I am installing two 12/3 uf cables in a continuous 1.5 pvc buried pipe from my basement to a backyard pull box for landscape lighting. How much must I derate the two 12/3 uf cables because they are in a pipe?

Other than for possible protection, why are you running in conduit? UF are direct bury cables. Are you running a 3 way switch too? Most landscape lighting is timered on a single pole switch with 12 or 14/2?

I am installing two 12/3 UF cables in continuous pvc pipe for extra protection for the wires and I would like to know if I have to derate the wires?

Yes I am installing the 12/3 UF cables in pipe for protection. No, I am not running a 3-way. The wires are for several SP switches and spares for future.

More than 3 current carrying conductors, up to six, will need to be derated 80%.

How many current carrying conductors in two 12/3 UF cables? Would you derate 80% of 25AMPS for UF cables?

When derating more than three current-carrying conductors in a raceway, does this apply the same if the conductors are installed in a cable? Example: romex installed through a contiuous pipe

You had about 7 posts on this yesterday, just kep adding your comments and questions at the bottom of your old post, on't start new ones each time

Explain how the 2- #12-3 cables will be used to determine how many are current carrying.

The simple answer is all will be current carrying, and protect each circuit with a 15 amp breaker.

Not sure I want to put my two cents worth in here but I'm going to do it anyway.

You do realize that with two 12-3 cables you will have four individual circuits protected by 15 amp breakers. If you use three circuits now and keep one for a spare, that will give you 45 amps now and 15 amps for future use. If one were to assume 100 watt bulbs in each fixture those 45 amps would permit as many as 54 fixtures. Must be one heck of a landscape lighting project.

You said you want multiple single pole switches but didn't say how many or what kind of fixtures.

You said 12-3 cable, you do realize that 12-3 means 12 gauge wire with 3 conductors. Two hots and a neutral plus a ground wire. Meaning 12-3 has four wires.

Something tells me that you are terribly confused. Why not tell us what you want to do and then let someone tell you how to do it.

Tell us like this.
"I would like to run power from my breaker panel to my patio. At my patio I would like to have four switches. I would like to have one switch turn on the front yard lights, the second would turn on the back yard lights and the third would turn on the side yard. The fourth switch would be a spare in case I decide to put lights on my roof."

Just a suggestion.