 | 30–40 MFD | 5–7.5 MFD | 370V or 440V |
| 2.5 ton | 30,000 | 35–45 MFD | 5–7.5 MFD | 370V or 440V |
| 3 ton | 36,000 | 40–55 MFD | 5–10 MFD | 370V or 440V |
| 3.5 ton | 42,000 | 40–55 MFD | 5–10 MFD | 370V or 440V |
| 4 ton | 48,000 | 45–60 MFD | 7.5–10 MFD | 370V or 440V |
| 5 ton | 60,000 | 55–70 MFD | 10–15 MFD | 370V or 440V |
How to Test a Capacitor
1 Multimeter Capacitance Test
- Step 1. Turn off power to the unit and wait 5 minutes.
- Step 2. Discharge the capacitor using a resistor or discharge tool (see Safety section).
- Step 3. Set your multimeter to capacitance mode (symbol: —|(—).
- Step 4. Disconnect the capacitor wires and connect the multimeter leads across the terminals.
- Step 5. Compare the reading to the rated MFD. A capacitor reading more than 10% below the rated value is weak and should be replaced.
2 Visual Inspection
Before testing with a meter, a quick visual check can confirm a failed capacitor immediately:
- ✗ Bulging or swollen top — the top of the capacitor can is dome-shaped rather than flat. This is a definitive sign of failure.
- ✗ Oil leaking — brown or dark oily residue around the base or terminals indicates internal failure.
- ✗ Burn marks or melted casing — carbon deposits or melted plastic on the body or terminals.
- ✗ Corroded terminals — heavy corrosion on the terminals increases resistance and causes poor contact.
- ✓ Flat top, no residue, clean terminals — visual appearance is good; proceed with meter test to confirm.
Capacitor Replacement Guidelines
Rules for Selecting a Replacement
MFD: Match within ±5–10%
A 45 MFD cap can be replaced with 42–48 MFD. Tighter is always better. Never use a higher MFD value.
Voltage: Equal or higher
A 370V cap can be replaced with a 440V cap of the same MFD. Both are fine on a 240V system. Never use a lower voltage rating.
Temperature Rating: Equal or higher
85°C-rated capacitors are preferred for outdoor installations where the electrical panel can exceed 70°C on hot days.
Never use a higher MFD value
Over-sizing the capacitor causes the motor winding to draw excess current, generating heat and accelerating insulation breakdown.
Dual Run Capacitor Substitution
When an exact dual run capacitor (e.g., 45/5 MFD 440V) is unavailable, you can substitute two individual run capacitors:
Wiring substitution for 45/5 MFD 440V dual cap:
Cap A (45 MFD): HERM ↔ COM
Cap B (5 MFD): FAN ↔ COM
Both capacitor negatives share the common (COM) connection. The compressor start winding connects to HERM and the fan motor start winding connects to FAN.
When to Install a Hard Start Kit
A hard start kit (potential relay + start capacitor) is recommended when:
- The compressor is repeatedly failing to start on the first attempt
- You have low voltage conditions (<210V on a 240V circuit)
- The compressor is older and showing high locked-rotor amperage
- You want to extend compressor life in harsh conditions
Common Symptoms of a Bad Capacitor
Humming But Not Starting
The motor tries to start, hums loudly, then trips on thermal overload. Classic sign of a failed run or start capacitor — the motor cannot develop enough torque to spin up.
AC Blowing Warm Air
A weak compressor run capacitor reduces compressor efficiency. The system runs but moves less refrigerant than rated, resulting in inadequate cooling despite the compressor appearing to operate.
Excessively Hot Compressor
A weak capacitor causes the motor to draw higher-than-rated amperage. This excess current generates heat in the motor windings, causing the compressor shell to become dangerously hot.
Tripped Breaker
High motor current from a weak capacitor repeatedly trips the circuit breaker. If the breaker trips every time the AC starts but not while running, suspect a start capacitor or hard-start relay fault.
Short Cycling
The compressor starts, runs briefly, then shuts off on thermal protection before completing a full cooling cycle. This cycle repeats, causing high energy use and accelerated compressor wear.
Condenser Fan Not Spinning
If the condenser fan motor hums but the blade doesn't spin (or spins only when manually started with a stick), the fan section of the dual run capacitor has likely failed.
Safety Warnings
Capacitor Safety — Read Before Servicing
Capacitors hold charge after power-off. A 440V capacitor can deliver a lethal shock minutes after the unit is shut off. Never assume a capacitor is safe just because power is disconnected.
Always discharge before handling. Use a 20,000-ohm, 2–5 watt resistor with insulated leads. Hold each lead on a terminal for 5–10 seconds. Then verify zero voltage with your multimeter.
Never short the terminals directly. Shorting with a screwdriver destroys the terminals, creates a dangerous arc flash, and can send metal fragments into your face and eyes.
Wear PPE. Safety glasses, insulated gloves rated for the operating voltage, and non-conductive footwear should be worn when working inside the electrical panel of any HVAC unit.
Use a dedicated capacitor discharge tool. Commercial discharge tools (available from HVAC supply houses) are purpose-built and safer than improvised resistor arrangements.
Dispose properly. HVAC capacitors are oil-filled. Check local regulations for proper disposal. Do not puncture the casing or incinerate capacitors.
Frequently Asked Questions
Check the original capacitor label for its MFD (microfarad) rating, or look on the unit's data plate and wiring diagram. As a general reference, 1.5-ton units typically use 25–35 MFD run capacitors, 2-ton units 30–40 MFD, 3-ton units 40–55 MFD, 4-ton units 45–60 MFD, and 5-ton units 55–70 MFD. Always match the exact OEM specification when possible.
No. Always replace a capacitor with one that matches the original MFD rating within ±5–10%. Using a higher MFD value causes the motor to draw excess current, overheat, and fail prematurely. You may use a higher voltage rating (e.g., 440V instead of 370V), but never a higher MFD.
A run capacitor stays energized the entire time the motor operates, improving efficiency and power factor. A start capacitor provides an extra torque boost only during startup and is disconnected from the circuit once the motor reaches about 75% of running speed via a centrifugal switch or relay.
Yes. A dual run capacitor (e.g., 45/5 MFD) combines the compressor run capacitor and the fan motor run capacitor in one housing. You can substitute two individual capacitors—one matching each MFD section—wired to the correct terminals (HERM, FAN, COM). This is a common field repair when the exact dual capacitor is unavailable.
With the power OFF, use a capacitor discharge tool or a 20,000-ohm, 5-watt resistor with insulated leads. Place one lead on each terminal and hold for 5–10 seconds. Never short capacitor terminals directly together—the resulting arc can damage the terminals and create a shock hazard. Always verify zero voltage with a multimeter before touching the terminals.
A failing run capacitor reduces motor torque and efficiency, causing the compressor or fan motor to draw high amperage, run hot, and eventually fail. Symptoms include the unit humming but not starting, tripping the breaker, blowing warm air, and the compressor or condenser fan motor becoming excessively hot to the touch.