Project: Run a 15Amp and a 20Amp circuit to bathroom from breaker panel outside the house.
After much reading, I have determined that NEC requires adherence to the following. Any suggestions for improvement are most welcome.
Exterior run from panel to the spot where it enters the attic will be done in a conduit (Cantex PVC SCH 40).
Because outdoors is considered damp/wet area, only THHN/THWN-2 wire should be used at least until the wire enters the attic.
For a 3/4" ID conduit, the max fill area for 2 or more wires (or circular-equivalent cables) is 40% of the available or
0.213 sq inch.
I will have a total of 6 wires, 3 per circuit, half of which will be 14 Gauge and the remaining 12 gauge. Alternatively I could also run all wires at 12 gauge.
If I went with 12AWG (3) and 14AWG (3), the total occupied area will be
0.0097*3=0.0291 sq inch for 14 AWG, and
0.0133*3=0.0399 sq inch for 12 AWGfor a grand total of
0.07 sq inor
11.40% fill, which is well within the limits allowed under NEC 2017. I did this calculation assuming 3 conductors per circuit, but presumably the grounding wire is not considered a conductor, so the actual numbers are lower by
As for derating the conductors, it is my understanding based on this post that for 15/20 Amp circuits running less than 9 wires, the derating really is not a concern. Someone correct me if I am wrong.
So in nutshell, I should be good to run either six 12 gauge wires or three each of 12 and 14 gauge wires through a single conduit from panel to attic entry point. Here are my specific questions:
Since I will be converting the THHN/THWN-2 wire to Romex 12/2 or 14/2 cable, is it okay to join the green sheathed grounding wire of the former type with the bare copper one from the romex?
If I use 12 gauge THHN wire on both circuits, am I limited to using only 12 gauge romex cable inside the house? This is what I am assuming, but just want to make sure.
Is there a benefit to using stranded vs solid THHN?