Conduit size does not matter when it comes to de-rating. First you must calculate, based on circuit amperage size, what size wire you need. You can have three current carrying conductors in a conduit before you nee to de-rate. When you de-rate for 14,12, and 10 gauge you are allowed to use a higher starting current for your calculations. Example, #12 wire you can use 25 amps for your calculations. ( although you can never put #12 wire on larger than a 20A breaker) When you have up to and including 6 #12 current carrying conductors, you have to de-rate 80%. 80% of 25 amps is 20 amps, so, you can have up to 6 #12 current carrying conductors in a raceway before you have to up-size the wiring accordingly. Once you determine the amount of conductors in the conduit, and, whether they need to be up-sized and to which size, then you base conduit size on conductors contained therein. Whether or not a conductor is considered a current carrying conductor is a whole new topic for debate, but, nowadays with almost everything being electronic, it is wise to consider both ungrounded and grounded conductors as current carrying.