I have 1 1/4 flex conduit with 2 #14 thhn, 14 #12 thhn, & 1 #4 thhn wire feeding from new service to old service panel. The #1 is used to feed old neutral bus bar and the #14 & #12's are circuit conductors. I have been asked to derate wire.

I believe that you first must determine how many of the wires in the conduit are carrying voltage (ungrounded conductors)

But before you even get that far, you need to explain yourself and what you are trying to do.

For example are you sure you are only going to need one #4 (copper or alm?) What will you be using as a neutral return? Are you setting up a subpannel? Will you be using 120/240VAC circuits.

What are you really trying to do?

I believe that you first must determine how many of the wires in the conduit are carrying voltage (ungrounded conductors)
Grounded conductors (neutrals) also carry current.


Larry,
Derating is a very tricky thing. Some current carrying conductors (CCC's) do not count towards this. Most do.


I also ask; why only one #4? If this is your ground you did NOT need to run such a large wire for the ground.

Who did this work?
How long is this flex conduit?