How to Size a Transformer: Load Lists, Demand Factors, and Derating — A Field Engineer's Method

Every undersized transformer I've seen in the field shared the same root cause: someone added up nameplate kW, divided by 0.8, and picked the next standard size. That's not sizing. That's guessing.

A properly sized transformer balances three forces: the load it actually sees, the environment it sits in, and the regulation that governs it.

Here's the field method, built on IEC 60076-12 and about a dozen mistakes I've learned from.

Step 1: Build Your Load List (This Takes Longer Than You Think)

Before any math, create a spreadsheet with these columns:

Load ID	Equipment	Qty	kW (each)	Total kW	PF	Efficiency	Demand Factor (Kd)	Notes
L-01	Chiller compressor	3	150	450	0.85	0.93	0.85	Duty/standby: only 2 run simultaneously
L-02	Cooling tower fan	2	30	60	0.80	0.90	0.80	Seasonal
L-03	Production line motor	12	22	264	0.83	0.91	0.70	Not all stations run concurrently
L-04	Welding machine	8	35 kVA	280 kVA	0.50	—	0.35	Intermittent, high inrush
L-05	Lighting (LED)	—	—	45	0.95	—	0.90	—
L-06	HVAC AHU	4	18.5	74	0.82	0.90	0.75	—
L-07	UPS (IT load)	1	60	60	0.90	0.92	0.95	Always on
L-08	Miscellaneous small power	—	—	30	0.85	—	0.50	Sockets, cleaning equipment

Rules of thumb for the load list:

• Motors: use shaft kW from the nameplate, not absorbed kW. The motor draws rated current at full mechanical load.
• Welding equipment: list in kVA, not kW. The power factor is terrible (0.4–0.6) and the duty cycle matters.
• VFD-driven loads: the VFD corrects input PF to ~0.95. Don't use the motor nameplate PF.
• Standby equipment: if a pump has a 100% duty/standby pair, count only one.
• Intermittent loads (cranes, lifts, test benches): flag them separately. They affect peak sizing, not continuous thermal rating.

Step 2: Apply Demand Factors (Kd) — The Art Part

A demand factor tells you what fraction of connected load is actually drawn at peak. IEC 60076-12 (Loading Guide) doesn't give you a table of demand factors — it assumes you derive them from load studies. In practice:

Load Type	Typical Kd Range	Why
Continuous process motors	0.85–0.95	They run at rated load most of the time
Batch / intermittent motors	0.50–0.75	Cycling on/off
Welding machines	0.25–0.50	Low duty cycle, high peaks
HVAC chillers	0.70–0.90	Weather-dependent loading
Lighting (industrial)	0.85–1.00	Once on, stays on
Lighting (commercial)	0.70–0.90	Some areas unoccupied
Office socket outlets	0.30–0.50	Plug load diversity is high
UPS / IT equipment	0.90–1.00	Always on, constant draw

Do not confuse demand factor with diversity factor. Demand factor applies to a single equipment group. Diversity factor applies across groups. I'll address diversity in Step 3.

For each row: Calculated Demand kW = Total kW × Kd.

Step 3: Apply the Simultaneity (Diversity) Factor (Ks)

You now have N groups of loads, each with its own demand kW. But they don't all peak at the same time. The simultaneity factor (also called diversity factor, but IEC 60076-12 uses "simultaneity") accounts for this.

Standard approach from IEC 60076-12, Annex A:

Number of Main Load Groups	Simultaneity Factor (Ks)
2–3	0.90
4–5	0.85
6–9	0.80
10+	0.75

So:

Total Demand kW (after Kd) = Σ(Individual demand kW per group)
Peak Diversified kW = Total Demand kW × Ks

For our example above (8 groups):

• Sum of demand kW after Kd: approximately 620 kW
• Ks for 8 groups: 0.80
• Peak Diversified kW = 620 × 0.80 = 496 kW

Step 4: Convert kW to kVA

The transformer supplies apparent power (kVA), not just real power (kW). You need the weighted average power factor across all connected loads.

For each group, the reactive component is:

kvar = demand kW × tan(arccos(PF))

Then sum kW and kvar across groups, and:

Total Diversified kVA = √(Total kW² + Total kvar²)

Simplified (and acceptable for most projects): take the weighted average PF and use:

kVA = Peak Diversified kW ÷ Weighted Average PF

If your average PF is 0.85, then:

kVA = 496 ÷ 0.85 = 583.5 kVA

Step 5: Apply the Loading Margin (Don't Run at 100%)

IEC 60076-12 defines loading capability in terms of ambient temperature, thermal time constant, and load cycle. For a standard ONAN transformer at 30°C ambient with a normal cyclic load, continuous loading should not exceed 80–85% of rated kVA.

Why not 100%?

• Transformer aging doubles for every 6–8°C above rated winding temperature (Arrhenius law per IEC 60076-12).
• Load growth will happen. It always does.
• Motor starting inrush (typically 6× FLC for DOL starting) causes voltage dip. A larger transformer mitigates this.

So: Required Rated kVA = Calculated kVA ÷ 0.80

Required Rated kVA = 583.5 ÷ 0.80 = 729.4 kVA

Select next standard size: 800 kVA.

But we're not done.

Step 6: Derating for Site Conditions (IEC 60076-12)

Altitude Derating

Standard transformers are designed for installation at ≤ 1000 m above sea level. Above this, air density drops, reducing cooling effectiveness.

Per IEC 60076-2 / IEC 60076-12:

For dry-type (air-cooled), derate by:

• 0.5% per 100 m for natural cooling (AN)
• 0.3% per 100 m for forced air (AF)

For oil-immersed, derate by 0.4% per 100 m.

Example: 2000 m altitude, dry-type AN.

Altitude above 1000 m = 1000 m
Derating = 1000/100 × 0.5% = 5%
Adjusted required kVA = 800 × 1.05 = 840 kVA

Select: 1000 kVA (next standard above 840).

Temperature Derating

Standard ambient: 40°C maximum, 30°C daily average, 20°C yearly average (per IEC 60076-2).

For every 1°C above 40°C maximum:

Transformer Type	Derating
Dry-type (Class F, 155°C)	1.0% per °C
Dry-type (Class H, 180°C)	0.66% per °C
Oil-immersed (Class A, 105°C)	1.5% per °C

Example: 50°C ambient, oil-immersed.

Excess = 50 – 40 = 10°C
Derating = 10 × 1.5% = 15%

This is severe. At 50°C ambient, you should seriously consider forced cooling (ONAF) or oversizing significantly.

Worked Example: Full Calculation

Scenario: Industrial plant at 1800 m altitude, 45°C max ambient. Mixed motor + lighting + welding load. Dry-type cast resin transformer, indoor installation with forced air cooling.

Step 1–2: Load list with Kd Total connected: 1200 kW → After demand factors: 760 kW

Step 3: Ks = 0.80 for 6 groups Diversified kW = 760 × 0.80 = 608 kW

Step 4: Weighted PF = 0.83 → kVA = 608/0.83 = 732.5 kVA

Step 5: 80% loading margin → 732.5/0.80 = 915.6 kVA

Step 6a: Altitude derating. 1800 m → 800 m excess, AF cooling = 0.3%/100m. Derating = 800/100 × 0.3% = 2.4%. Adjusted = 915.6 × 1.024 = 937.6 kVA

Step 6b: Temperature. 45°C → 5°C excess, Class F dry-type = 1%/°C. Derating = 5 × 1% = 5%. Adjusted = 937.6 × 1.05 = 984.5 kVA

Final selection: 1000 kVA dry-type, or 1250 kVA if significant future load growth is planned.

Note: with forced-air (AF) cooling and Class H insulation, the temperature derating drops from 5% to 3.3%, giving ~967 kVA — still 1000 kVA standard.

Quick Sizing Shortcuts (When You Don't Have a Load List)

Application	Rule of Thumb	Basis
Commercial office	80–120 VA/m²	Includes lighting, HVAC, sockets
Light industrial workshop	120–200 VA/m²	Motors + lighting + small power
Heavy industrial plant	200–400 VA/m²	Large motors, welding, process heat
Data center (IT only)	4–10 kW/rack	Depends on rack density
Hospital	100–150 VA/m²	High reliability, backup generation implied
Shopping mall	100–160 VA/m²	Lighting + HVAC dominant
Residential apartment	6–12 kVA per dwelling	National codes vary (NEC 220, IEC 60364)

Warning: These are for feasibility budgets only. Never use shortcuts for an RFQ.

Common Sizing Mistakes I've Seen

1. Adding Up MW, Not MVA

A 1000 kW motor with PF 0.80 draws 1250 kVA. If the transformer is sized at 1000 kVA, it's already overloaded before the first machine starts.

2. Ignoring Motor Starting

A 200 kW DOL-started motor draws ~1200 kVA momentarily. On a 1000 kVA transformer, that's a 120% inrush — voltage dip may exceed 10%, tripping contactors and VFDs. Soft starters or VFDs fix this, but the specifier must account for it.

3. Assuming Unity PF After PFC

Power factor correction capacitors reduce reactive current on the *supply side*, not on the transformer secondary. The transformer still sees the load PF. PFC at the transformer LV bus saves you on utility penalties; it doesn't make the transformer smaller.

4. Forgetting Harmonics

VFDs, UPS systems, and LED drivers generate harmonic currents (3rd, 5th, 7th, 11th). Harmonics cause additional heating in windings — IEC 60076-12 requires derating per IEEE C57.110 or by specifying a K-factor transformer. If THD-i > 15%, derate by at least 10%.

5. No Load Growth Buffer

I've seen a factory expand within 18 months of commissioning. The 1250 kVA transformer they "sized perfectly" was at 102% load by Year 2. Adding a second transformer cost 3× what upsizing from 1250 to 1600 kVA would have cost at procurement.

Frequently Asked Questions

FAQ

Q: "My contractor says I should size at 60% load. Why not 80%?"

*— Asked on r/ElectricalEngineering, 2024*

A: 60% loading is conservative and appropriate when future expansion is certain (e.g., Phase 2 already approved) or when you expect high harmonics (data centers). At 60%, you're paying for a larger transformer and higher no-load losses. At 80%, you're balancing capital cost with headroom. At 90%+, you're gambling. The number isn't universal — it depends on load profile and growth certainty.

Q: "Can I connect more load than the transformer rating if I have PFC?"

*— Asked on Electronics Stack Exchange, 2023*

A: PFC reduces reactive current drawn *from the grid*, so your utility meter sees better PF. But the transformer's rated current is based on apparent power (kVA). If you pull 1000 kVA through an 800 kVA transformer, the windings overheat regardless of PF at the primary terminals. PFC at the LV bus can reduce transformer loading if you correct the actual load PF — but that's a different calculation from utility-side correction.

Q: "How do I account for future EV charger installation?"

*— Asked on Quora, 2025*

A: EV chargers are nonlinear loads with high harmonic content and near-unity PF. For a commercial parking garage, add 7–22 kW per charging bay, with a demand factor of 0.6–0.8 (not all bays used simultaneously). For fast DC chargers (50–350 kW), treat each unit as a dedicated load with Kd = 1.0 and additional harmonic derating of 10–15% on the transformer.

Q: "Does altitude affect oil-immersed transformers the same way as dry-type?"

*— Asked on ResearchGate, 2022*

A: Oil-immersed is less sensitive to altitude than dry-type because oil cooling is relatively independent of air density. The derating is ~0.4% per 100 m above 1000 m vs 0.5% for dry-type AN. However, at extreme altitudes (>3000 m), the reduced dielectric strength of air affects external bushings — you may need extended creepage distance bushings per IEC 60076-3.

Q: "Should I use two smaller transformers instead of one large one?"

*— Asked on LinkedIn Engineering group, 2024*

A: Two transformers at 50–60% loading provide N+1 redundancy: if one fails, the other runs at 100–120% (temporarily overloaded) while you repair. This design is common in hospitals, data centers, and critical process plants. The premium is 20–40% on total transformer cost and additional switchgear. For non-critical loads, one properly sized unit is almost always more cost-effective.

Q: "What's the difference between IEC 60076-12 and IEEE C57.91 for loading?"

*— Asked on Engineering Tips, 2023*

A: Both cover loading beyond nameplate rating using thermal models. IEC 60076-12 is more conservative at high ambient temperatures, while IEEE C57.91 (Clause 7) provides more detailed guidance for North American conditions. In practice, use whichever your local authority requires. The underlying physics — Arrhenius aging rate — is the same.

Standards & References

Standard	Title
IEC 60076-1	Power Transformers — General
IEC 60076-2	Temperature Rise for Liquid-Immersed Transformers
IEC 60076-11	Dry-Type Transformers
IEC 60076-12	Loading Guide for Dry-Type and Liquid-Immersed Transformers
IEEE C57.91	Guide for Loading Mineral-Oil-Immersed Transformers
IEEE C57.110	Recommended Practice for Establishing Capability Under Nonsinusoidal Loads
NEC (NFPA 70)	Article 220 — Branch-Circuit, Feeder, and Service Load Calculations

*Du Fu sizes transformers for a living. He's seen both undersized units sweating at 110% and oversized units wasting capital — neither is good engineering.*