Size of Transformer_s Circuit Braker

The size of a transformer’s circuit breaker or fuse is governed by the National Electrical Code (NEC) section 450.3, which specifies the maximum overcurrent protection for transformers. The rules outlined in NEC 450.3 aim to protect both the transformer and the downstream circuits from damage caused by overcurrent conditions. Below is a summary of the key requirements:

NEC 450.3 Overview

NEC 450.3 addresses the overcurrent protection of transformers and varies based on the following factors:

Location of Overcurrent Protection Devices (Primary and Secondary side).
Type of Transformer (Dry-type, liquid-filled, etc.).
Voltage Class (600 volts or less, or over 600 volts).

The code distinguishes between overcurrent protection on the primary side and secondary side of the transformer. In some cases, only primary protection is required, while in others, secondary protection is necessary.

Primary Side Protection

The primary overcurrent protection device protects the transformer’s primary winding from faults or overload conditions. The size of the fuse or circuit breaker is determined as a percentage of the transformer's primary current.

For Transformers Rated 600 Volts or Less:
- Primary protection size is typically based on the percentage of the primary full-load current (FLC).
  - For transformers with secondary overcurrent protection:
    - If the transformer has secondary protection, the primary fuse or circuit breaker size can be up to 250% of the transformer's primary FLC.
  - For transformers without secondary overcurrent protection:
    - If there is no overcurrent protection on the secondary side, the primary protection device size can be up to 125% of the transformer's primary FLC.
  - If a standard size fuse or breaker isn't available, the next higher standard size can be used according to NEC 240.6.

Secondary Side Protection

Secondary protection is required in cases where the primary side protection alone is not sufficient to protect the transformer or downstream loads from faults.

For Transformers Rated 600 Volts or Less:
- Secondary protection size depends on whether or not primary overcurrent protection is present and the transformer’s full-load current rating.
- When secondary protection is used in conjunction with primary protection, it is sized at 125% of the secondary full-load current.

Practical Example

For a 45 kVA, 480V to 208Y/120V transformer:

Primary Current Calculation:
- $\text{Primary Current} = \frac{\text{kVA} \times 1000}{\text{Primary Voltage}}$
- For a 45 kVA, 480V primary:
  - $\text{Primary Current} = \frac{45000}{480} = 93.75 \text{ Amps}$ .
Primary Overcurrent Protection:
- With secondary protection, the maximum size would be 250% of 93.75 A:
  - $93.75 \times 2.5 = 234.38 \text{ Amps}$ .
- Choose the next standard size, which would be 250 Amps.
Secondary Current Calculation:

For 45 kVA, 208V secondary:

$\text{Secondary Current} = \frac{45000}{208 \times \sqrt{3}} \approx 125 \text{ Amps}$

Secondary Overcurrent Protection:
- At 125% of the secondary current:
  - $125 \times 1.25 = 156.25 \text{ Amps}$ .
- Choose the next standard size, which would be 175 Amps.

This example illustrates how to size both the primary and secondary overcurrent protection based on NEC 450.3 guidelines.

Considerations

The choice between fuses and circuit breakers can impact the sizing. Fuses generally allow for a more compact setup but require replacement after a fault, whereas circuit breakers can be reset after tripping.
Coordinating protection between the primary and secondary sides ensures that faults are isolated as close to the source as possible.
Always verify against the latest NEC code and consult with a qualified electrician or engineer for specific applications and compliance with local codes.

For a 1600 kVA transformer with a voltage rating of 11 kV/415 V, the selection of the circuit breaker and fuse size is guided by the National Electrical Code (NEC) 450.3. This standard outlines the requirements for overcurrent protection of transformers, ensuring safety and reliability.

Key Factors for Sizing Circuit Breakers and Fuses:

Primary Current Calculation (11 kV Side):

The primary current can be calculated using the formula: $\text{Primary Current (I\_primary)} = \frac{\text{Transformer kVA}}{\sqrt{3} \times \text{Primary Voltage (kV)}}$

For a 1600 kVA transformer with a primary voltage of 11 kV, the primary current would be: $I\_primary = \frac{1600}{\sqrt{3} \times 11} \approx 84 \, \text{A}$

Secondary Current Calculation (415 V Side):

The secondary current can be calculated using the formula: $\text{Secondary Current (I\_secondary)} = \frac{\text{Transformer kVA}}{\sqrt{3} \times \text{Secondary Voltage (V)}}$
$I\_secondary = \frac{1600}{\sqrt{3} \times 0.415} \approx 2225 \, \text{A}$

Overcurrent Protection per NEC 450.3:

Primary Overcurrent Protection:
- According to NEC 450.3(B), for primary-only protection of transformers over 600 V (such as 11 kV), the maximum size of the overcurrent protective device is typically 125% of the full load current.
- For a primary current of 84 A, the maximum breaker or fuse size would be: $\text{Max Primary Fuse/Breaker Size} = 84 \times 1.25 \approx 105 \, \text{A}$
- The available standard rating nearest to this would be 100 A or 125 A, depending on local code interpretation.
Secondary Overcurrent Protection:
- For secondary protection, NEC 450.3(A) provides guidelines for transformers rated 1000 V or less (such as 415 V). It typically allows 125% of the transformer's secondary full-load current for overcurrent protection.
- For a secondary current of 2225 A, the maximum overcurrent device size would be: $\text{Max Secondary Fuse/Breaker Size} = 2225 \times 1.25 \approx 2781 \, \text{A}$
- Standard breaker sizes would be 2500 A or 3000 A.

Practical Selection:

Primary Side: Use a circuit breaker or fuse rated at 100 A or 125 A on the 11 kV side.
Secondary Side: Use a circuit breaker rated at 2500 A or 3000 A on the 415 V side, depending on the specific application needs and local code interpretations.

Always verify with the latest NEC guidelines and consult with a licensed electrical engineer to ensure proper sizing and compliance with all safety standards and regulations.

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Size of Transformer_s Circuit Braker _ Fuse-NEC 450.3 Excel