Quick answer
For a balanced three-phase load, apparent power is √3 × line-to-line volts × line amps ÷ 1,000. Estimated real power is kVA × power factor. That arithmetic is only a screening step. Generator selection also depends on motor starting, voltage and frequency dip, nonlinear loads, single-phase load balance, neutral current, duty rating, site derating, fault performance, and the exact available voltage configuration. Have a qualified designer and the generator manufacturer validate the final system.
Three-phase output is more than three energized conductors
In a balanced three-phase AC system, the three voltage waveforms have equal magnitude and are separated by 120 electrical degrees. The generator winding and connection determine the available line-to-line and line-to-neutral voltages. Common labels such as 120/208V or 277/480V describe related voltages in a particular system, not two independent generator ratings.
Record the load equipment voltage, number of phases, frequency, conductor and neutral requirements, and grounding arrangement. A 480V motor cannot be selected from a general 480V label without checking the exact connection and equipment nameplate. Reconnection or voltage-selector work belongs to trained, authorized personnel following the model instructions.
Keep kVA, kW, and power factor separate
Kilovolt-amperes describe apparent power. Kilowatts describe real power. Power factor connects the two for a load under defined conditions. For a balanced three-phase load using line-to-line voltage, kVA equals 1.732 multiplied by volts and line current, divided by 1,000. Multiplying that result by power factor gives estimated kW.
Do not assume one power factor for every load or use this equation to predict motor starting. Starting power factor and kVA can differ sharply from running values. Nonlinear loads can also require alternator capacity that a simple fundamental-frequency calculation does not show.
| Quantity | What it describes | What to verify |
|---|---|---|
| Line-to-line voltage | Voltage measured between phases | Generator configuration and load nameplate |
| Line current | Current in each phase conductor | Highest phase and operating sequence |
| kVA | Apparent power demand | Running and starting values |
| kW | Real power demand | Engine capacity and duty rating |
| Power factor | Relationship between real and apparent power | Running, starting, and nonlinear behavior |
Select the voltage configuration from the loads and distribution
Start with the equipment, not the generator selector switch. List every required line-to-line and line-to-neutral voltage. Identify transformers, transfer equipment, switchgear, distribution boxes, breakers, receptacles, and feeders that must carry the source. Changing generator voltage can also change available current and invalidate downstream assumptions.
Some mobile sets support reconnectable or selectable arrangements. The available kW, kVA, current, neutral treatment, receptacles, and protection can depend on that arrangement. Keep a one-line diagram and the approved configuration at the generator. Do not move a selector, reconnect windings, or alter terminals under load.
Measure and plan phase balance instead of averaging it away
A three-phase generator can serve three-phase loads and, in some configurations, single-phase loads. Those single-phase loads must be assigned across the phases within the generator and system limits. A reasonable total kVA can still overload one phase, increase neutral current, or create unacceptable voltage unbalance.
Create a phase schedule for lighting, receptacles, controls, heaters, office loads, and other single-phase equipment. Review the highest line current and neutral current for each operating state, not only the average. Cummins sizing guidance notes that software assumptions about balanced single-phase loads must match the actual design.
Motor starts and electronic loads can control the generator choice
Across-the-line motors can demand high starting kVA and cause voltage and frequency dip. The required generator depends on motor horsepower, locked-rotor or starting current, starting method, acceleration time, driven load, permissible dip, and other loads already online. Sequence starts where the process allows it.
Variable-frequency drives, UPS systems, welders, battery chargers, and switching power supplies can create harmonic current and transient demands. The alternator, excitation system, voltage regulator, engine response, and protection must be evaluated together. Provide detailed load data to the manufacturer sizing tool rather than applying one generic reserve percentage.
Specify the complete three-phase power path
The project scope should name the generator duty rating, site-rated output, voltage and frequency, neutral arrangement, grounding and bonding, transfer equipment, fault-current requirements, feeders, distribution, protection, cable routes, environmental enclosure, cooling, exhaust, fuel, monitoring, and commissioning tests.
Acceptance testing should confirm phase rotation, line and phase voltages, current on every line, frequency, load steps, transfer sequence, alarms, protection, and intended load shedding. Record the as-left configuration. Temporary and rental systems need the same clarity even when they operate for only a few days.
- Verify every load voltage and phase
- Separate running kW, running kVA, and starting kVA
- Create an operating and starting sequence
- Balance single-phase loads across the permitted phases
- Commission the complete generator and distribution system
Frequently asked questions
Common questions
How do I calculate three-phase generator kVA?
For a balanced load, multiply line-to-line volts by line amps by 1.732, then divide by 1,000. Validate the input voltage and current and do not use the result alone for generator selection.
What is the difference between generator kW and kVA?
kW is real power. kVA is apparent power. Power factor relates them. A generator and alternator must satisfy the applicable real, apparent, starting, and transient requirements.
Can a three-phase generator run single-phase loads?
Some configurations can, within manufacturer limits. The loads must use the correct voltage and be distributed so no phase, winding, neutral, or protection limit is exceeded.
Is a 480V generator always three phase?
Do not infer the complete system from a voltage label. Confirm phase, winding connection, line-to-line and line-to-neutral voltages, frequency, neutral, and the exact load requirements.
Primary references
CordFind summarizes connector specifications and public safety guidance. Always follow the instructions supplied with your equipment.