Fill in the blank with the appropriate answer.
When installing a ventilating duct in a transformer vault, per Part III of Art. 450, the duct shall be constructed of material that is ____________.
A. acceptable to the Authority Having Jurisdiction (AHJ)
C. fire resistant
D. flame retardant
Part III of Art. 450 covers the topic of transformer vaults. The location, arrangement, size, covering, damper type, and duct material requirements for ventilation opening rules are listed in 450.45. As noted in 450.45(F), "Ventilating ducts shall be constructed of fire-resistant material."
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