kW to VA Calculator

Example 1: Window AC Unit - Transformer Sizing

Title: Window AC Unit - Transformer Sizing Description: Convert air conditioner real power to apparent power for service upgrade Scenario: 3.5 kW window AC with 0.

1. Power: 3.

2. Power Factor: 0.

Result: Apparent Power: 4,118 VA or 4.12 kVA (3.5 kW / 0.85 = 4.12 kVA). Transformer adequacy: Yes, 5 kVA transformer sufficient (82% loaded, within 80-90% optimal range). Starting surge: 15-20 kVA momentary (3-5× running, hard-start kit reduces to 2×). Current draw: 17.2A at 240V (4,118 VA / 240V). Circuit: 20A breaker, 12 AWG wire minimum per NEC 440.

Notes: Formula: VA = kW / PF × 1000. Power factor for HVAC: Compressors 0.75-0.85, fan only 0.95-1.0, combined 0.80-0.90. Transformer loading: Optimal 70-85% for efficiency and headroom. Starting current: Locked rotor amps (LRA) 6× running, time delay relay or soft-start reduces. Efficiency: EER 10-12 typical (12,000 BTU ÷ 3,500W = 3.4 EER, low), modern inverter units EER 14-16. Annual cost: 1,200 hours/summer × 3.5 kW × $0.12/kWh = $504/year.

Example 2: Office Transformer - Load Analysis

Title: Office Transformer - Load Analysis Description: Convert office building load to apparent power for transformer selection Scenario: 75 kW office load (lighting, computers, HVAC) with 0.92 power factor. Need to calculate apparent power for transformer sizing Difficulty: intermediate Industry: Commercial Electrical

Steps:

1. Real Power (kW): 75 kW

   • Office building load

2. Power Factor: 0.92

   • Combined load power factor

Result: Apparent Power: 81,522 VA or 81.5 kVA (75 kW / 0.92 = 81.5 kVA). Transformer: 112.5 kVA standard size (next size up from 81.5). Loading: 72% (good for N-1 redundancy). Three-phase: 480V primary, 208Y/120V secondary. Current: 226A secondary (81,500 / (√3 × 208)). Losses: 1.5-2.5 kW no-load, 1.5-2.0 kW at 75% load.

Notes: Standard transformer sizes: 45, 75, 112.5, 150, 225, 300 kVA. Sizing: Select next standard size above calculated kVA with 20-30% margin for growth. Efficiency: Peak at 50-70% load, DOE 2016 standards require 98.0-98.8% at 50% load. Temperature rise: 150°C insulation class common, 80°C rise at full load. Lifespan: 30-40 years, insulation degrades exponentially with temperature. Loading: Per IEEE C57.96, continuous overload 1.2× for well-ventilated indoor transformers. Paralleling: Two 75 kVA for redundancy better than one 150 kVA (N+1 operation during maintenance).

Example 3: Data Center - Transformer Cascading

Title: Data Center - Transformer Cascading Description: Calculate apparent power requirements for multi-tier power distribution Scenario: 2 MW data center with 0.95 power factor. Need to calculate apparent power for transformer cascading and multi-tier power distribution Difficulty: advanced Industry: Data Center

Steps:

1. Real Power (kW): 2,000 kW (2 MW)

   • Data center load

2. Power Factor: 0.95

   • Combined load power factor

Result: Apparent Power: 2,105,263 VA or 2.11 MVA (2,000 kW / 0.95 = 2.11 MVA). Transformer configuration (2N): 4× 1.5 MVA (two parallel per side), each loaded 35% during normal operation. Redundancy: Loss of any two transformers maintains full capacity. Utility service: 13.2 kV primary, dual feeds from separate substations (A/B feeds). Annual losses: 60-80 kW transformer losses = 525-700 MWh/year = $52,500-70,000 at $0.10/kWh.

Notes: Data center power hierarchy: Utility (13.2-35 kV) → main transformers (480V) → PDUs → rack (208V). 2N topology: Two independent systems (A + B), each sized for 100% load, highest reliability. Fault tolerance: N, N+1, 2N, 2(N+1) increasing redundancy. Medium voltage: 13.2 kV or 33 kV reduces I²R losses for multi-MW loads. Harmonic mitigation: K-rated transformers (K-13, K-20) for nonlinear loads, or 12-pulse rectification. Selective coordination: Fuse/breaker timing prevents cascade failures. Monitoring: EPMS (electrical power monitoring system) tracks voltage, current, harmonics, PF in real-time. TCO optimization: Amorphous core transformers reduce no-load losses 70%, pay back in 5-7 years for 24/7 operation.