Generator Sizing Calculator

Size standby and emergency generators, motor starting surge kVA, fuel/altitude derates, and transfer switches per NEC Articles 445 & 702.

NEC edition

Inputs

kW
kW
kW
ft

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

Sizing an electrical generator requires balancing continuous running load with high instantaneous starting inrush (surge) from electric motors, AC compressors, and pumps. Under NEC §702.4(B), an optional standby generator with an automatic transfer switch (ATS) must have adequate capacity to supply the full calculated load, unless an automated energy management load-shedding system is installed.

Key Sizing Criteria per NEC Articles 445 & 702:

  • 1. Continuous Running Load: Sum of all simultaneously operating base loads (lighting, appliances, electronics, refrigeration).
  • 2. Motor Inrush Starting Surge (Locked Rotor Amps - LRA): Electric motors draw 3× to 6× their running watts during startup. The generator must provide this momentary surge without dropping voltage below 85% of nominal.
  • 3. Altitude and Temperature Deratings: Combustion engines lose ~3.5% power per 1,000 ft above sea level (over 500 ft) and ~1% per 10°F above 77°F ambient.
  • 4. Fuel Type Factor: Natural gas produces approximately 10% to 15% less power than LP (liquid propane) or diesel on identical displacement engines.
  • 5. Transfer Switch Sizing (§702.5): Must be rated for not less than the maximum calculated load or the full main service rating when switching the entire service.

Formulas & equations

Total Running kW = Base Continuous kW + Motor Running kW Peak Surge Starting kW = Base Running kW + Largest Motor Starting Surge kW Derated Generator Capacity = Nameplate kW × Altitude Factor × Temp Factor × Fuel Factor Transfer Switch Amps (1-Phase 240V) = (Generator Watts ÷ 240V) or Full Service Rating (§702.5)

NEC reference table

Appliance / Motor Typical Running Watts Starting Surge Watts NEC Sizing Rule
Central A/C (3-Ton / 36k BTU)3,500 W10,500 W (3×)NEC Art. 440
Central A/C (5-Ton / 60k BTU)5,500 W16,500 W (3×)NEC Art. 440
Well Pump (1 HP)1,500 W4,500 W (3×)NEC Art. 430
Sump Pump (1/2 HP)800 W2,200 W (2.75×)NEC Art. 430
Refrigerator / Freezer600 W1,800 W (3×)Continuous
Electric Water Heater (50 Gal)4,500 W4,500 W (1× resistive)Non-motor

Worked example, step by step

Example: Whole-House Standby Generator for 2,400 sq.ft Home

Loads: 6 kW base continuous load, 4-Ton AC (4,500W running, 13,500W starting surge), 1 HP Well Pump (1,500W running, 4,500W surge).

  • 1. Total Running Load: 6,000W + 4,500W + 1,500W = 12.0 kW running.
  • 2. Peak Starting Surge: Base (6,000W) + Well Pump running (1,500W) + AC starting surge (13,500W) = 21.0 kW Peak Surge.
  • 3. Natural Gas Fuel Adjustment (12%): 21.0 kW ÷ 0.88 = 23.9 kW required capacity.
  • 4. Recommendation: Install a standard 24 kW / 26 kW Air-Cooled Standby Generator with a 200A Automatic Transfer Switch (ATS).

Frequently asked questions

Under NEC §702.4(B)(2), an Automatic Transfer Switch (ATS) must be sized to carry the full calculated load of the entire house unless an automated load-shedding controller is installed. A manual transfer switch or mechanical panel interlock kit allows the homeowner to manually choose which circuits to energize during an outage, so the generator can be sized for only the selected critical circuits.

Because air density decreases at higher elevations, internal combustion generator engines lose oxygen for combustion. Standard derating is approximately 3.5% of nameplate power for every 1,000 feet above 500 feet elevation.

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