Transformer Breaker Size Chart
Primary & Secondary MCCB Sizing — Single-Phase & Three-Phase — 15 VA to 1000 kVA — 480V/208V/120V
Most competitors offer only PDF documents that require downloading. This page gives you a live online transformer breaker size chart — searchable, printable, and updated to the 2023 NEC cycle. Whether you’re sizing a 75 kVA three-phase unit or a small 500 VA control transformer, these tables cover your exact application.
In nearly 20 years of building and commissioning transformers at our factory, I’ve seen breaker mis-sizing cause more nuisance shutdowns than almost any other single factor. The math is straightforward once you know the NEC rules — and that’s exactly what this guide delivers.
Download: Transformer Breaker Size Chart PDF
2-page A4 quick-reference · NEC 450.3(B) · Single & Three-Phase · 15VA–1000kVA
1. NEC 450.3(B) Rules for Circuit Breakers
NEC Article 450.3(B) governs overcurrent protection for transformers rated 600V or less on both sides. The same percentage multipliers that apply to fuses also apply to inverse-time circuit breakers (ITCBs) — the difference is how the device achieves that protection.
Primary-ONLY Protection
- Primary FLA ≥ 9A → breaker ≤ 125%
- Primary FLA 2A – 9A → breaker ≤ 167%
- Primary FLA < 2A → breaker ≤ 300%
Round up to next standard MCCB frame size if calculated value falls between ratings.
Primary + Secondary Protection
- Primary breaker ≤ 250% of primary FLA
- Secondary breaker ≤ 125% of secondary FLA (if ≥ 9A)
- Secondary breaker ≤ 167% of secondary FLA (if < 9A)
Upsizing the primary breaker to 250% reduces nuisance trips from transformer inrush current (8–12× FLA).
2. FLA Calculation Formulas
Calculate full-load amps (FLA) before using any chart. Round up to the next standard breaker size after applying the NEC multiplier.
I (A) = kVA × 1,000 ÷ V
I (A) = kVA × 1,000 ÷ (V × 1.732)
Standard MCCB rating sequence: 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000A
3. Single-Phase Transformer Breaker Chart — Primary 480V
Use the yellow-highlighted row (37.5 kVA) as a common reference for light commercial installations.
| kVA | Pri FLA (480V) | Pri CB 125% only | Pri CB 250% (w/sec) | Sec FLA (240V) | Sec CB 125% | Sec FLA (120V) | Sec CB 125% |
|---|---|---|---|---|---|---|---|
| 5 kVA | 10.4A | 15A | 30A | 20.8A | 30A | 41.7A | 60A |
| 10 kVA | 20.8A | 30A | 60A | 41.7A | 60A | 83.3A | 110A |
| 15 kVA | 31.3A | 40A | 80A | 62.5A | 80A | 125.0A | 175A |
| 25 kVA | 52.1A | 70A | 150A | 104.2A | 150A | 208.3A | 300A |
| 37.5 kVA | 78.1A | 100A | 200A | 156.3A | 200A | 312.5A | 400A |
| 50 kVA | 104.2A | 150A | 300A | 208.3A | 300A | 416.7A | 600A† |
| 75 kVA | 156.3A | 200A | 400A | 312.5A | 400A | 625.0A | 800A† |
| 100 kVA | 208.3A | 300A | 600A† | 416.7A | 600A† | 833.3A | 1200A† |
| 167 kVA | 347.9A | 450A | 900A† | 695.8A | 900A† | — | — |
| 250 kVA | 520.8A | 700A† | LVPCB | 1041.7A | LVPCB | — | — |
† Above 600A: consult AHJ; current-limiting fuses or LVPCBs (Low Voltage Power Circuit Breakers) are often preferred. | All values per NEC 450.3(B) 2023.
For the corresponding transformer specifications, see our 100 kVA single-phase transformer spec page and the transformer fuse size chart if your installation uses fuses instead.
4. Three-Phase Transformer Breaker Chart — Primary 480V → Secondary 208Y/120V
The 112.5 kVA row (highlighted) is the most common rating in US commercial buildings for 480V→208Y/120V step-down service.
| kVA | Pri FLA (480V) | Pri CB 125% only | Pri CB 250% (w/sec) | Sec FLA (208V) | Sec CB 125% |
|---|---|---|---|---|---|
| 15 kVA | 18.0A | 25A | 50A | 41.6A | 60A |
| 30 kVA | 36.1A | 50A | 100A | 83.3A | 110A |
| 45 kVA | 54.1A | 70A | 150A | 124.9A | 175A |
| 75 kVA | 90.2A | 125A | 250A | 208.2A | 300A |
| 112.5 kVA | 135.3A | 175A | 350A | 312.4A | 400A |
| 150 kVA | 180.4A | 250A | 500A | 416.4A | 600A† |
| 225 kVA | 270.6A | 350A | 700A† | 624.6A | 800A† |
| 300 kVA | 360.8A | 500A | 1000A† | 832.7A | 1200A† |
| 500 kVA | 601.4A | 800A† | 1600A† | 1387.8A | LVPCB |
| 750 kVA | 902.1A | 1200A† | LVPCB | 2081.7A | LVPCB |
| 1000 kVA | 1202.8A | 1600A† | LVPCB | 2775.7A | LVPCB |
LVPCB = Low Voltage Power Circuit Breaker (required when standard MCCBs cannot achieve the required rating or AIC). † Verify AIC against available fault current — standard MCCBs typically 22–65 kA.
For large three-phase units, see our 1000 kVA three-phase padmount transformer spec and 112.5 kVA three-phase padmount specifications.
5. Control Transformer Breaker Chart — 480V Primary, 120V Secondary
Small control transformers (50 VA – 1000 VA) use miniature circuit breakers (MCBs) rather than MCCBs. Time-delay trip characteristics are important to survive inrush.
| VA Rating | Pri FLA (480V) | NEC Rule | Pri CB max | Sec FLA (120V) | Sec CB 125% |
|---|---|---|---|---|---|
| 50 VA | 0.10A | <2A → 300% | 1A | 0.42A | 1A |
| 100 VA | 0.21A | <2A → 300% | 1A | 0.83A | 1A |
| 250 VA | 0.52A | <2A → 167% | 1A | 2.08A | 4A |
| 500 VA | 1.04A | 2–9A→167% | 2A | 4.17A | 6A |
| 750 VA | 1.56A | 2–9A→125% | 2A | 6.25A | 10A |
| 1000 VA | 2.08A | ≥9A →125% | 3A | 8.33A | 15A |
Use Type D (high inrush) or Type K MCBs for control transformer primary protection. Standard Type B/C MCBs may nuisance-trip on energization. Always verify MCB is rated for 480V AC.
6. MCCB Adjustable Trip Settings for Transformer Service
Electronic-trip MCCBs offer field-adjustable long-time delay (LTD) and short-time delay (STD) bands. Proper setting prevents both nuisance trips and slow response to real faults.
Recommended MCCB Settings
| Setting | Recommended Value | Purpose |
|---|---|---|
| Long-Time Pickup (LTD) | 1.0–1.15× transformer FLA | Continuous overload protection |
| Long-Time Delay | Maximum (longest delay) | Survive transformer inrush (8–12× FLA) |
| Short-Time Pickup (STD) | 6–10× transformer FLA | Above inrush, below fault level |
| Short-Time Delay | 0.1–0.2 s | Allow downstream breakers to clear first |
| Instantaneous | Disabled or max setting | Avoid inrush-caused instantaneous trips |
7. Fuse vs. Breaker: Which Is Right for Your Transformer?
Both fuses and circuit breakers comply with NEC 450.3(B). The choice depends on application, fault current levels, and operational requirements.
| Criterion | Fuse (Class T / RK5) | MCCB (Inverse-Time CB) |
|---|---|---|
| AIC Rating | Up to 200 kA (Class T) | 22–85 kA (standard MCCB) |
| After fault operation | Must replace fuse element | Reset and re-close |
| First cost | Lower purchase price | Higher upfront cost |
| Inrush tolerance | Time-delay fuses handle well | Adjustable trip handles well |
| Adjustability | Fixed rating (change fuse) | Electronic trip: adjustable |
| Current-limiting | Yes (Class CC, T, RK5) | Only specially rated CLCBs |
| Space in panel | Compact fuse holder | Larger frame footprint |
| Best for | High fault-current locations | Frequent switching / resets |
For fuse-specific sizing tables, refer to our companion page: Transformer Fuse Size Chart – NEC 450.3.
8. 5 Common Breaker Sizing Mistakes (Factory Experience)
The most dangerous error I see: selecting an MCCB rated 22 kA on a switchboard bus with 50 kA available fault current. The breaker may explode rather than safely interrupt. Always calculate AFC at the installation point and select an MCCB with AIC ≥ AFC.
Setting the breaker exactly at transformer FLA causes constant nuisance trips from inrush on every energization. Per NEC 450.3(B), you’re allowed — and required — to multiply FLA by 125% (or 250% with secondary protection). Nine times out of ten when I troubleshoot a “bad transformer,” it’s actually an undersized breaker.
Standard residential MCBs (Type B/C, typically rated 230V or 400V) are not rated for 480V AC industrial circuits. Always use 480V-rated MCCBs or fused disconnects on 480V transformer primaries.
For transformers ≥ 75 kVA, skipping secondary breakers forces you to use a smaller primary breaker (125%), which is more prone to inrush trips. Adding a secondary breaker lets you run a 250% primary breaker — far more forgiving on energization. Our factory always specifies both sides for units over 30 kVA.
Some installers permanently disable the instantaneous trip to stop inrush nuisance trips. While this works for inrush, it eliminates protection against bolted faults — which can destroy a transformer before the long-time delay trips. Use the adjustable STD band instead; set it above inrush (8–12× FLA) for 0.1–0.2 seconds.
FAQ
Primary FLA = 75,000 ÷ (480 × 1.732) = 90.2A. With primary-only protection: 90.2 × 125% = 112.8A → use a 125A MCCB. With secondary protection (250% primary): 90.2 × 250% = 225.5A → use a 250A MCCB primary plus 300A secondary MCCB (208V side FLA = 208.2A × 125% = 260.3A → 300A). See our 112.5 kVA three-phase oil-filled spec for related ratings.
Only for small control transformers (typically <1 kVA, 120V secondary side). For distribution transformers (15 kVA+), you need MCCBs with higher AIC ratings (22–65 kA minimum) and appropriate frame sizes. Standard MCBs max out at 6 kA or 10 kA interrupting capacity — far below the available fault current on most 480V distribution buses.
If protecting only the primary side, the MCCB must be no more than 125% of primary FLA (for FLA ≥ 9A). If you add a secondary breaker rated at 125% of secondary FLA, you can upsize the primary breaker to 250% — this extra headroom is critical for surviving transformer inrush current (8–12× FLA) without nuisance trips.
Set long-time delay (LTD) pickup at 1.0–1.15× transformer FLA with maximum delay. Set short-time delay (STD) pickup at 6–10× transformer FLA for 0.1–0.2 seconds. Set instantaneous trip to the maximum setting or disable it (if permitted by AHJ) — relying on STD for fault protection instead. This configuration handles inrush while protecting against sustained faults.
Electronic trip MCCBs are preferred for transformers ≥ 75 kVA because adjustable LTD and STD bands can be tuned to the transformer’s inrush characteristic. Thermal-magnetic MCCBs work for smaller units but may nuisance-trip if the inrush multiplier exceeds 10×. For 15–30 kVA units, a time-delay fuse is often the most economical and reliable choice.
No — FLA values differ because FLA = kVA × 1000 ÷ voltage. A 75 kVA three-phase transformer at 600V primary has FLA = 72.2A (vs 90.2A at 480V). Recalculate FLA for your actual primary voltage, then apply the same 125%/250% NEC multipliers. The breaker percentage rules are the same; the ampere values change with voltage.
Related Transformer Resources
- Transformer Fuse Size Chart – NEC 450.3 Reference
- 10 kVA Single-Phase Transformer Specifications
- 100 kVA Single-Phase Transformer Specifications
- 112.5 kVA Three-Phase Padmount Transformer
- 1000 kVA Three-Phase Padmount Transformer
Need a Custom Transformer with Protection Coordination?
Our factory engineers can specify the correct primary and secondary MCCB ratings alongside your transformer order — ensuring NEC compliance from day one.