Transformer protection wrapper
Why you care (60 seconds)
Transformer protection mistakes can damage equipment and trigger inspection issues. The protection rules are specific and often overlooked.
Where people lose time
- Sizing protection based on nameplate only without rules.
- Forgetting that primary and secondary protection can differ.
- Ignoring manufacturer instructions on protection settings.
This is
The rule in plain language
Transformers require overcurrent protection sized according to transformer rules and the equipment listing.
When it applies
Any time you install or modify a transformer and its protection.
What you must do (checklist)
- Identify transformer type and rating.
- Apply the correct protection sizing method.
- Confirm primary and secondary protection requirements.
- Follow manufacturer instructions and listings.
Quick examples
- A transformer may allow higher primary protection than the secondary.
- Protection settings must align with the transformer listing.
This is not
Common misreads
- Treating transformer protection like standard feeder protection.
- Assuming a single protection device covers both sides.
What it doesn't cover
- Detailed transformer installation and ventilation requirements.
- Grounding and bonding specifics for all transformer types.
False friends
- A transformer nameplate does not override code protection rules.
Exceptions & edge cases
- Special transformer types can have unique protection allowances.
- Overcurrent devices may need coordination for downstream loads.
Cross-references (NEC map)
- Primary: 450.3
- Secondary: 110.3(B)
Exam traps
- Using incorrect protection percentages for the transformer type.
- Missing secondary protection when required.
Field notes
- Record protection settings and device ratings in commissioning notes.
- Keep transformer documentation accessible for inspection.
AHJ / Local amendments notes (placeholder)
- Add local amendments or interpretations here.
Revision notes
- Draft wrapper created for transformer protection fundamentals.