Answer:

 

This air tube is pointed at the motor starter inside of this motor control center. Someone had replaced an existing 20-hp motor in a cooling tower with a new 40-hp motor. However, they did not replace the circuit conductors feeding the motor. When this new load caused the overloads in the starter to function as designed, someone simply removed the enclosure door and replaced it with this expanded steel cover, extending an air conditioning duct to keep the undersized starter and circuit conductors cool. They claimed it has been working like this for years!

As per 430.6, Ampacity and Motor Rating Determination, “The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with 310.15(B) or shall be calculated in accordance with 310.15(C).” This rule goes on to state, “The required ampacity and motor ratings shall be determined as specified in 430.6(A), (B), and (C).

See Example D8 on page 70-723 of the 2005 NEC for additional information.

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