Choosing a Factory Transformer Right — kVA Sizing, Oil-immersed vs Dry-type, and the Loss That Runs 24/7

The transformer is the "gateway" that takes the utility's medium voltage (22 kV from PEA or 24 kV from MEA) down to the 400/230 V your machines use. Choose the wrong size or type and the cost doesn't end at the price tag — it follows you as losses that draw power 24 hours a day across a 20–30 year life, plus the risk of not being able to expand or to pass grid connection. This guide covers choosing right the first time.

What a factory transformer does — and what it must pass

• Steps 22 kV (PEA) / 24 kV (MEA) → 400/230 V (feeding motors / the plant electrical system)
• Must comply with TIS + IEC 60076 and be on the utility's accepted-product list (PEA/MEA product acceptance) — if it isn't, you may not be allowed to connect

Sizing (kVA): not too big, not too small

Transformer size is in kVA, calculated from real load, not guessed:

1. Sum the connected load → convert to kVA
2. Apply a demand factor (not everything runs at once) + diversity
3. Add ~20–25% margin for future expansion
4. Compare against the contract demand requested from the utility

Too big = you pay for no-load loss (drawn 24/7) you don't need + over-invest Too small = overload, high temperature, fast insulation aging, short life + no room to grow

Running a transformer at ~50–80% of rating is the sweet spot (headroom + good efficiency).

Oil-immersed vs Dry-type — the main decision

Factor	Oil-immersed	Dry-type (cast resin)
Location	Outdoor / outside the building	Indoor / near people
Fire safety	Needs an oil bund + fire separation	Hard to ignite, suits risk areas
Initial price	Cheaper	Higher
Efficiency/loss	Excellent (low loss)	Slightly higher
Maintenance	Oil testing required (DGA)	Low (no oil)
Environment tolerance	Sealed in a tank	Needs dust/moisture protection (IP enclosure)
Large size / high voltage	✓ Suited	More limited

In short: outdoor / tight budget / large size → oil · indoor / near people / fire-risk areas (malls, high-rises, staffed warehouses) → dry-type.

Loss and TCO (the most common mistake)

A transformer has two losses:

• No-load loss (iron loss): present whenever the transformer is energized — drawn 24 h/day, all year, even with no load
• Load loss (copper loss): varies with load²

Because no-load loss runs 24/7 for 20–30 years → it costs more than the price tag over the long run. You must think in TCO (Total Owning Cost), not just purchase price:

TCO ≈ transformer price + (factor A × no-load loss) + (factor B × load loss)

A high-efficiency transformer (≥98%, TIS/DEDE) costs more up front but saves on energy over its life — usually a fast payback for continuously running factories.

Specs to state when ordering

• Rating (kVA) + voltage (22/24 kV → 400/230 V)
• Vector group — Thai factories commonly use Dyn11
• % Impedance — typically ~4–6% (affects fault current + parallel operation)
• Tap changer — off-load DETC, typically ±2×2.5% for voltage adjustment
• Cooling — ONAN/ONAF (oil) or AN/AF (dry)
• TIS + on the utility's accepted-product list

Hot, humid Thailand — don't forget heat

• A transformer's rating is quoted at a standard ambient temperature — hot Thailand needs derating + transformer-room ventilation
• Oil: needs an oil bund to contain leaks + electrical clearances + periodic oil testing
• Dry-type: needs dust/moisture protection (right IP enclosure) + adequate ventilation

The selection flow in brief

flowchart TD
  A["Sum load → kVA
× demand factor + 20-25% margin"] --> B{"Where is it installed?"}
  B -->|"Outdoor / outside"| C["Oil-immersed
(cheaper, low loss)"]
  B -->|"Indoor / near people / fire-risk"| D["Dry-type
(hard to ignite, low upkeep)"]
  C --> E{"Runs continuously?"}
  D --> E
  E -->|"Yes"| F["Pick a high-efficiency model
≥98% — think TCO, not price tag"]
  E -->|"Intermittent"| G["Balance price vs loss"]
  F --> H["Specify Dyn11 + Z% + tap
+ must pass TIS/PEA-MEA"]
  G --> H

A complete factory power intake

Transformer = power in · genset = backup when the grid fails — they can be designed together (see sizing a backup genset per ISO 8528).

Let Sahawatthanakit design + source it

Our engineering team handles factory electrical EPC — load survey → kVA calculation → type (oil/dry) + spec selection → utility coordination → installation and testing — choosing a right-sized, low-TCO transformer so you don't pay for unnecessary losses.

• Phone: 02-096-2118 / 061-541-6939 (Khun Chin)
• LINE: @sahawatt1988
• Email: info@sahawatthanakit1988.com
• Free quote / consultation: click here

Frequently Asked Questions

What kVA transformer does my factory need? Base it on the real connected load × demand factor (not everything runs at once) + 20–25% for expansion, then compare to the contract demand requested from the utility. Don't oversize (24/7 no-load loss) or undersize (overload, short life).

Oil-immersed or dry-type — which should I pick? Outdoor, tight budget, or large size → oil (cheaper, low loss). Indoor, near people, or fire-risk areas → dry-type (hard to ignite, low maintenance, no oil) — but it costs more.

Why does no-load loss matter more than the price tag? Because no-load loss draws power 24 h/day, every day, for 20–30 years, even with no load — it accumulates to more than the purchase-price difference, so consider TCO and high-efficiency models (≥98%).

What standards must a transformer meet to connect to the utility? It must comply with TIS + IEC 60076 and be on the PEA/MEA accepted-product list — buy from approved manufacturers, or your connection application may be rejected.

What is vector group Dyn11 and why specify it? It's the winding configuration (delta-star, 330° phase shift) common in Thai distribution — important when paralleling transformers or matching an existing system's phase. Always specify it to match the existing system.