Applying NEC Table 450.3(B): Transformer Overcurrent and Conductor Protection

NEC Table 450.3(B) provides clear rules for overcurrent protection of transformers rated 1,000 volts or less with secondary currents of 9 amps or more. These rules govern the maximum overcurrent protective device (OCPD) ratings for both the primary and secondary sides of transformers, ensuring protection for both the transformer and associated conductors.

This article explains how to apply Table 450.3(B), protect primary and secondary conductors, and utilize 240.4(B) where appropriate.

Understanding NEC Table 450.3(B)

Key Rules for Transformer OCPD Sizing

• Primary-Only Protection: The maximum OCPD size is 125% of the transformer’s primary current. Note 1 allows rounding up to the next standard OCPD size per 240.6(A).
• Primary and Secondary Protection:
  • The primary OCPD can be up to 250% of the transformer’s primary current, but rounding up is not permitted (Note 1 does not apply).
  • The secondary OCPD is limited to 125% of the transformer’s secondary current, with Note 1 allowing rounding up to the next standard size.
• Conductor Protection:
  • Primary conductors are protected by the transformer’s primary OCPD, provided their ampacity matches or exceeds the OCPD rating. If the ampacity is less, conductors must be upsized or 240.4(B) can be applied if conditions allow.
  • Secondary conductors, governed by 240.21(C), must meet or exceed the rating of the OCPD they terminate into. 240.4(B) does not apply to secondary conductors.
• Special Transformers: For transformers with coordinated thermal overload protection, Note 3 permits exceeding the 250% primary OCPD limit.

Protecting Primary Conductors

Primary Conductors Are Not Feeder Taps

The conductors between the primary OCPD and the transformer are not feeder taps because they terminate at the transformer rather than a branch-circuit OCPD. These conductors are part of the transformer circuit and governed by Article 450, not 240.21(B).

How to Protect Primary Conductors

• The primary OCPD, sized per Table 450.3(B), typically protects both the transformer and the primary conductors.
• If the conductor ampacity is less than the OCPD rating:
  • Upsize the conductors to match or exceed the OCPD rating.
  • Apply 240.4(B) to use the next standard OCPD size if conditions are met.

Protecting Secondary Conductors

Secondary conductors are governed by 240.21(C), which requires their ampacity to meet or exceed the rating of the OCPD they terminate into. 240.4(B) does not apply to secondary conductors.

Examples of Applying NEC Table 450.3(B)

Example 1: Primary-Only Protection

Scenario:

• Transformer: 75 kVA, 480V primary, 208/120V secondary (three-phase).
• Primary Current:
  I_Primary = 75,000 / (480 × 1.732) ≈ 90.3 amps.
• Primary OCPD:
  90.3 amps × 1.25 = 112.88 amps.
  Note 1 allows rounding up to 125 amps.

Primary Conductor Protection:

• Conductor Ampacity: 110 amps.
• Load: 90.3 amps.
• 240.4(B) Application: Since the conductor ampacity (110 amps) exceeds the load and is between standard sizes, the 125-amp OCPD can be used.

Conclusion: Install a 125-amp OCPD and protect the 110-amp primary conductors using 240.4(B).

Example 2: Primary and Secondary Protection

Scenario:

• Primary OCPD: Sized per 250% Rule:
  90.3 amps × 2.5 = 225.75 amps.
  The OCPD must not exceed 225 amps.
• Secondary Current:
  I_Secondary = 75,000 / (208 × 1.732) ≈ 208.2 amps.
• Secondary OCPD: Sized per 125% Rule:
  208.2 amps × 1.25 = 260.25 amps.
  Note 1 allows rounding up to 300 amps.

Secondary Conductor Protection: Secondary conductors must have an ampacity of at least 300 amps to match the OCPD rating. If the secondary conductors are rated for less (e.g., 225 amps), they must be upsized.

Conclusion:

• Install a 225-amp OCPD on the primary side.
• Use a 300-amp OCPD on the secondary side, ensuring secondary conductors are upsized to 300 amps or more.

Key Takeaways

• Primary Conductors Are Not Feeder Taps: Conductors between the primary OCPD and transformer are part of the transformer circuit and governed by Article 450.
• 240.4(B) and Primary Conductors: 240.4(B) allows the next standard OCPD size if conductor ampacity supports the load and is between standard sizes.
• Note 1 for Secondary OCPD: Secondary OCPDs sized at 125% of secondary current can use the next standard size under Note 1, but conductors must meet or exceed the OCPD rating.
• Secondary Conductor Protection: Secondary conductors must comply with 240.21(C), ensuring they are adequately protected without relying on 240.4(B).

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