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Since there are multiple factors in determining the operating temperature of a conductor, the use of ampacity tables follows detailed instructions, found in Section 310.15(B), which include the use of:ambient temperature correction factors and adjustment factors for more than three conductors.
They also provide specific information about bare or covered conductors, neutral conductors and grounding and bonding conductors.
The instructions include a separate special section and table, both numbered 310.15(B)(7), for ampacities of conductors that are used strictly to supply the entire load of a dwelling unit. This rule typically allows for a smaller conductor because of the historical information that has been gathered about that specific type of occupancy.
The result of using the table method is what's called an allowable ampacity.Allowable ampacity is not a precisely calculated ampacity, but an ampacity that is considered to be adequate, that will not present a significant risk of overheating and that may have some safety factor built into it.
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