How many wires can I put in conduit before I have to derate?
Three current-carrying conductors share a raceway with no adjustment. Four to six get 80%, seven to nine get 70%, and ten to twenty get 50% per NEC Table 310.15(C)(1). Neutral counting depends on system type and harmonics. Conduit fill limits in Chapter 9 are separate from ampacity derating, so you need to check both.
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