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The cable manufacturer will provide derating factors for several possibilities i.e., number of installed cables in a layer, number of layers, cable spacing horizontally and vertically. The actual condition is a combination of cable installation and cable manufacturer factors are not sufficient.When do you need to derate a conduit wire?
Determine if conductors require derating for both ambient temperature and the number of bundled conductors. For example: A conduit carrying six TW type 12-gauge wires travels through an attic that reaches 110 degrees F (43 C) in the summertime.How big should a derating conductor be for a raceway?
Where the connected load is over 800kVA, the Table allows the service conductors to be sized for a demand load of 410 + (0.5 x 28) 424kVA. Dividing this figure by 208 x 1.73 results in a current of about 1,178A.Can a voltage drop cause a cable to derate?
Other Engineers may also derate based on voltage drop over the length of the wire. Or using a larger gauge wire to reduce the current and voltage drop of the interconnection.