How to Select the Correct kVA Rating

Sizing Guide

How to Calculate and Select the Correct kVA Rating for a Servo Stabilizer

Published by Hindustan Stabilizers Technical Team | February 2025 | 10 min read

Selecting the wrong kVA rating is the most common mistake when purchasing a servo voltage stabilizer. An undersized stabilizer will overheat, trip on overload, and fail prematurely. An unnecessarily oversized stabilizer wastes money on initial purchase and consumes more no-load power throughout its life. This guide provides a systematic method to calculate the exact kVA requirement for any single-phase or three-phase installation.

Understanding kVA vs kW vs Power Factor

Before sizing, understand the relationship between the three key quantities: kW (kilowatt) is real power — the actual power consumed by the load to do work. kVA (kilovolt-ampere) is apparent power — the total power the supply system must provide, including both real power and reactive power. Power Factor (PF) is the ratio of kW to kVA, always between 0 and 1. The relationship is: kVA = kW / Power Factor. A servo stabilizer must be rated in kVA because it must supply the apparent power to the load, not just the real power.

Step-by-Step kVA Sizing Calculation

1. Step 1 — List all connected loads: List every electrical machine, equipment, or appliance that will be connected to the stabilizer output. Include their rated power in watts (W) or kilowatts (kW). If the nameplate shows amperage, calculate watts using: W = V x A x PF (single-phase) or W = 1.732 x V x A x PF (three-phase).
2. Step 2 — Calculate total connected load in kW: Add up the wattage of all connected loads. Convert to kW by dividing by 1000. Example: 10 machines at 5 kW each = 50 kW total connected load.
3. Step 3 — Apply demand factor: In practice, not all machines run at full load simultaneously. A demand factor of 0.75 to 0.9 is typically applied for industrial installations. For pharmaceutical and hospital installations where loads run near full capacity, use 0.9 to 1.0. Example: 50 kW x 0.85 demand factor = 42.5 kW demand load.
4. Step 4 — Add future expansion margin: Add 25% to 30% to the demand load to accommodate future load additions and prevent operating the stabilizer continuously at maximum capacity. Example: 42.5 kW x 1.25 = 53.1 kW.
5. Step 5 — Convert kW to kVA using power factor: Divide the kW figure by the system power factor. For mixed industrial loads (motors, drives, lighting), use 0.8. For predominantly resistive loads (heaters, ovens), use 0.95 to 1.0. Example: 53.1 kW / 0.8 PF = 66.4 kVA. Select the next standard size: 75 kVA.
6. Step 6 — Account for motor starting current (if applicable): If the largest motor connected will be started direct-on-line (DOL), add its locked rotor current (typically 6 to 7 times rated current, lasting 3 to 8 seconds) to the design. If motors use VFDs or star-delta starters, starting current surge is greatly reduced and step 5 result is sufficient.

Worked Example: CNC Machine Shop

Equipment	Qty	kW Each	Total kW
VMC (CNC Machining Center)	4	11 kW	44 kW
CNC Turning Center	6	7.5 kW	45 kW
Wire EDM	2	5 kW	10 kW
Coolant Pumps	5	1.5 kW	7.5 kW
Lighting and Miscellaneous	1	5 kW	5 kW
Total Connected Load			111.5 kW
Demand Factor (0.80)			89.2 kW
Expansion Margin (+25%)			111.5 kW
Power Factor Conversion (0.80)			139 kVA
Selected Standard Size			150 kVA

Single-Phase vs Three-Phase Stabilizer

If your total load above results in less than 25 kVA and all equipment is single-phase, a single-phase stabilizer is appropriate. If the total exceeds 25 kVA, or if any equipment is three-phase, select a three-phase stabilizer. For installations where three-phase loads are significantly unbalanced between phases (for example, a mix of single-phase and three-phase equipment), select a three-phase independent (column type) servo stabilizer to ensure each phase is individually regulated.