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Sizing & Specs · 11 min read · Published 2026-05-15

How to Size a Commercial Transformer in Arizona (kVA Calculation Guide)

Step-by-step method for engineers, EPCs, and electrical contractors to size a commercial distribution transformer in Arizona — connected load math, diversity factors, harmonic derating, high-ambient derating, and growth contingency.

TL;DR: A correctly sized transformer in Arizona is the connected load (in kVA), times the diversity factor (typically 0.6–0.8), plus 25% growth contingency, then derated for high-ambient and harmonics. For a 100,000 sq ft commercial building, that usually lands at 500–1,500 kVA depending on use case. Oversize is wasted capital; undersize is overheating, derating, and replacement. This guide walks through the math.

The basic equation

The transformer sizing equation in simplest form:

Required kVA = (Connected Load × Diversity Factor + Growth Allowance) ÷ Derating Factor

Where:

  • Connected Load (kVA) = sum of all load on the transformer's secondary, in kVA.
  • Diversity Factor = fraction of connected load running simultaneously (0.6–0.8 typical for commercial).
  • Growth Allowance = 20–30% additional capacity for future load.
  • Derating Factor = combined effect of high ambient + harmonics + altitude.

Step 1 — Calculate connected load

Connected load is the sum of every electrical device that can draw current from the secondary side of the transformer. Don't include redundant/standby equipment that won't operate simultaneously.

Typical commercial load buckets

  • Lighting — 1–3 W/sq ft for LED, 1.5–4 W/sq ft for fluorescent retrofits.
  • HVAC — 2–6 W/sq ft for office, 6–12 W/sq ft for restaurants, 8–15 W/sq ft for data center / labs.
  • Plug load (general receptacles) — 1.5–3 W/sq ft for office, 5–10 W/sq ft for retail with point-of-sale.
  • Specialty loads — kitchen equipment (commercial), industrial process, EV chargers, server rooms.
  • Motors — convert HP to kW × 0.85 power factor → kVA (e.g., 50 HP motor ≈ 44 kVA at 85% PF).

Convert kW to kVA

Transformers are rated in kVA (apparent power), not kW (real power). Convert with: kVA = kW ÷ Power Factor. Typical PF: 0.85–0.95 for modern commercial loads, but inductive loads (motors, fluorescent ballasts) push PF down to 0.7–0.8.

Step 2 — Apply diversity factor

Not all connected load runs simultaneously. Diversity factor accounts for the fact that lights aren't on 100% of the time, HVAC cycles, and motors don't all start at once.

Building typeTypical diversity factor
Office building0.65 – 0.75
Retail (open)0.70 – 0.80
Restaurant (full kitchen + dining)0.75 – 0.85
Warehouse / light industrial0.50 – 0.65
Data center / server room0.85 – 0.95
Hospital / clinic0.70 – 0.80
School / education0.55 – 0.70
Multi-tenant commercial0.65 – 0.80

Demand Load (kVA) = Connected Load (kVA) × Diversity Factor

Step 3 — Add growth allowance

A transformer is a 25–35 year asset. Plan for the building's load to grow over that horizon. Standard growth allowance:

  • 20% minimum for any new commercial install.
  • 25–30% for buildings expecting tenant turnover with potentially higher-load uses.
  • 30–40% for solar-ready buildings, EV charging buildout, or expansion-likely sites.

Design Load (kVA) = Demand Load × (1 + Growth Allowance)

Step 4 — Apply Arizona-specific deratings

This is where Arizona projects diverge from generic NEC tables. Three derating factors stack:

High-ambient derating

Standard transformer ratings assume 30°C ambient. Sustained operation above 30°C requires derating. Phoenix-metro summer ambients (peak monthly average 35–40°C in July) push transformers into derating territory.

  • 30°C standard: 100% rated capacity.
  • 40°C ambient: ~92% rated capacity (8% derate).
  • 50°C ambient (in-cabinet at peak summer): ~83% rated capacity (17% derate).

Two options: (a) order the transformer pre-derated for 50°C ambient (JST Power's standard Arizona configuration), or (b) order standard rating and oversize by 15–20% to compensate.

Harmonic derating (K-factor)

Modern non-linear loads (variable frequency drives, LED drivers, computer power supplies, electronic ballasts) generate harmonics that cause additional heating in transformer windings. Standard K-1 transformers can't handle high harmonic loads without derating.

  • K-1 (no harmonic rating): use for resistive loads, incandescent lighting, simple motors.
  • K-4 transformer: handles loads with ~33% harmonic content (typical office).
  • K-13 transformer: handles loads with ~50% harmonic content (data center, server room).
  • K-20 transformer: handles loads with 60%+ harmonic content (industrial with many VFDs).

Spec a higher K-factor (K-4, K-13, K-20) rather than oversizing a K-1 transformer — it's more cost-effective and runs cooler.

Altitude derating

Above 3,300 ft (1,000 m), transformer cooling efficiency drops because air density decreases. Required derating:

  • Phoenix metro (~1,100 ft): no altitude derate needed.
  • Tucson (~2,400 ft): no altitude derate needed.
  • Prescott (~5,400 ft): ~2% derate, or specify altitude-rated transformer.
  • Flagstaff (~7,000 ft): ~5% derate, or specify altitude-rated transformer.
  • White Mountain communities (~7,500–9,000 ft): 6–8% derate, altitude-rated transformer recommended.

Step 5 — Round to standard kVA size

Transformers come in standard sizes. Round up to the next standard size after applying derates.

Standard pad-mount three-phase kVA sizes: 75 · 112.5 · 150 · 225 · 300 · 500 · 750 · 1,000 · 1,500 · 2,000 · 2,500 · 3,000 · 3,750 · 5,000.

Standard pad-mount single-phase kVA sizes: 25 · 37.5 · 50 · 75 · 100 · 167 · 250.

Standard pole-mount three-phase kVA sizes: 30 · 45 · 75 · 112.5 · 150 · 225 · 300 · 500.

Worked example — Office building, Phoenix

20,000 sq ft single-tenant office building, Phoenix:

  • Lighting: 2.0 W/sq ft × 20,000 = 40,000 W = 40 kW → 42 kVA at 0.95 PF
  • HVAC: 4.0 W/sq ft × 20,000 = 80 kW → 89 kVA at 0.90 PF
  • Plug load: 2.5 W/sq ft × 20,000 = 50 kW → 56 kVA at 0.90 PF
  • Specialty (server room ~25kW): 25 kW → 28 kVA

Connected load total: 215 kVA

Diversity factor (office): 0.70 → Demand Load = 150.5 kVA

Growth allowance: 25% → Design Load = 188 kVA

Phoenix ambient derate: divide by 0.92 (40°C) → 204 kVA

Harmonic K-4 derate: minimal additional if spec'd K-4 transformer (~2% to be safe) → 209 kVA

Altitude (Phoenix): no derate.

Round to standard size: 225 kVA pad-mount three-phase.

Common sizing mistakes

  • Using nameplate capacity without diversity factor. Result: oversized by 30–40%, wasted capital, and high no-load losses.
  • Skipping ambient derate. Result: undersized for Arizona summer, transformer runs hot, life expectancy drops from 30 years to 10–15.
  • Ignoring harmonics in modern offices. Result: transformer overheats even though connected load fits the nameplate.
  • Not planning for EV chargers. A 50-stall parking lot with future Level 2 charging adds 200–400 kVA. Easier to size now than upgrade later.
  • Specifying only kW, not kVA. Result: undersized when actual loads have lower power factor than assumed.
  • Following the developer's "preliminary" load letter. Tenant fit-out almost always exceeds the developer's initial load estimate. Add a buffer.

When to oversize and when to undersize

Slight oversizing (one standard size up) is usually safer than tight sizing — the cost difference between 300 and 500 kVA is often 10–15%, while the consequences of undersizing are dramatic (premature failure, replacement cost, downtime).

That said, severely oversizing is wasteful:

  • Higher no-load losses (constant 24/7 cost).
  • Higher initial capital.
  • Lower efficiency at light loads (transformers are most efficient at 30–60% of rated capacity).
  • Larger vault and concrete pad cost.

Rule of thumb: target 50–70% of rated capacity at full design load. That gives growth headroom without sacrificing efficiency.

Common questions

Should I size for "peak demand" or "average load"?

Size for peak demand (highest 15-minute average). Transformers can handle short-term overloads of 1.2–1.5× nameplate for limited durations, but sustained peak demand above nameplate accelerates winding insulation degradation. Always size for the peak.

What size transformer for a 100,000 sq ft warehouse in Phoenix?

Typical: 300–500 kVA pad-mount three-phase. Warehouses have lower W/sq ft than offices (mostly lighting + a few high-bay heaters or coolers). Add for forklift charging stations if applicable.

How do I size a transformer for a solar PV installation?

For utility-scale or large commercial PV, size the step-up transformer to the PV array's AC output rating, plus 10–15% for inverter overload. Add growth allowance if the site might add capacity later. JST Power makes solar EPC step-up transformers in 1,500 / 2,500 / 3,750 / 5,000 kVA configurations commonly specified for Arizona projects.

Can a transformer be too large?

Yes. Significantly oversized transformers operate at light load most of the time, where efficiency curves drop and no-load losses dominate. For a transformer expected to run at 10–20% of nameplate, those no-load losses can become a noticeable operating cost over 25 years.

Does Tech Energy America provide sizing engineering support?

Yes. Free preliminary sizing review when you submit a load schedule or single-line diagram. For complex projects (data centers, hospitals, solar EPC interconnections), we coordinate directly with the project engineer to confirm sizing and configuration before committing to a JST factory order. Contact us via the Electrical Services page or call (480) 910-0867.

Related reading

Need sizing support for a commercial transformer in Arizona?

Tech Energy America provides free preliminary sizing review for any commercial or industrial transformer specification. Submit your load schedule or single-line and we'll confirm the kVA, K-factor, BIL, and JST configuration that fits — with confirmed factory lead time.

📞 Call (480) 910-0867✉ Email Sizing Review