Field calculator
Three-phase power calculator (kW, kVA, amps)
Three-phase power ties together voltage, current, and power factor, and the field needs to move between them constantly to size a service, check a load, or read a nameplate. Apparent power in kVA equals the square root of three times the line-to-line voltage times the line current, divided by 1000, and real power in kW equals that kVA times the power factor. Enter the line-to-line voltage (208, 240, 480, or 600 are common), the line current in amps, and the power factor, which is 1.0 for a resistive load and lower for motors and other inductive loads. To go the other way and solve current from a known kW load, amps equals kW times 1000 divided by the square root of three, the voltage, and the power factor. These are the core relationships; use the equipment nameplate and the real power factor, and apply NEC conductor sizing, continuous-load, and derating rules to the actual installation.
Result
Three-phase power: apparent power kVA = sqrt(3) × line-to-line volts × line amps / 1000, and real power kW = kVA × power factor. Enter the line-to-line voltage (such as 208, 240, 480, or 600), the line current in amps, and the power factor (1.0 for resistive, lower for motors). To find amps from a known kW load instead, amps = kW × 1000 / (sqrt(3) × volts × PF). These are the basic relationships; use the equipment nameplate and actual power factor, and apply NEC conductor sizing, continuous-load, and derating rules for the real install.
Worked example
A 480 V three-phase feeder carries 100 A at 0.9 power factor. What is the real power?
- Voltage (line-line)480 V
- Current100 A
- Power factor0.9
- Apparent power = √3 × V × I = 1.732 × 480 × 100 = 83.1 kVA.
- Real power = kVA × PF = 83.1 × 0.9 = 74.8 kW.
83.1 kVA and 74.8 kW. The √3 factor is what separates three-phase from single-phase power math.
anvilfield.com/calculators/three-phase-power-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
More Electrical calculators
Three-phase power 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.