Emergency generator sizing is the process of calculating the maximum simultaneous running load plus the highest single motor starting surge to select an alternator and engine that won't stall or drop voltage under peak demand. This calculation fundamentally changes the physical alternator frame size, the engine horsepower required, the automatic transfer switch (ATS) amp rating, and the AWG gauge of the feeder conductors between the unit and your main panel. The most common mistake DIYers and junior electricians make is confusing the physical size of the main service breaker (e.g., 200A) with the actual connected load, or assuming a generator's marketing "surge watt" rating can handle indefinite, high-inertia motor-starting events without collapsing the voltage waveform.
The Core Physics: Running Watts vs. Locked Rotor Amps (LRA)
When sizing a generator, you are actually sizing two distinct components: the engine (which provides continuous mechanical horsepower to sustain running watts) and the alternator (which provides the magnetic mass to absorb instantaneous electrical surges). Resistive loads like incandescent lights, space heaters, and water heaters are straightforward; their starting current is identical to their running current. Inductive loads, specifically AC motors in HVAC compressors, well pumps, and refrigerator compressors, are where sizing gets complex.
When an AC motor starts from a dead stop, the rotor isn't yet spinning to generate back-EMF (electromotive force). For the first 200 to 500 milliseconds, the motor winding acts almost like a dead short circuit. This inrush current, known as Locked Rotor Amps (LRA), is typically 3 to 6 times the Full Load Amps (FLA). Pushing a heavy truck from a dead stop requires massive initial torque, but keeping it rolling at 30 mph takes a fraction of the energy; electric motors behave exactly the same way.
If the alternator's physical copper and iron mass is too small to supply this instantaneous LRA demand, the magnetic field inside the alternator collapses. This causes a severe voltage dip. According to NEMA MG-1 standards, a 35% voltage dip during motor starting is technically permissible for the alternator's health, but in practice, if your home's voltage drops below 80% of nominal (roughly 192V on a 240V system), the magnetic coils inside your HVAC contactors will weaken and drop out, killing the start cycle.
Where You Meet This In Practice: Transfer Switches and Feeder Wire
Proper emergency generator sizing dictates your physical installation materials. Once you determine the maximum continuous current and the physical generator output, you must select your feeder wire and ATS based on NEC Article 310 and Article 445.
- Feeder Wire Sizing: If you size a 22kW generator (like the Generac Guardian 7042) for a 200A panel, the generator's maximum continuous output is roughly 91A at 240V. However, if you are using the generator to backfeed a 200A panel via a service-rated ATS, NEC 310.16 (75°C column) requires you to size the conductors for the ATS rating. This means pulling 2/0 AWG copper or 4/0 AWG aluminum for the four feeder conductors (L1, L2, Neutral, Ground) if the run is under 100 feet.
- ATS Sizing: The Automatic Transfer Switch must be rated for the upstream overcurrent device or the full load current of the generator, whichever is applicable based on whether it is a service-entrance-rated unit or a sub-panel branch ATS.
- Breaker Coordination: The generator's internal output breaker must be sized to protect the alternator windings from continuous thermal overload, usually set at 100% to 115% of the alternator's continuous nameplate amperage.
Worked Numeric Example: Sizing a 200A Residential Panel
Let's run a real-world NEC Article 220 style load calculation for a standard 3,000 sq ft home to see how emergency generator sizing plays out on paper.
The Loads:
- Base Continuous: LED lighting, router, refrigerator, TV, and gas furnace blower = 3,500W.
- HVAC: 3-ton central AC unit. Nameplate FLA = 18A at 240V (4,320W running). Nameplate LRA = 95A.
- Well Pump: 1HP, 240V submersible. FLA = 8A (1,920W running). LRA = 35A.
The Calculation:
First, sum the running watts: 3,500W (base) + 4,320W (HVAC) + 1,920W (pump) = 9,740W continuous running load.
Next, calculate the worst-case starting surge. Assume the well pump is already running when the AC compressor kicks on. The surge demand is the base load + running pump + AC starting surge.
AC Starting Watts = LRA × Voltage = 95A × 240V = 22,800W.
Total Peak Surge = 3,500W (base) + 1,920W (pump) + 22,800W (AC surge) = 28,220W (28.2kW).
Real-World Scenario Walkthrough: The HVAC Stall Failure
The Setup: A homeowner purchases a 20kW air-cooled generator for a 200A service, assuming "20kW is plenty for a normal house." They have a 5-ton central AC unit with an LRA of 120A (28,800W surge demand). They bypass professional load calculation software to save money.
The Numbers: The 20kW generator's alternator is physically rated to deliver about 24kW of surge for a fraction of a second before voltage regulation fails. The AC demands 28.8kW.
The Outcome: The grid drops. The generator starts, and the ATS transfers the house to generator power. The thermostat calls for cooling, and the AC contactor pulls in to send 240V to the outdoor compressor.
What Went Wrong: The 28.8kW surge demand vastly exceeded the alternator's magnetic capacity. The voltage at the ATS plummeted from 240V to 135V for 400 milliseconds. Because the voltage dropped below the 160V holding threshold of the AC contactor coil, the contactor physically dropped out, cutting power to the compressor. The AC never started. Meanwhile, the massive current draw bogged the generator's engine down, dropping the frequency below 55Hz. The generator's internal controller registered a low-frequency fault, shut the engine down entirely, and locked out the system. The house sat in the dark, and the homeowner had to pay for a manual reset and a larger alternator retrofit.
FAQ: Emergency Generator Sizing Edge Cases
Q: Can I just size the generator to match my 200A main breaker?
A: No. A 200A breaker at 240V equals 48,000W (48kW). Very few homes actually pull 48kW continuously. Sizing a generator to the main breaker rather than the calculated load will result in buying a massively oversized, $25,000+ liquid-cooled commercial unit that will wet-stack (accumulate unburned fuel in the exhaust) because it never runs under enough load to reach optimal operating temperature.
Q: How do solar panels and battery backups change generator sizing?
A: If you have a grid-tied solar inverter, the generator must be large enough to "sink" the excess solar production if the batteries are full and the home load drops. A common rule of thumb from NFPA 70 (NEC) guidelines and inverter manufacturers is that the generator's continuous rating must be at least 1.25 times the maximum continuous output of the solar inverter to prevent the solar inverter from backfeeding and tripping the generator's reverse-power protection.
Q: What is a "hard-start kit" and does it change my sizing math?
A: A hard-start kit adds a start capacitor and a potential relay to an AC compressor. It provides a massive phase-shifted torque boost to get the rotor spinning in milliseconds, effectively reducing the time the LRA is drawn. While it doesn't strictly lower the absolute peak LRA number on the nameplate, it reduces the energy (watt-seconds) required to start, which is often enough to allow a smaller 22kW generator to start a 4-ton or 5-ton AC unit without dropping voltage below the contactor's threshold. Always verify with the generator manufacturer's sizing software before relying on this workaround.
