What size generator is needed to run a 3-ton AC and well pump?

A standard 3-ton central AC draws ~75A LRA at 240V, creating an initial startup spike of roughly 18,000 watts. With a soft starter (-65% inrush), it can run comfortably with a well pump on an 8,500W to 9,500W generator.

Do I need a 30-amp or 50-amp generator transfer switch?

A 30-amp transfer switch supports up to 7,200 continuous watts at 240V. Running loads over 7,200W or central AC units 2.5 tons or larger require a 50-amp transfer switch (CS6375) rated for up to 12,000W.

Whole-House & Supplemental Generator Sizer

1. Enter Heavy Inductive Loads

Central A/C Condenser Size: Select system size to load typical Locked Rotor Amps (LRA).

A/C Locked Rotor Amps (LRA Data Tag): Found stamped directly on outdoor condenser data plate.

Central A/C Electronic Soft Starter:

240V Large Window / Through-Wall A/C Units: Heavy 240V room conditioners used in shops, additions, or suites.

120V Standard Window A/C Units: Standard 15-amp bedroom/living room window units.

Deep Well Submersible Pump:

2. Critical Household Circuits

Kitchen Refrigerator & Deep Freezer (800W Run / 1,600W Surge) Basement Sump Pump 1/2 HP (1,050W Run / 2,200W Surge) Gas/Propane Furnace Blower (800W Run / 1,400W Surge) Electric Water Heater Element (4,500W Continuous / No Surge) LED Lighting, Network Router & Home Office (750W Continuous)

3. Electrical Sizing & Hardware Specs

Recommended Generator Capacity Rating:

0.0 kW Running / 0.0 kW Peak

Select equipment loads to calculate capacity.

Total Continuous Running Load (with 1.25× Safety Buffer):

0 W (0.0 kW)

Includes NEC 125% continuous-duty capacity buffer.

Worst-Case Motor Startup Surge Demand:

0 W (0.0 kW)

Calculated assuming highest concurrent motor startup.

Required Transfer Switch & Power Inlet Box Tier:

30 Amp (7,200W Max)

NEMA Configuration: L14-30P Twist-Lock (30A 120/240V)

Central Air Conditioning Starting Inrush & Generator Matching Chart

Air conditioning compressors pull severe instantaneous inrush current (LRA) upon ignition. Review standard loads versus soft-starter mitigated capacity:

A/C Condenser Size	Average LRA	Raw Startup Surge (240V)	Surge w/ Soft Starter (-65%)	Min Generator (Peak / Run)	Required Inlet
2.5 Ton (30k BTU)	60 Amps	14,400 Watts	5,040 Watts	8,500W Peak / 6,500W Run	30A (L14-30P)
3.0 Ton (36k BTU)	75 Amps	18,000 Watts	6,300 Watts	10,500W Peak / 8,000W Run	50A (CS6375)
3.5 Ton (42k BTU)	88 Amps	21,120 Watts	7,390 Watts	12,000W Peak / 9,500W Run	50A (CS6375)
4.0 Ton (48k BTU)	105 Amps	25,200 Watts	8,820 Watts	14,000W Peak / 10,500W Run	50A (CS6375)
5.0 Ton (60k BTU)	135 Amps	32,400 Watts	11,340 Watts	18,000W Peak / 13,000W Run	50A (CS6375)

Frequently Asked Questions

What size generator is needed to run a 3-ton AC and a well pump?

A standard 3-ton compressor spikes to 75A LRA (18,000W surge). With a soft starter, surge drops to ~6,300W, allowing both the 3-ton AC and a 0.75 HP pump to run on a 9,500W generator with a 50A transfer switch.

When is a 50-amp transfer switch necessary over a 30-amp?

A 30A inlet limits total load to 7,200W. If continuous loads exceed 7,200W or you run multiple heavy 240V appliances (central AC, 240V window units, water heater), a 50A inlet (CS6375, 12,000W max) is required.

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Pre-wired subpanel interlock kit matching your calculated electrical service amperage tier.

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Safety Disclaimer: Calculations are approximations for planning. Always verify breaker sizing and interlocks with a licensed electrician before connecting generators.