I have a 200A service panel main in the basement. I'll be installing a subpanel
Main lug in the attic with a dual pole 60A breaker in the basement main feeding this
Attic new subpanel. Using a #6/3 romex wire from basement to attic
(about 35 - 40 feet total wire length).

Along with this #6/3 romex wire (from basement to attic), I'll be bringing 2 more 14/2 romax wire from a couple of HOT switches (HOT goes to switch first) located in the lower
Levels, to the attic to control a copule of sets of flood lights (150W for each set).
These flood lights are installed outside of the house to light the front and back yard.

Question:
1. Since I have 3 wires at long lengths,
#6/3 romex at 40' on a 2pole 60A breaker
#14/2 romex at 50' on a 15A breaker
#14/2 romex at 60' on the same very 15A breaker
Coming from basements to attic, do I need to derate my wires?

2. I Would appreciate it greatly, if someone explain in details,
In what conditions one needs to derate, and what does derating really do ?
e.g. is it voltage drop a concern that derating addresses,
Is it wire over heating conditions that derating addresses, etc.
Thanks in advance
-Ab

Derating is related to the quantity of current carrying conductors bundled, such as when individual wires are pulled into a conduit.

When 4 or more current carrying conductors, and I keep saying that because in most cases the neutral is current carrying, derating is necessary to minimize the load on these wires in an effort to keep the heat created minimized.

Cables only need to be derated if they are bundled, such as tyrapped into a bundle. I do not believe this is your case, and derating is not necessary in your situation.

Derating is done by either of two methods. Either redcue the size circuit breaker, if applicable, or mostly, by increasing the size.

I am going to stop here, as there are several more details that will get lengthy to explain, having to do with the ampactiy rating of a certain wire at a certain temp ratng.