Accurate primary and secondary protection sizing is critical for electrical system safety and compliance with NEC standards. Proper calculations prevent equipment damage and ensure reliable operation.
This article thoroughly explores NEC-based protection sizing, including formulas, tables, and real-world examples. Learn to apply NEC guidelines effectively for optimal protection device selection.
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- Calculate primary and secondary fuse sizes for a 480V, 200A transformer.
- Determine secondary protection for a 75kVA, 208Y/120V transformer.
- Find primary fuse rating for a 150kVA transformer with 600V primary voltage.
- Compute secondary breaker size for a 112.5kVA transformer with 240V secondary voltage.
Comprehensive Tables for Primary and Secondary Protection Sizing According to NEC
Table 1: NEC Recommended Primary Protection Sizing for Transformers (NEC 450.3)
| Transformer kVA Rating | Primary Voltage (V) | Primary Full Load Current (A) | Primary Fuse Size (A) (NEC 450.3) | Notes |
|---|---|---|---|---|
| 15 | 480 | 18.1 | 25 | Standard fuse size, 125% of full load current |
| 30 | 480 | 36.3 | 50 | Typical primary fuse rating |
| 75 | 480 | 90.1 | 125 | Fuse size per NEC 450.3(B) |
| 112.5 | 480 | 135.1 | 175 | Common primary fuse size |
| 150 | 480 | 180.2 | 225 | Fuse size rounded to next standard size |
| 225 | 480 | 270.3 | 350 | Large transformer primary protection |
| 300 | 480 | 360.4 | 400 | High capacity primary fuse |
Table 2: NEC Recommended Secondary Protection Sizing for Transformers (NEC 450.3(B))
| Transformer kVA Rating | Secondary Voltage (V) | Secondary Full Load Current (A) | Secondary Overcurrent Device Size (A) | Notes |
|---|---|---|---|---|
| 15 | 120/240 | 62.5 | 80 | 125% of full load current per NEC 450.3(B) |
| 30 | 120/240 | 125 | 150 | Standard breaker size |
| 75 | 208Y/120 | 208 | 250 | Secondary breaker size per NEC |
| 112.5 | 240 | 468.8 | 600 | Large secondary breaker |
| 150 | 240 | 625 | 700 | Breaker size rounded up |
Table 3: Common Transformer Full Load Currents (FLC) for Various kVA and Voltages
| Transformer kVA | Primary Voltage (V) | Secondary Voltage (V) | Primary FLC (A) | Secondary FLC (A) |
|---|---|---|---|---|
| 15 | 480 | 120/240 | 18.1 | 62.5 |
| 30 | 480 | 120/240 | 36.3 | 125 |
| 75 | 480 | 208Y/120 | 90.1 | 208 |
| 112.5 | 480 | 240 | 135.1 | 468.8 |
| 150 | 600 | 240 | 144.3 | 625 |
Essential Formulas for Primary and Secondary Protection Sizing According to NEC
1. Transformer Full Load Current (FLC) Calculation
The fundamental starting point for protection sizing is calculating the transformer’s full load current on both primary and secondary sides.
Secondary FLC (A) = Transformer kVA × 1000 / (√3 × Secondary Voltage (V))
- Transformer kVA: Rated power of the transformer in kilovolt-amperes.
- Primary Voltage (V): Rated primary voltage of the transformer.
- Secondary Voltage (V): Rated secondary voltage of the transformer.
- √3: Square root of 3 (~1.732), used for three-phase systems.
For single-phase transformers, the formula simplifies to:
2. Primary Protection Sizing (NEC 450.3(B))
NEC 450.3(B) provides guidelines for sizing primary overcurrent protection devices (OCPD) for transformers.
- The 125% multiplier accounts for inrush current and transformer heating.
- OCPD size must not exceed the next standard fuse or breaker size above this value.
3. Secondary Protection Sizing (NEC 450.3(B))
Secondary protection sizing also follows the 125% rule but must consider the transformer’s secondary full load current.
- Secondary OCPD protects the transformer and downstream equipment.
- Must be coordinated with primary protection to avoid nuisance tripping.
4. Maximum Primary OCPD Size (NEC 450.3(B))
NEC limits the maximum primary OCPD size to prevent excessive fault current.
- This ensures the primary fuse or breaker does not exceed a value that could damage the transformer.
5. Minimum Secondary OCPD Size (NEC 450.3(B))
NEC also specifies a minimum secondary OCPD size to avoid nuisance tripping during transformer energization.
This is the same as the secondary protection sizing formula but serves as a minimum threshold.
Detailed Real-World Examples of Primary and Secondary Protection Sizing
Example 1: Sizing Protection for a 75 kVA, 480V/208Y/120V Transformer
Calculate the primary and secondary overcurrent protection device sizes for a 75 kVA three-phase transformer with a 480V primary and 208Y/120V secondary.
Step 1: Calculate Full Load Currents
Primary FLC:
Secondary FLC:
Step 2: Calculate Primary OCPD Size
Choose the next standard fuse size: 125 A.
Step 3: Verify Maximum Primary OCPD Size
125 A is less than 468.75 A, so the fuse size is acceptable.
Step 4: Calculate Secondary OCPD Size
Choose the next standard breaker size: 275 A or 300 A (depending on availability).
Summary:
- Primary fuse size: 125 A
- Secondary breaker size: 275 A or 300 A
Example 2: Protection Sizing for a 150 kVA, 600V/240V Transformer
Determine the primary and secondary protection sizes for a 150 kVA transformer with a 600V primary and 240V secondary.
Step 1: Calculate Full Load Currents
Primary FLC:
Secondary FLC:
Step 2: Calculate Primary OCPD Size
Next standard fuse size: 200 A or 225 A (225 A preferred for margin).
Step 3: Verify Maximum Primary OCPD Size
225 A is well below 937.5 A, so acceptable.
Step 4: Calculate Secondary OCPD Size
Next standard breaker size: 475 A or 500 A.
Summary:
- Primary fuse size: 225 A
- Secondary breaker size: 475 A or 500 A
Additional Technical Considerations for NEC Protection Sizing
- Inrush Current: Transformers draw a high inrush current during energization, often 6-12 times full load current. The 125% sizing factor helps accommodate this.
- Coordination: Primary and secondary protection devices must be coordinated to ensure selective tripping and minimize system downtime.
- NEC Exceptions: NEC 450.3(B) allows exceptions for certain transformer types or applications, such as transformers with integral primary protection.
- Fuse Types: Time-delay fuses are preferred for transformer protection to handle inrush currents without nuisance tripping.
- Voltage Ratings: Protection devices must be rated for the system voltage and interrupting capacity per NEC 110.9 and 240.60.
- Temperature Ratings: Ambient temperature and conductor insulation ratings affect protection device sizing and must be considered.
Authoritative References and Further Reading
- National Electrical Code (NEC) – NFPA
- Transformer Protection Guidelines – Schneider Electric
- Fuse Selection and Sizing – Eaton Corporation
- International Electrotechnical Commission (IEC) Standards
By following NEC guidelines and applying these calculations, engineers can ensure safe, reliable transformer protection. Proper sizing minimizes risk and optimizes system performance.