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Transformer secondary conductors wrapper

Why you care (60 seconds)​

Transformer secondary conductors have special protection allowances. If you apply general feeder rules, you may over- or under-protect the conductors.

Where people lose time​

  • Treating transformer secondary conductors as standard feeder taps.
  • Ignoring the distance and protection limits for secondary runs.
  • Forgetting the relationship between transformer OCPD and secondary conductors.

This is​

The rule in plain language​

Transformer secondary conductors can be protected by the primary or secondary overcurrent device if the secondary conductors meet specific length and installation conditions.

When it applies​

Conductors on the secondary side of a transformer feeding a panel or equipment.

What you must do (checklist)​

  • Identify which secondary conductor rule applies.
  • Keep secondary conductor length within allowed limits.
  • Ensure overcurrent protection aligns with transformer and conductor ratings.
  • Provide required physical protection and termination ratings.

Quick examples​

  • A short secondary run to a nearby panel is permitted under specific rules.
  • A longer secondary run requires stricter protection and routing conditions.

This is not​

Common misreads​

  • Using feeder tap rules without checking the transformer-specific section.
  • Assuming the transformer OCPD always protects secondary conductors.

What it doesn't cover​

  • Transformer sizing or primary OCPD selection.
  • Grounding and bonding of transformer systems.

False friends​

  • A "secondary feeder" still needs to meet the transformer tap rules.

Exceptions & edge cases​

  • Different length limits apply based on installation type and enclosure.
  • Secondary conductors in a raceway may have different allowances.

Cross-references (NEC map)​

  • Primary: 240.21(C)
  • Secondary: 450.3, 310.16

Exam traps​

  • Mixing transformer secondary tap rules with general feeder tap rules.
  • Missing the required length limits.

Field notes​

  • Measure secondary runs before ordering conductors.
  • Document which transformer tap rule applies for inspectors.

AHJ / Local amendments notes (placeholder)​

  • Add local amendments or interpretations here.

Revision notes​

  • Draft wrapper created for transformer secondary conductors.

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