How much derating will occur if I run 3 2awg and 1 4awg in a 1.5" EMT conduit? Breaker is likely to be a 75Amp. 2 conductors will be hot legs.
You only derate for current carrying conductors, not conduit size. If this circuit is fed from a single phase 120/240 V source, then you only have two current carrying conductors, so no derating is required. Over three conductors is when you begin to derate.