450.9 Ventilation.
Code Change Summary: New code requirements require marking on transformer covers.
Horizontal surfaces tend to be a magnet for storage, especially a transformer in an electrical room. Transformers mounted on the floor are an inviting place to set boxes or other things but unfortunately, transformers tend to give off a lot of heat which is why they require ventilation.
In the 2020 NEC®, language was added to require marking on transformers with a horizontal surface and installed in readily accessible locations to prohibit storage.
The new code language does not provide a word for word quote that must be on the label or any size or color of the label, but most folks will come up with something creative that will do the trick.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.
2017 Code Language:
450.9 Ventilation. The ventilation shall dispose of the transformer full-load heat losses without creating a temperature rise that is in excess of the transformer rating.
Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions. The required clearances shall be clearly marked on the transformer.
2020 Code Language:
450.9 Ventilation. The ventilation shall dispose of the transformer full-load heat losses without creating a temperature rise that is in excess of the transformer rating.
Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions. The required clearances shall be clearly marked on the transformer. Transformer top surfaces that are horizontal and readily accessible shall be marked to prohibit storage.