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Conductor ampacity wrapper

Why you care (60 seconds)​

Ampacity errors create overload risk, failed inspections, and nuisance trips. This wrapper keeps sizing decisions tied to actual installation conditions.

Where people lose time​

  • Forgetting that conductor temperature rating can be limited by terminals.
  • Applying derating after selecting the device instead of before.
  • Miscounting current-carrying conductors in a raceway.

This is​

The rule in plain language​

Choose conductor sizes using the correct ampacity table, then apply any adjustments and corrections based on how and where the conductors are installed.

When it applies​

Any time you size or verify conductors for branch circuits, feeders, or services.

What you must do (checklist)​

  • Identify conductor material, insulation type, and temperature rating.
  • Use the appropriate ampacity table for the conductor.
  • Apply ambient temperature corrections when required.
  • Apply adjustments for number of current-carrying conductors.
  • Confirm termination temperature limits do not reduce allowable ampacity.

Quick examples​

  • A raceway packed with conductors requires adjustment before choosing an OCPD.
  • A 90C conductor may still be limited by 75C terminals.

This is not​

Common misreads​

  • Assuming the largest table value is always usable.
  • Treating adjustments as optional for short runs.

What it doesn't cover​

  • Special equipment rules that override general ampacity tables.
  • Full load calculations for complex systems.

False friends​

  • Mixing conductor ampacity with allowable load on a device.

Exceptions & edge cases​

  • Certain equipment types have dedicated sizing rules.
  • Short taps or special installations may follow separate requirements.

Cross-references (NEC map)​

  • Primary: 310.15, 310.16
  • Secondary: 110.14(C)

Exam traps​

  • Choosing a conductor size before applying corrections.
  • Ignoring termination temperature when both conductors and lugs are involved.

Field notes​

  • Record ambient assumptions and conductor counts in the job file.
  • Keep a quick reference card for adjustment and correction inputs.

AHJ / Local amendments notes (placeholder)​

  • Add local amendments or interpretations here.

Revision notes​

  • Draft wrapper created for ampacity and derating fundamentals.

Related

Prerequisites