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Do transformers require Arc flash labels, and what voltages are arc flash labels required for in low voltage panels?
Section 110.16 of the NEC addresses your question and states Electrical equipment, such as switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures, and motor control centers, that is in other than dwelling units, and is likely to require examination, adjustment, servicing, or maintenance while energized, shall be field or factory marked to warn qualified persons of potential electric arc flash hazards. The marking shall meet the requirements in 110.21(B) and shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment. The key information to focus on in this rule is if the transformer is equipment likely to be worked on while energized. Transformers are not mentioned, at least directly. The other bit of useful information in this rule worth noting is the phrase "shall be field or factory marked to warn qualified persons of potential electric arc flash hazards." Often manufacturers include not only danger markings on this type of equipment, but also arc-flash hazard warnings from the factory. Applicable voltages are not mentioned in Section 110.16.
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