3-Phase Commercial Generator Sizing Calculator

Calculate required 3-phase standby and prime generator capacity (kVA, kW, and Phase Amps) with motor starting kVA (SkVA), power factor, and NEC continuous load standards.

1. System Configuration

3-Phase AC

Line-to-Line Voltage (V_LL)

Power Factor (PF)

Duty Rating

Safety / Future Expansion Margin 25% (Recommended)

0% (Tight) 20% (NEC 80% Rule) 25% (Standard) 50% (High Expansion)

2. 3-Phase Electric Motor Loads

Motor Application

Rating (HP)

Starter Type

Qty

3. Steady-State Linear Loads

Balanced 3-Phase kW

Commercial HVAC / Heating (kW)

Facility Lighting (kW)

Server Room / IT / UPS (kW)

Pumps & General Outlets (kW)

Calculated Generator Size

3-Phase Capacity

Standby

Apparent Power

125 kVA

Standard commercial rating

Real Active Power

100 kW

@ 0.80 Power Factor

Full Load Current 150 A Per phase @ 480V

Peak Starting Surge 180 SkVA Motor locked rotor surge

Recommended Electrical Hardware

Main Breaker: 200 Amp

Automatic Transfer Switch: 200A 3-Phase ATS

Standard ISO Standard Match:

Nearest standard production frame: 125 kVA / 100 kW 3-Phase Diesel/Gas Generator Set.

3-Phase Calculation Formulas (NEC Standards)

Apparent Power: kVA = (V_LL × I × √3) ÷ 1,000

Active Power: kW = kVA × Power Factor (PF)

Phase Current: I = (kW × 1,000) ÷ (V_LL × √3 × PF)

Motor Power: 1 HP ≈ 0.746 kW (Running)

Detailed 3-Phase Load Breakdown

Load Component	Quantity / Spec	Running Load (kW)	Starting Surge (kVA)	Notes

Frequently Asked Questions: 3-Phase Generator Sizing

What is the difference between kVA and kW in a 3-phase generator?

kW (kilowatts) measures the actual real work done by the system, while kVA (kilovolt-amperes) measures apparent power. 3-phase commercial generators are typically rated at 0.8 Power Factor (PF), meaning a 100 kVA generator provides 80 kW of real continuous electrical power.

Why do 3-phase electric motors require starting multipliers (SkVA)?

Induction motors draw 3 to 6 times their rated running current upon starting across the line (Direct-On-Line). Using Variable Frequency Drives (VFDs) or electronic soft starters reduces this surge to 1.5x - 3.0x, allowing you to install a significantly smaller generator.

What standard sizes do commercial 3-phase generators come in?

Standard industrial production sizes include: 20 kVA, 30 kVA, 50 kVA, 80 kVA, 100 kVA, 125 kVA, 150 kVA, 200 kVA, 250 kVA, 300 kVA, 400 kVA, 500 kVA, 750 kVA, and 1000+ kVA.