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This guide explains what voltage your AC needs, how low voltage damages the system, and how to check voltage safely. If your AC is not cooling or your outside unit is not running, voltage may be one cause.
Residential central AC typically requires 208–230 volts (for 240V systems) or 115–120 volts (for 120V systems). The acceptable range is usually ±10% of the nameplate rating. Sustained voltage below 198V (for 240V) or 108V (for 120V) can damage the compressor.
| Symptom | Likely Cause |
|---|---|
| AC runs but cools poorly | Low voltage or refrigerant — see AC not cooling |
| Breaker trips when AC runs | Low voltage causing high amp draw — see AC breaker guide |
| AC starts then shuts off | Voltage drop under load |
| Compressor hums but doesn't start | Low voltage or capacitor |
AC motors are designed to run at a specific voltage. When voltage drops, the motor draws more current to maintain output. That creates heat. Repeated low-voltage cycles overheat windings, break down insulation, and shorten compressor life. A compressor replacement can cost $2,000–$4,000—protecting against low voltage is cost-effective.
Low voltage often accompanies capacitor and contactor strain. If your AC is not cooling, voltage is one of the first electrical checks.
⚠️ WARNING: Working with live voltage is dangerous. If unsure, hire a licensed electrician or HVAC technician.
If you have a multimeter and are comfortable:
Call a licensed electrician or HVAC technician if:
| Repair | Typical Cost |
|---|---|
| Voltage check (diagnostic) | $75–$150 |
| Dedicated circuit / wiring | $200–$500 |
| Compressor replacement (if damaged) | $2,000–$4,000+ |
Residential AC typically needs 208–230V (or 115–120V). Sustained below 198V (240V) or 108V (120V) can damage the compressor.
Yes. Low voltage forces the compressor to draw more current, overheat, and fail over time.
Use a multimeter at the disconnect. If unsure, hire an electrician or HVAC technician.
Load on the circuit. If it drops significantly, you may need a dedicated circuit or wiring upgrade.
Yes. Low voltage increases amp draw; combined with other loads, it can trip the breaker.
If your HVAC system is experiencing electrical issues and you're unsure how to proceed, schedule a licensed inspection before further damage occurs.
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Last updated: March 2026 · Reviewed for technical accuracy
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