OCPD sizing wrapper
Why you care (60 seconds)
OCPD sizing decisions affect conductor protection, nuisance trips, and inspection acceptance. This wrapper keeps the sizing logic clear and traceable.
Where people lose time
- Mixing up conductor ampacity with device rating.
- Skipping the standard ratings list before selecting a device.
- Treating adjustment and correction factors as optional.
This is
The rule in plain language
A method for selecting overcurrent protection that fits conductor limits and standard device ratings.
When it applies
Whenever you size a breaker, fuse, or device protecting conductors or equipment.
What you must do (checklist)
- Establish conductor ampacity after adjustments and corrections.
- Compare to standard device ratings.
- Verify any allowances for rounding are applicable.
Quick examples
- Confirm that a conductor ampacity supports the selected breaker size.
- Use standard ratings instead of custom values.
This is not
Common misreads
- Assuming a device rating can exceed conductor ampacity without conditions.
- Using equipment nameplate only and skipping conductor limits.
What it doesn't cover
- Full motor and HVAC protection rules.
- Detailed coordination studies.
False friends
- Confusing ampere rating of a device with the allowable load on conductors.
Exceptions & edge cases
- Special equipment articles can override general sizing rules.
- Ambient temperature and bundling can change allowable ampacity.
Cross-references (NEC map)
- Primary: 240.4, 240.6
- Secondary: 310.16
Exam traps
- Choosing the next size up without confirming the conditions allow it.
- Ignoring derating that pushes you below a standard rating.
Field notes
- Document the derating inputs used for final OCPD selection.
- Keep a quick reference of standard device ratings on-site.
AHJ / Local amendments notes (placeholder)
- Add local amendments or interpretations here.
Revision notes
- Draft wrapper created for OCPD sizing fundamentals.