U.S. National Electrical Code allows *SIX* #10 wires in 1/2" conduit, with
appropriate insulation -- e.g. THHN.

What was the derating factor?



Derating in this application is not important. You would derate to 80% but THHN
#10 cu is good for 40a and 240.4(D) limits you to 30a so you still have a
safety factor, even with the derate.
You only have to derate based on current carrying conductors and you don't need
to derate the conductors that only carry the unbalanced current of a multiwire
circuit so it usually takes a lot of wire in a pipe before a THHN run is
affected.