Transformer Protection per NEC Calculator – NEC

Transformer protection per NEC calculator is essential for ensuring electrical safety and compliance. It determines appropriate protective device ratings based on transformer specifications and NEC guidelines.

This article covers detailed NEC requirements, formulas, tables, and real-world examples for transformer protection calculations. It is designed for engineers, electricians, and professionals seeking precise NEC-compliant transformer protection.

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• Calculate primary fuse size for a 75 kVA, 480V transformer.
• Determine secondary breaker rating for a 150 kVA, 208Y/120V transformer.
• Find maximum primary overcurrent device for a 500 kVA, 2400V transformer.
• Compute transformer inrush current and select protective device accordingly.

Common Transformer Protection Values per NEC

Key Formulas for Transformer Protection per NEC

Transformer protection calculations rely on several fundamental electrical formulas derived from NEC Article 450. These formulas help determine the appropriate sizing of overcurrent protective devices (OCPDs) for both primary and secondary sides of transformers.

1. Full Load Current (FLC) Calculation

The full load current of a transformer is the current drawn when the transformer operates at its rated kVA and voltage.

• I_FL: Full load current (Amperes)
• Transformer kVA: Transformer rating in kilovolt-amperes
• Voltage: Line-to-line voltage on the side being calculated (Volts)
• √3: Square root of 3 (≈1.732), used for three-phase systems

For single-phase transformers, the formula simplifies to:

2. Primary Overcurrent Device Sizing per NEC 450.3

NEC 450.3(B) allows the primary OCPD to be sized up to 125% of the transformer’s full load current for protection against overloads.

• I_{Primary OCPD}

: Maximum primary overcurrent protective device rating (Amperes)

• I_FL: Full load current on the primary side (Amperes)

3. Secondary Overcurrent Device Sizing per NEC 450.3

For the secondary side, NEC 450.3(A) requires the OCPD to be sized at 125% of the full load current or the next standard size above.

• I_{Secondary OCPD}

: Maximum secondary overcurrent protective device rating (Amperes)

• I_FL: Full load current on the secondary side (Amperes)

4. Transformer Inrush Current Considerations

Transformer inrush current can be 6 to 12 times the full load current momentarily. Protective devices must accommodate this without nuisance tripping.

• Use time-delay fuses or inverse-time breakers.
• NEC allows sizing OCPDs up to 250% of full load current for transformers with inrush current.

5. Calculating Full Load Current Example

For a 75 kVA, 480 V three-phase transformer:

Detailed Real-World Examples of Transformer Protection per NEC

Example 1: Sizing Primary Fuse for a 75 kVA, 480 V Transformer

A 75 kVA, 480 V three-phase transformer requires primary side protection. Determine the appropriate fuse size per NEC 450.3.

• Step 1: Calculate full load primary current.

• Step 2: Calculate maximum fuse size (125% of full load current).

• Step 3: Select the next standard fuse size above 113 A.

Standard fuse sizes are 100 A, 110 A, 125 A, etc. The 125 A fuse is the appropriate choice.

• Step 4: Verify inrush current accommodation.

Since inrush current can be up to 12 times full load current, use a time-delay fuse rated at 125 A to prevent nuisance tripping.

Example 2: Secondary Breaker Sizing for a 150 kVA, 208Y/120 V Transformer

A 150 kVA transformer with a 208Y/120 V secondary requires a secondary breaker. Calculate the breaker size per NEC 450.3.

• Step 1: Calculate full load secondary current.

• Step 2: Calculate maximum breaker size (125% of full load current).

• Step 3: Select the next standard breaker size above 520.1 A.

Standard breaker sizes include 500 A, 600 A, etc. The 600 A breaker is selected.

• Step 4: Confirm breaker trip characteristics to handle inrush current.

Use a breaker with an inverse-time delay to accommodate transformer magnetizing inrush current.

Additional Technical Details and NEC References

NEC Article 450 governs transformer protection, specifying requirements for sizing overcurrent devices to protect transformers from overloads and faults. Key points include:

• NEC 450.3(B): Primary OCPD can be sized up to 125% of full load current.
• NEC 450.3(A): Secondary OCPD must not exceed 125% of full load current.
• NEC 450.4: Protection against faults and overloads must be provided on both primary and secondary sides.
• NEC 240.21: Overcurrent device ratings must coordinate with conductor ampacity and transformer ratings.

Transformer protection devices must also consider coordination with upstream and downstream devices to ensure selective tripping and minimize system downtime.

Summary of Transformer Protection Device Selection Process

• Calculate full load current for both primary and secondary sides.
• Apply NEC 125% rule to determine maximum OCPD size.
• Select the next standard device rating above the calculated value.
• Consider transformer inrush current and select time-delay or inverse-time devices.
• Verify coordination with conductors and other protective devices.
• Ensure compliance with NEC Articles 240 and 450.

Authoritative External Resources

• National Fire Protection Association (NFPA) – NEC Official Site
• EC&M: Transformer Protection Basics
• Schneider Electric: Transformer Protection Guide