Replacing a Run Capacitor or Contactor: How to Pick the Right Part
Capacitors and contactors are the most replaced parts on a condensing unit. Getting the replacement right takes a minute and saves a callback.
Safety: Turn off power at the disconnect and verify it's off. Discharge the capacitor before touching the terminals. Take a photo of the wiring before you remove anything.
Picking a run capacitor
| Rating | Rule |
|---|---|
| Capacitance (µF) | Match the original exactly. A dual like 45/5 must match both HERM (45) and FAN (5). |
| Voltage | Equal or higher. 440 V can replace 370 V; 370 V should not replace 440 V. |
| Type | Run capacitor (oil-filled, continuous duty). Don't use a start capacitor in a run circuit. |
| Shape and size | Round or oval doesn't matter electrically; it must fit the bracket and clear the panel. |
| Temperature rating | Equal or better than the original. |
Two singles for one dual works if each matches its side: one capacitor for the compressor (HERM) and one for the fan. Wire carefully: the commons tie together.
Universal multi-value capacitors can replace many sizes by jumpering taps. Follow the label exactly and double-check the values with a meter after wiring.
Before replacing it, confirm the capacitor is actually bad: see how to test a run capacitor and the capacitor check calculator.
Picking a contactor
| Rating | Rule |
|---|---|
| Coil voltage | Match it, usually 24 VAC on residential systems. |
| Poles | Match: single-pole (one leg switched, a shunt on the other) or two-pole. Two-pole can replace single-pole if wired correctly. |
| Amp rating (FLA / inductive) | Equal or higher than the original, and above the compressor RLA plus fan FLA. |
| Resistive amps | Higher than FLA; matters for heat strips, not compressors. |
| Mounting and terminals | Same footprint and terminal layout makes the swap faster and neater. |
A single-pole contactor leaves one leg of power connected to the compressor and fan when it's off. Treat everything in the unit as live until you've shut off the disconnect.
Before replacing it, confirm it's the problem: see how to test a contactor.
While you're there
- Look for why it failed. Burned contacts and swollen capacitors often follow high head pressure from a dirty condenser, a weak fan motor, low voltage, or short cycling.
- Check the wiring for burned terminals and loose connections, and replace damaged spade connectors.
- Measure amps after the repair: compressor and fan amps against the nameplate.
- Check voltage at startup: a big drop points to supply problems that kill contactors and capacitors.
Hard start kits
A hard start kit (a start capacitor plus a relay or a device that takes it out of the circuit) can help a compressor that struggles to start on a weak voltage supply or with unequal pressures. It's a fix for starting trouble, not for a failing compressor. Explain that to the customer if you install one. See AC compressor not starting.
Common mistakes
- Using a lower voltage capacitor because it was on the truck.
- Matching only one side of a dual capacitor.
- Swapping HERM and FAN wires.
- Using a lower amp contactor than the load needs.
- Replacing the part and leaving the cause, like a dirty condenser or failing fan motor.
FAQ
Can I use a higher µF capacitor?
No. Too much capacitance makes the motor run hotter and draw more current. Match the rating.
Can I replace a 370V capacitor with a 440V one?
Yes, as long as the µF matches and it fits. The higher voltage rating gives more margin.
Do I need an OEM contactor?
No, as long as the coil voltage, poles, amp rating and mounting match or exceed the original.
Sources
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Tools for this job
Related guides
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