AC LRA Calculator
Accurately convert Locked Rotor Amps (LRA) to AC Starting Surge Watts, determine continuous running wattage, calculate Soft Start kit reduction savings, and find the perfect backup generator size for your central air conditioner or heat pump.
Enter Air Conditioner Specifications
Calculated Surge & Generator Sizing
Required AC Starting Surge Power:
Instantaneous Peak Current: 78.0 Amps (Surge @ 240V)
🎉 Soft Start saves 12,168 Watts (50.7 A) of surge capacity!
US Central AC & Heat Pump LRA Reference Chart
Standard 240V single-phase values for standard efficiency (14–16 SEER) compressors.
| AC Size (Tons / BTU) | Running Amps (RLA) | Standard LRA | Standard Starting Surge | Soft Start LRA (Surge) | Rec. Portable Gen (w/ Soft Start) |
|---|---|---|---|---|---|
| 1.5 Ton (18,000 BTU) | 7 - 9 A | 45 A | 10,800 W | 15 A (3,600 W) | 5,000W - 6,500W |
| 2.0 Ton (24,000 BTU) | 9 - 12 A | 55 A | 13,200 W | 18 A (4,320 W) | 6,500W - 7,500W |
| 2.5 Ton (30,000 BTU) | 11 - 14 A | 65 A | 15,600 W | 22 A (5,280 W) | 7,500W - 8,500W |
| 3.0 Ton (36,000 BTU) ⭐ | 13 - 16 A | 78 A | 18,720 W | 26 A (6,240 W) | 8,500W - 9,500W |
| 3.5 Ton (42,000 BTU) | 15 - 19 A | 90 A | 21,600 W | 30 A (7,200 W) | 9,500W - 11,000W |
| 4.0 Ton (48,000 BTU) | 17 - 22 A | 105 A | 25,200 W | 35 A (8,400 W) | 11,500W - 13,000W |
| 5.0 Ton (60,000 BTU) | 22 - 28 A | 135 A | 32,400 W | 45 A (10,800 W) | 13,500W - 15,000W |
The AC Starting Watts Formula
To size an emergency generator or battery inverter system properly, you must convert the Locked Rotor Amperage into Apparent Surge Power (Volt-Amperes/Watts):
*For 3-phase commercial systems (208V/480V): Starting Watts = LRA × Volts × √3 (1.732).
Why LRA Spikes Cause Generator Overload
When an AC condenser compressor starts against head pressure:
- Zero Counter-EMF: For the first 100-300 milliseconds, the rotor is motionless, drawing maximum current like a direct short.
- Voltage Sag: Standard portable generators cannot maintain 240V under instantaneous 70A–130A draws, causing generator stalls or breaker trips.
- Soft Start Fix: Electronic soft starters ramp up voltage in a smooth sine-wave, eliminating the heavy torque surge and light flicker.
Soft Start vs. Hard Start Capacitor: What is the Difference?
Many homeowners confuse a cheap $25 "Hard Start Kit" (capacitor + relay) with an advanced electronic Microprocessor Soft Starter. Here is why the difference is critical for backup generators:
Hard Start Kit (Capacitor)
~$20 - $40Adds extra starting torque by increasing initial inrush current through a capacitor. Helps aging compressors start faster, but DOES NOT significantly reduce LRA surge. Often worsens generator overload.
Microprocessor Soft Starter
~$280 - $350Learns compressor characteristics over 5 starts, gradually applying voltage and optimizing phase angles to cut locked-rotor surge by 65% to 70% while extending compressor lifespan.
How to Read Your AC Condenser Nameplate
Every outdoor condensing unit or heat pump manufactured in the United States features a metal data plate containing critical electrical ratings required by the National Electrical Code (NEC):
The momentary startup surge current drawn by the compressor motor. Usually between 45A and 150A.
Continuous steady-state operating current of the compressor under full 95°F ambient cooling load.
Determines the minimum copper wire gauge (AWG) required to feed the AC disconnect and breaker.
The maximum circuit breaker or HACR fuse rating allowed in your main electrical service panel.
Frequently Asked Questions (FAQ)
What is LRA (Locked Rotor Amps) on an Air Conditioner?
Can a 10,000 Watt generator start a 3-Ton or 4-Ton AC?
With a Soft Start: Yes! A soft start kit reduces the 3-ton startup surge to ~6,200W (26 LRA), allowing a standard 10,000W / 8,000W generator to easily start and run your whole AC unit with headroom for refrigerators and lights.
How do I find my AC tonnage from the model number?
18 = 1.5 Tons (18,000 BTU), 24 = 2 Tons, 30 = 2.5 Tons, 36 = 3 Tons, 42 = 3.5 Tons, 48 = 4 Tons, and 60 = 5 Tons.