Selecting the correct silicon steel grade for a transformer core is not a catalog exercise — it is a loss-budget engineering decision that locks in your transformer’s efficiency for its entire 25–40 year service life. This guide walks you through the grade selection framework used by Zhongxin Steel’s technical team when advising transformer manufacturers across Europe, North America, and Southeast Asia.
Core Key Points
- Silicon steel grades for transformers are designated by thickness (B23 = 0.23 mm, B27 = 0.27 mm, B30 = 0.30 mm, B35 = 0.35 mm) plus a core loss number (e.g., B27G120 = 0.27 mm, ≤ 1.20 W/kg at 1.7 T, 50 Hz).
- Hi-B (High Induction) grades achieve magnetic induction B₈ ≥ 1.88 T vs. ≥ 1.80 T for conventional grades — allowing ~5–8% smaller core cross-sections.
- Transformer efficiency classes (IEC 60076 AA0/A0/A/B) directly map to required CRGO core loss levels — always start from the efficiency class requirement.
- Thinner laminations reduce eddy-current losses but increase stacking time and cost — 0.23 mm is rarely justified for 50 Hz transformers below 25 MVA.
- Always request mill test certificates (MTCs) with actual measured P₁.₇/₅₀ values, not just grade designation compliance.
The Grade Designation System Explained
The IEC 60404-8-7 standard designates grain-oriented electrical steel grades using a structured naming system. Understanding it is the first step to confident grade selection.
Example: B27G120
| Code | Meaning |
|---|---|
| B | Cold-rolled (Bandstahl = strip/sheet) |
| 27 | Nominal thickness: 0.27 mm |
| G | Grain-oriented |
| 120 | Maximum core loss: 1.20 W/kg at 1.7 T, 50 Hz |
Hi-B grades use the letter P (for Permeability) or H suffix in some manufacturer systems. Zhongxin’s Hi-B grades specify B₈ ≥ 1.88 T alongside the core loss value.
Common grade families supplied by Zhongxin Steel:
| Grade | Thickness | P₁.₇/₅₀ Max (W/kg) | B₈ Min (T) | Type |
|---|---|---|---|---|
| B23G090 | 0.23 mm | 0.90 | 1.88 | Hi-B |
| B23G095 | 0.23 mm | 0.95 | 1.88 | Hi-B |
| B27G100 | 0.27 mm | 1.00 | 1.88 | Hi-B |
| B27G120 | 0.27 mm | 1.20 | 1.80 | Standard |
| B30G110 | 0.30 mm | 1.10 | 1.88 | Hi-B |
| B30G120 | 0.30 mm | 1.20 | 1.80 | Standard |
| B35G130 | 0.35 mm | 1.30 | 1.80 | Standard |
| B35G155 | 0.35 mm | 1.55 | 1.77 | Economy |
Step 1: Define Your Loss Budget
Every transformer design starts with a maximum no-load loss (P₀) and load loss (Pk) budget, typically specified by the buyer or mandated by regional efficiency standards.
Key efficiency regulations in 2026:
- EU EcoDesign Regulation (EU) 2019/1781 — Tier 2 (effective 2025) mandates AA0 class or better for distribution transformers ≥ 25 kVA.
- US DOE 10 CFR 431 — TP-2 efficiency standards for liquid-filled transformers.
- China GB/T 17468-2019 — S20 energy efficiency rating system.
- India IS 1180 — Star-3 and Star-5 ratings for distribution transformers.
According to the International Electrotechnical Commission (IEC) IEC 60076-20 (Energy Efficiency of Transformers), AA0 class transformers must achieve no-load losses at least 20% lower than A0 class. Achieving AA0 essentially requires Hi-B CRGO at 0.27–0.30 mm.
Practical calculation:
If your specification requires P₀ ≤ 200 W for a 630 kVA transformer, and your core cross-section and mean path length yield approximately 170 kg of core material, your allowed specific core loss is:
P₀ / core weight = 200 W / 170 kg ≈ 1.18 W/kg
This maps directly to B27G120 or better — the grade ceiling for your design.
Step 2: Choose Between Standard CRGO and Hi-B
The Hi-B vs. standard choice affects not only material cost but core geometry:
Standard CRGO (B₈ = 1.80 T):
- Lower material cost (typically 8–12% less than equivalent Hi-B)
- Sufficient for IEC 60076 A and B class transformers
- Widely available; shorter lead times
Hi-B CRGO (B₈ ≥ 1.88 T):
- Higher magnetic flux density means the core can be designed ~5–8% smaller
- Reduced core weight: direct material savings on copper windings due to shorter mean turn length
- Required for AA0 and A0 class transformers
- Narrower supply base; verify lead time before committing to design
When to specify Hi-B:
- Efficiency class AA0 or A0 is required
- Physical size constraints limit core cross-section
- Buyer pays a premium for transformer efficiency (e.g., utility procurement)
When standard CRGO is sufficient:
- Efficiency class A or B
- Cost-competitive bidding context where first cost dominates
- Replacement/retrofit transformers matching existing core geometry
Step 3: Select Thickness Based on Operating Frequency
Eddy-current losses increase with the square of lamination thickness and with the square of frequency. The correct thickness choice depends on your operating frequency:
| Frequency | Recommended Thickness | Typical Application |
|---|---|---|
| 50 Hz | 0.27–0.35 mm | Power/distribution transformers |
| 60 Hz | 0.23–0.30 mm | US/Japan power transformers |
| 200–400 Hz | 0.15–0.20 mm | Industrial power supplies |
| 400–2,000 Hz | 0.10–0.15 mm | Railway traction, high-freq. inductors |
For standard 50 Hz grid-connected distribution transformers, 0.30 mm (B30Gxxx) represents the optimal balance between eddy-current loss, stacking cost, and material price. Moving to 0.27 mm yields approximately a 10% reduction in eddy-current losses — justifiable for AA0 class designs.
Step 4: Verify Against Your Efficiency Class
Cross-reference your grade selection against the IEC 60076 efficiency class table:
| IEC Class | No-Load Loss Level | Recommended CRGO Grade | Notes |
|---|---|---|---|
| AA0 | −40% vs A0 | B23G090 / B27G100 (Hi-B) | Highest efficiency |
| A0 | −25% vs A | B27G100 / B30G110 (Hi-B) | Common for EU Tier 2 |
| A | Baseline | B30G120 / B27G120 (Standard) | Most common globally |
| B | +25% vs A | B35G130 / B35G155 | Economy designs |
Once you have selected a grade designation, request actual MTCs from your supplier. Verify that the measured P₁.₇/₅₀ value on each coil meets or beats the grade maximum — don’t rely on grade name alone.
Grade Quick-Reference Table
| Design Scenario | Recommended Grade | Rationale |
|---|---|---|
| 50 Hz, AA0 class, ≤ 630 kVA | B27G100 (Hi-B) | Lowest loss at practical cost |
| 50 Hz, A0 class, 1–10 MVA | B30G110 (Hi-B) | Good cost/performance balance |
| 50 Hz, A class, 100–1000 kVA | B30G120 or B27G120 | Standard grade, cost-effective |
| 60 Hz, US market, DOE TP-2 | B23G090 or B27G100 | 60 Hz requires thinner for eddy loss |
| 400 Hz industrial | B15Gxxx (0.15 mm) | Thin gauge required |
| Special: 3-phase core-type large power (≥ 100 MVA) | B23G090 (Hi-B) | Premium grade, lowest total losses |
Common Mistakes and How to Avoid Them
Mistake 1: Specifying grade name without checking MTC values.
Grade designation is a maximum — actual material performance often beats the spec. Always request and review actual measured values.
Mistake 2: Using 0.23 mm for 50 Hz designs to “get the best grade.”
At 50 Hz, the eddy-current reduction from 0.23 mm vs. 0.27 mm is marginal (< 5%) but stacking cost and material cost increase significantly. Use 0.23 mm only where 60 Hz operation or strict loss budgets demand it.
Mistake 3: Ignoring the stacking factor in core loss calculations.
A stacking factor of 0.97 (typical for 0.30 mm CRGO) must be applied when calculating effective core cross-section — neglecting it will cause you to underestimate no-load losses by 3%.
Mistake 4: Selecting Hi-B grade without verifying coating compatibility.
Hi-B grades often use a different inorganic coating (C5/C6 class per IEC 60404-11) that requires stress-relief annealing at specific temperatures. Confirm coating grade compatibility with your core-building process before ordering.
FAQ
What is the difference between B27G120 and B27P120 CRGO grades?
In most manufacturer designations, both refer to 0.27 mm cold-rolled grain-oriented steel with a maximum core loss of 1.20 W/kg at 1.7 T, 50 Hz. The letter difference (G vs. P) is manufacturer-specific. Some producers use P to indicate a higher magnetic induction (Hi-B / high-permeability) variant. Always check the datasheet’s B₈ value — Hi-B requires B₈ ≥ 1.88 T.
Can I mix CRGO grades in a single transformer core?
Mixing grades is strongly discouraged. Different grades have different magnetic induction levels and thermal expansion coefficients. Mixing creates uneven flux distribution, hot spots, and audible noise (magnetostriction mismatch). Use a single grade throughout the core.
How much does upgrading from B30G120 to B27G100 (Hi-B) reduce no-load losses?
Based on Zhongxin Steel’s case studies with transformer manufacturers, the upgrade from B30G120 (standard) to B27G100 (Hi-B) typically reduces measured no-load losses by 18–24% in well-designed step-down distribution transformers, depending on core geometry and flux density design point.
What is the typical lead time for Hi-B CRGO from China?
Lead times for Hi-B CRGO (B27G100, B23G090) from Zhongxin Steel are typically 4–8 weeks for standard widths (800–1,250 mm) and 6–10 weeks for custom-slit widths. Standard grades (B30G120, B35G130) have 2–4 week lead times from stock.
Do I need a stress-relief anneal after cutting CRGO laminations?
Yes, for most transformer applications. Cutting (shearing or laser) introduces residual stress that increases core loss by 10–30%. Stress-relief annealing at 800–850°C in a controlled atmosphere recovers most of this loss. Some grades with specialized coatings require specific annealing cycles — always consult the MTC.