Field calculator

Generator sizing calculator (running + surge)

A generator is undersized if it cannot carry everything running at once plus the jolt when the biggest motor starts. This calculator adds the total running watts of the loads that will be on together to the momentary starting surge of the largest motor, then applies a headroom percentage to land on a recommended size. Motors are the catch: an air conditioner, well pump, or compressor can pull several times its running watts for a moment on start, and that inrush sets the peak the generator must swallow without stalling or sagging the voltage. Enter the total running load in watts, the extra starting surge of the largest motor, and a headroom percentage. Build the running total from actual nameplate watts, use the locked-rotor or starting figures for the surge, and confirm the size, fuel type, and power factor against the equipment and the generator manufacturer, along with NEC sizing for the conductors, overcurrent, and transfer equipment.

Worked example

A site runs 6 kW steady with a 3 kW motor-starting surge. Size the generator with 25% headroom.

Running load6 kW
Starting surge3 kW
Headroom25%

Peak demand = running + surge = 6 + 3 = 9 kW.
With 25% headroom: 9 × 1.25 = 11.25 kW.

About 11.25 kW. Motor and compressor inrush drives the surge, so size for the peak, not just the running load.

Change the numbers in the calculator above to run your own.

anvilfield.com/calculators/generator-sizing-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Generator sizing FAQ

What is an electrical load calculation?

An electrical load calculation totals a building's connected load, applies the NEC Article 220 demand factors, and returns the minimum service or feeder size in amps. It accounts for the fact that no building runs every load at once, so the diversified demand sizes the gear, not the sum of all nameplates.

What is a demand factor in a load calculation?

A demand factor is the fraction of a connected load you may count toward the service, because that load will not run at full output with everything else. The Article 220 tables set them by category from metered data. The dwelling general-lighting demand, for example, takes the first 3000 VA at 100 percent and the rest at 35 percent.

What is the optional method for a dwelling load calculation?

The optional method, NEC 220.82, lumps general lighting, small-appliance, laundry, and appliance nameplates together, then takes 100 percent of the first 10,000 VA and 40 percent of the remainder. It is allowed for a single 120/240 V service rated 100 A or more and usually yields a smaller service than the standard method.

How do you add an EV charger to an existing service?

Use NEC 220.87 to find the existing load from the metered annual peak, or a 30-day recording, taken at 125 percent. Subtract that from the service rating for the spare capacity. The EV charger, a continuous load under Article 625, enters at 125 percent of its rating and has to fit inside the headroom.

Do you count both heating and air conditioning in a load calculation?

No. You count only the larger of the two, because heating and air conditioning are noncoincident under NEC 220.60 and cannot run at the same time. Compare the heat load against the AC load and carry the larger into the total. Counting both is a common error that oversizes the service.

More in the Load calculation, NEC 220 field guide.